A very good observation. Yes, thunderstorms occur in the late afternoon and early evening in many parts of the United States, including my home in the Midwest. During the morning hours, the air near the surface of the land is humid and cool. As the Sun comes up, it starts to heat up the ground, which then heats the air above it, and any moisture on the ground in the form of dew evaporates into the air.
It takes the hottest part of the day to generate the most unstable air, caused by a lifting action as warm air rises. In the process, the warm air cools, and cool air cannot hold as much moisture as warm air can. So the relative humidity shoots up. Relative humidity is the ratio of moisture in the air to the amount of moisture the air can hold (100 percent humidity). As the air rises and cools, it has less ability to hold moisture.
At the same time, the dew point temperature decreases. The dew point is the temperature at which the air can no longer hold moisture as vapor, so the vapor must condense into liquid water. At a certain altitude, typically about ten thousand feet, the vapor in the air condenses onto dust particles or oxygen molecules and forms a little visible bead. Those little droplets of moisture make up a cloud.
This lowest altitude of condensation is generally where we see the bottom of those fluffy white cumulus clouds, usually in the summer sky. If enough of this lifting and condensing occurs, the cloud may build into a cumulonimbus cloud, or thunderhead, which is the type of cloud that produces thunderstorms. A line of these thunderheads is called a squall line. Every thunderstorm needs moisture, unstable air, and lifting capability to form.
The average thunderstorm is fifteen miles across and lasts thirty minutes. Nearly two thousand thunderstorms are occurring at any moment around the world. Winds in thunderstorms can exceed 100 mph, and lightning kills an average of fifty-three people per year in the United States.
Snow is a bunch of ice crystals stuck together. It’s a very complex arrangement. To understand why snow is white, we must be familiar with what happens to light when it strikes any material. The color of anything, including snow, depends on how light interacts with it (see question 37).
Visible light consists of a rainbow of colors, the ROY G BIV colors of red, orange, yellow, green, blue, indigo, and violet that were assigned by Isaac Newton. When photons of light strike an object, they may bounce back (reflection), bounce to the sides (scattering), pass right through (transmission), or give up their energy (absorption). Grass is green because it reflects the green light to our eyes and absorbs all the other colors. Red apples reflect red light to our eyes and absorb the wavelengths of all the other colors.
When light goes into snow, it hits all those ice crystals and air pockets and bounces around, and then some of the light comes back out. Snow reflects all the colors; no it doesn’t absorb, transmit, or scatter any single color or wavelength more than any other. The “color” of all the light wavelengths combined equally is white. So all the colors coming out are the same colors that go in, combining to make white light.
A few years ago, I visited the famous Mendenhall Glacier in Juneau, Alaska. The glacial ice looked bluish. Ice is just very compact snow, without a lot of light-scattering bubbles. Light can penetrate much deeper into ice than into snow. The deeper the light goes, the more the longer wavelengths, toward the red end of the spectrum, get scattered out, and eventually the reds dissipate, leaving only blue colors to be reflected back to us. So the ice takes on a beautiful, eerie blue tone.
The record snowfall for any one year in the United States is 1,140 inches (95 feet) at Mount Baker Ski Area in northwestern Washington, during the 1998–1999 snow season.
Snow is beautiful. It coats everything in a pure white blanket. It helps farmers and is good for the land, because it has a ton of air pockets. Even though the snow itself is cold, the air that they hold in insulates the ground, protecting seedlings and preventing the frost from going too deep. Snow that falls in the mountains later melts and helps fill the depleted reservoirs of the American West.
You may have noticed how quiet it is outside after a fresh snowfall. In addition to making snow fluffy those air pockets absorb sound, just like the ceiling tile in my classroom. After a few days, sound travel returns to its normal pattern. Many of the fluffed-up ice crystals melt and compact somewhat, so the tiny pockets that absorbed sound are gone.
When I was a kid on the farm, my dad planted fields of oats in April. One year the oats were up about three inches when we got one of those late-spring snowstorms with four or five inches of snow. I thought all the oats would be dead. Strangely, my dad didn’t seem to be concerned. Turned out those were some of the best oats we ever had. I remember him mentioning something about the snow adding nitrogen to the soil, which makes sense because moisture helps plant seedlings fix nitrogen in the soil.
The Great Salt Lake is quite salty because it is a terminal lake, which means it does not have an outlet. Even though the water in the rivers feeding it doesn’t seem salty, they have small amounts of salt dissolved in them, which they constantly bring into the Great Salt Lake. This water has no place to go, so much of it evaporates from the lake surface, leaving the salt to accumulate. The lake’s three tributary rivers deposit over a million tons of minerals in the lake each year. The water flowing down from the mountains carries these dissolved mineral salts that have been removed from rocks and soil. The Great Lakes in the Midwest would also be salty if they did not have an outlet. Fortunately, the Great Lakes empty into the Atlantic Ocean by way of the St. Lawrence Seaway.
The Great Salt Lake is the largest salt lake in the Western Hemisphere and is located in northern Utah. Its size fluctuates depending on rainfall and snowfall. Its smallest area was about one thousand square miles in 1963, and its largest size was more than three thousand square miles in 1987. The shoreline keeps changing. In late summer, the lake is smaller because much water has evaporated away. Snowmelt makes the lake bigger in the spring. So the shores of the Great Salt Lake have remained fairly undeveloped, with widespread wetlands that attract migrating birds.
Because of all the salt, the water in the Great Salt Lake is denser than the human body. You cannot sink in it; you’ll float like a cork. Only brine shrimp and some algae have managed to live in the lake; it’s too salty to support fish. Mono Lake in California, just east of Yosemite National Park, is another very salty lake. It is a terminal lake in a low spot that has no outlet.
But which body of water is the saltiest? The ocean is about 3.5 percent salt, the Great Salt Lake varies from 5 to 25 percent salt, and Mono Lake is 10 percent salt. Even saltier is the Dead Sea, located between Jordan and Israel. It is 31.5 percent salt, and it also has the lowest elevation on earth, at 1,400 feet below sea level. In comparison, Death Valley in California is 282 feet below sea level. But the prize for “saltiest” goes to the Don Juan Pond in Antartica. It’s eight times saltier than the Dead Sea!
The lowest possible temperature is termed absolute zero, which is −460°F, the same as −273°C, and also the same as zero Kelvin. Scientists have come within a few hundredths of a degree of absolute zero in the laboratory.
Temperature measures the average kinetic energy of molecules. The greater the motion or vibration of molecules in an object, the hotter it is. So outer space has no temperature, because it is a vacuum. The few particles floating around out there would have a temperature of about three Kelvin, close to absolute zero.
On Earth, temperatures have never gotten anywhere near absolute zero. The lowest temperature ever recorded on Earth was −129°F, which is 184° above absolute zero, on July 21, 1983, at Vostok, a Soviet station in the Antarctic. Keep in mind that July is actually wintertime in the Southern Hemisphere. The coldest temperature ever recorded in the United States was −80°F, at Prospect Creek Camp, Alaska, on January 23, 1971. Prospect Creek is along the Alaskan oil pipeline and is just north of the Arctic Circle. And the lowest temperature recorded in the contiguous forty-eight states was −70°F, at Rogers Pass in Montana on January 20, 1954.
Many physics classes, such as my own, feature demonstrations using liquid nitrogen. The temperature of liquid nitrogen is −321°F, or −196°C. In one experiment, we would put a constant-volume, hollow, stainless-steel sphere in four different liquids, including liquid nitrogen, and record the pressure of the air inside the sphere. We would then graph pressure versus temperature and use extrapolation to determine the value of absolute zero. We would also make ice cream using liquid nitrogen; we’d study the behavior of materials, ranging from balloons to flowers to a rubber handball, in liquid nitrogen; and we would demonstrate superconductivity by levitating a magnet above a super-cold pellet. This superconducting pellet, the size of a checker on a checkerboard, was made of yttrium, barium, and copper. Cooled to 77K, or −196°C, or −320°F, the pellet wafer became a superconductor, losing all resistance to electrical flow.
Magnetic levitation occurs due to the Meissner effect. A magnetic field surrounds the magnet. This magnetic field induces a current in the pellet. Surrounding the current carrying pellet is a magnetic field. These two magnetic fields, one from the magnet and one from the pellet, oppose each other, creating levitation.
Here are my favorite “How cold was it?” jokes:
The world we live in is both beautiful and surprising. And sometimes what we see can mislead us. How can the sky meet the Earth? First of all, let’s agree that the horizon is the line at which the sky and Earth appear to meet.
Children will often draw a strip of blue sky and a strip of brown ground with a few rudimentary figures on it, but the rest of the page is white. They do not draw the blue sky touching the ground. Teachers sometimes take young students outside to show them that the sky touches the ground.
So it’s not easy to believe what we see. We look straight up and see the sky. But in the distance the sky does appear to meet the Earth. It’s all a matter of perspective. Long ago, people on shore would watch ships leave the docks and sail away. The last things that disappeared over the horizon were the tops of the masts, which was a clue that the Earth is round, or spherical.
Nature can fool us in other ways, too. The Moon looks gigantic rising over the horizon but seems much smaller when it’s directly overhead. This is a result of the well-known Ponzo illusion (see question 88). The brain interprets the sky as being farther away near the horizon and closer near the zenith, or directly overhead. We notice this on a cloudy day. The overhead clouds may be a few thousand feet away, but near the horizon they might be hundreds of miles away. The sky appears sort of bowl-shaped. The Moon on the horizon is interpreted by the brain as being farther away. Because it is the same apparent size as when it’s high up, the brain figures it must be physically larger. Otherwise, the distance would make it look smaller.
And railroad tracks seem to converge and come together when we stand between the rails and look down the tracks. As objects’ distances increase, their image size on the retina of the eye decreases, until the farthest objects seem to meet at what is called the vanishing point. Objects seem so small they are no longer visible. So parallel lines, such as railroad tracks, are perceived as coming together until they meet, or converge, at the vanishing point.
There is a formula that tells us how far we can see to the horizon. Another way to put it is that we can calculate how far away the horizon is. The distance to the horizon equals ninety times the square root of the distance, in miles, between your eyes and the ground beneath you. So if you are standing on the beach of the ocean or a big lake and you are six feet tall, 6 divided by 5,280 will give you the distance you are above the sand in miles, and if you take the square root of this number and multiply it by 90, you’ll find that the horizon is about 3 miles away. If you go to the top of the Willis Tower (formerly called the Sears Tower), which is about 1,450 feet tall, the horizon will be nearly 50 miles from you.
When I was a kid growing up on a hilltop farm in Crawford County, Wisconsin, I often wondered what it would look like if the sky met the water. The biggest hunk of water I had seen at the time was the Mississippi River, and I could see to the other side. It wasn’t until I left home and joined the military at eighteen that I saw the Atlantic Ocean. And, sure enough, the sky touched the water, with nothing in between. It really could happen after all.
I have a theory about why the sky is blue and grass is green. If the sky were green, we wouldn’t know when to stop mowing (a little science joke)!
Diamonds measure ten on the Mohs scale of hardness, making them the hardest natural substance on Earth (see question 74). Cutting diamonds is both an art and a science that goes back centuries. Cleaving, the basic diamond-cutting process, is the separation of a rough diamond into separate pieces, to be finished as separate gems. The jeweler places a chisel at a point of weakness in the stone and taps it with a mallet, causing the diamond to split. Any misjudgment by the diamond cutter could lead to losing a valuable gem.
A polishing wheel, called a scaif, was invented in 1456. The diamond is held in a dop, a padded holder that protects the diamond while the jeweler works on it. The polishing wheel is kept lubricated with olive oil and coated with diamond dust. It takes a diamond to cut a diamond. The scaif allows diamond cutters to create symmetrical and even facets, which bring out the sparkle and shine of a diamond.
A diamond saw was developed in the 1900s. Diamond saws are steel blades lubricated with olive oil and edged with diamond dust. Some blades are made of a phosphor bronze alloy. The material lost in cutting and polishing is often over half the weight of the rough diamond.
The value of a diamond depends on the four Cs: cut, clarity, color, and carat. The cut refers to the diamond’s geometric proportions: its faces, facets, and finished shape. Clarity refers to the flaws in the diamond. Color can range from milky white to yellow. Carat is a measure of weight and size.
The 3,107-carat Cullinan was the largest diamond ever found. Mined in 1905, it was presented to Edward VII of England. Later, it was cut into nine major stones. The most famous diamond in America is the Hope diamond, on display at the Smithsonian in Washington, DC. It was 112 carats in the year 1668 but has also been recut. A carat is 200 milligrams, or 0.2 grams. There are 454 grams in a pound.
Diamond is pure carbon in an organized, highly compressed form. Carbon is plentiful; our bodies are, for example, 18 percent carbon by weight. Diamonds are formed deep inside the Earth, where extreme pressure and heat turn carbon into diamonds. They are brought closer to the surface of the Earth by volcanic eruptions.
The diamond market is dominated by a single entity, the De Beers cartel in South Africa.
The Earth rotates counterclockwise, as seen from above. “Seen from above” means from a position above the North Pole. The rotation of Earth is vitally important to all aspects of life, because as the Earth spins on its axis, every part of the planet has a chance to face the Sun and receive warmth over an interval that repeats in a relatively short period of time.
We can trace the Earth’s rotation back to the way stars and planets are born. A newborn star gathers a disk of dust and gas around itself, and the star’s gravity sets that dust and gas spinning. The disk begins as a large mass of gas and molten liquid. The center of the disk becomes the Sun, and the outer rings and clumps of matter cool and condense to take on solid form. Any clumps within the larger mass of spinning dust and gas are going to have their own rotation. These clumps form the planets. As each clump collapses onto itself, its volume decreases and its density increases, causing it to spin faster and faster, a concept of motion known as “conservation of angular momentum.” Figure skaters make use of this principle when they bring their arms in closer to their bodies to speed up their spin rate. Because space is a vacuum, no force or friction is there to stop the rotation. The Sun and its planets will just keep on spinning forever. For this rule, we can credit Newton’s law of inertia: objects in motion tend to stay in motion.
The Moon’s gravity affects Earth’s rotation. It causes the waters of the oceans to wash up the shore and back down the shore. This tidal friction slows down the rotation of the Earth. Because of the Moon, the spin of the Earth is slowing down about one millisecond (0.0015 seconds) per year. About every eighteen months a leap second is added to keep our planetary time consistent with atomic clocks and astronomical observations. At the time of the dinosaurs, a day was about twenty-three hours long.
To make up for the slower rotation rate, the Moon is moving away from the Earth about 1.5 inches per year. As the time for rotation of the Earth grows longer, the Moon increases its orbital radius. It’s that same figure-skater idea in reverse: slow rotation, arms out. We can think of the Earth and the Moon as being a system. The body of the skater is the Earth, and the arms are the Moon. As the skater (Earth) slows down, the arms (Moon) move away from the skater.
Millions of years from now, the Moon will look smaller in the sky and a day will be twenty-five or twenty-six hours. We’ll all get more done!
Most rational explanations for incidents in the Bermuda Triangle involve pilot errors, a swift and turbulent Gulf Stream, and environmental factors—namely, weather.
The Bermuda Triangle is an area off the southeastern coast of the United States. The corners of the triangle are Bermuda; Miami, Florida; and San Juan, Puerto Rico. It’s really an “imaginary” area, not recognized by the United States Coast Guard. But the area has become notorious for the seemingly high frequency of unexplained losses of ships, small boats, and planes. For example, the loss of five TBM Avenger airplanes (Flight 19) and the rescue aircraft sent after them, on December 5, 1945, is legendary.
However, the Bermuda Triangle is an area of heavy sea and air traffic covering a huge chunk of ocean. One would expect to have losses along such a heavily traveled route.
Another possible reason behind the Bermuda Triangle’s link to disappearances is that compasses in the triangle don’t point anywhere near to true north. Instead, they point to magnetic north, which is about eleven hundred miles away from true north. That magnetic variation is off by only a few degrees here in Tomah, Wisconsin, but it is off by nearly twenty degrees in the Caribbean. If navigators don’t take that anomaly into account, they can be in deep trouble.
The Bermuda Triangle also happens to be a naturally danger-filled area. It falls within the course of the Gulf Stream, which is swift and turbulent. Storms can arise suddenly and unpredictably, which not only leads to accidents but also rapidly erases any evidence of plane crashes or shipwrecks. Most of the hurricanes that have made the news lately travel through the Bermuda Triangle. And waterspouts, which are tornadoes over water, are frequent. The terrain itself is also challenging to navigate. The ocean floor has some of the deepest trenches on the planet, but also extensive shoals and reefs. Many a ship has lost its bottom on those treacherous formations.
Lloyd’s of London, which provides the insurance for most of the world’s shipping, claims that losses are no higher in the Bermuda Triangle than in any comparable area of the world’s oceans. Insurance rates are the same for Bermuda Triangle traffic as for ships that traverse any other area of the world.
Conspiracy theories abound in our media, running the gamut from Bigfoot and the Loch Ness monster to UFOs and aliens from outer space. These theories all sell books, magazines, and TV programs, but what they lack is credible evidence.
It would seem logical and natural for a river to run straight, but rivers that flow over gently sloping ground begin to curve back and forth. The rambling routes of what we call meandering rivers come about through erosion and sediment deposit.
Due to some asymmetry or obstruction in the riverbed, such as rocks, weed growth, or fallen trees, the speed of the flowing water between the two banks differs. The faster side of the river carries more sediment along, so less is deposited. And because the water is flowing faster, more erosion takes place. The slower side of the river deposits more sediment from erosion. Slower-moving water allows more time for soil particles to settle.
You can probably tell what is happening here. The faster-moving water is eating into the bank, making a small curve. Once the curve is established, the water on the outside of the curve must travel faster than that on the inside because of the greater distance the water must travel. This erodes the outside of the curve more, the water moves still faster, and the process perpetuates itself. As the river erodes soil from the outer curve, it deposits the sediment on the inner curve. This causes the meanders to grow larger and larger over time; the bend gets more and more pronounced. Consequently, the slower side of the river will continue to get slower and the faster side to get faster. Thus, more sediment gets deposited on the slow side and more erosion occurs on the fast side.
This process continues until the curve is so sharp that the river cuts through the bend and reestablishes a straight path. This can cut off a meander from the rest of the river, most often at a time of flooding. The cutoff part of the river forms an oxbow, named for the U-shaped part of the yoke for oxen, because the meander could be viewed as a pronounced U-shaped bend in the river. Over years, many oxbows fill in with sediments and plant growth.
From a high vantage point, one can see the oxbows from decades past. Some newly formed oxbows harbor water for years and form lakes if large enough. Carter Lake, Iowa, was created this way in 1877 after severe flooding shifted the Missouri River over a mile to the southeast.
The town of Horseshoe Lake, Arkansas, is built on the eastern tip of a U-shaped body of water with the same name, formed by changes in the course of the Mississippi River that created an oxbow. The lake is no longer connected to the Mississippi River.
The low-lying area around a river is termed a floodplain. Sediment is deposited in such an area after heavy rains and spring flooding, yielding some very rich land for crops. We had a few acres of that kind of farmland down on the farm I grew up on in Wisconsin. That soil was so rich that if you dropped a seed corn in a small furrow and covered it up with dirt, you had to get your head out of the way, lest the fast-growing stalk hit you in the face. Now, that is rich farmland!
Ben Franklin gets credit for the invention of the lightning rod. A lightning rod is a rather simple device, a metal rod with a ball or a point at its end attached to a building’s roof, chimney, or steeple. The one-inch-diameter rod is attached to a copper or aluminum wire connected to a conductive metal grid buried deep in the ground.
The purpose of the lightning rod is not to attract the lightning. It is actually supposed to discharge the clouds above it. Lightning is an electrical discharge between two points, namely cloud and ground or from cloud to cloud. You and I build up an electrical charge when we walk across a carpeted floor wearing socks and touch a metal doorknob. We also experience mini-lightning when we bring cotton and wool clothes out of the clothes dryer. The same thing happens when we pull a wool sweater off a cotton shirt we’re wearing (see question 207).
Sometimes the electrical potential between the clouds and the ground is so strong that, despite the presence of a lightning rod, lightning strikes anyway. So the lightning rod provides a low-resistance path to the ground that can carry the tremendous current safely to the Earth.
Lightning is very finicky and can jump around. Lightning can strike and then seek a path of least resistance. Usually, but not always, it is the highest object closest to the clouds, such as a steeple or tower. In open areas, that could be nearby trees. Growing up on a farm, we would hear stories of lightning striking a grove of trees and killing cattle seeking refuge from a storm.
Lightning is nothing to fool around with. More than five hundred people get struck by lightning every year in the United States, and about a tenth of them die. The following are all bad ideas during a lightning storm: being out in the open, sitting in a boat on water, operating farm machinery, golfing, talking on the phone (although cordless and cell phones are okay), and taking a bath or shower.
Being inside a house away from windows is good. Sitting in your car (unless it’s a convertible) or truck is ideal—just don’t touch any metal parts that connect with the outside of the car. You’re safe in the car because the lightning travels around the outside of the metal. It’s called a Faraday cage. Contrary to popular belief, it is not the tires that save you.
In 1836, the English scientist Michael Faraday demonstrated that an electrical charge stays on the outside of a conductor. A metal box will keep electromagnetic radiation from penetrating the interior of that metal box. A box made of metal mesh or screen will accomplish the same thing, if the holes in the mesh are small compared to the length of the electromagnetic waves trying to get in.
There could be several reasons that your house, barn, or any structure, for that matter, is repeatedly struck by lightning. To protect it, the lightning rod wire must be attached securely to a wire cable. The wire cable should be buried as deeply as possible in the ground. But lightning is very unpredictable and capricious, so maybe you’re just having bad luck!
Roy Sullivan, a US park ranger in Shenandoah National Park in Virginia, was struck by lightning seven times between 1942 and 1977. He holds the record. Two of the strikes even set his hair on fire. Strangely enough, it wasn’t the lightning that killed him; he took his own life by a self-inflected gunshot wound in 1983, at age seventy-one, reportedly over a woman.
At first thought, it might seem that that a mountaintop should be hotter than the lower valleys, because we know that hot air rises, and, additionally, mountaintops are closer to the Sun. The Sun beats down on the Earth, and air at the surface of the Earth is warmed.
The real reason is that, yes, warm air rises, but rising air expands. And as the rising air expands, it cools. Think of air molecules as tiny balls hitting against each other. A ball would pick up speed when another ball that is approaching hits it. But when a ball collides with another ball that is moving away or receding, its rebound speed is reduced. Or think of it this way. A Ping-Pong ball picks up speed when hit by an approaching paddle; it loses speed when hit by a receding paddle.
Higher up in the atmosphere, in the area of expanding air, molecules collide with more molecules that are receding than approaching. So the average speed of the molecules is decreasing. Hence, air is cooling because temperature is the average kinetic energy or velocity of molecules. How much does air cool as it rises? This is called the adiabatic cooling rate. It depends on the moisture in the air. On average, air cools about 3.5°F or 2°C for every one thousand feet in altitude.
The temperature on Mount Everest, altitude slightly less than thirty thousand feet, can be as low as –100°F. But on a good day in May, a climber can expect around –15°F.
Last summer, my wife, Ann, and I toured New Mexico and Arizona. Our rental car had a thermometer readout that gave the temperature of the air outside the car. When we were at Page, Arizona, altitude four thousand feet, the temperature was 92°F. A couple of hours later, we were at the North Rim of the Grand Canyon, altitude about nine thousand feet, and the temperature was 74°.
Arizona highways have signs that tell the altitude at every one-thousand-foot level. It was satisfying to keep track of the temperature versus the altitude along Highway 89 and then Highway 67 and watch the temperature drop about 3.5 degrees for every thousand-foot rise in elevation.
When we think of mountains in the United States, we think Rocky Mountains. The pioneers on the Oregon Trail in the mid-1800s would first glimpse the snow-covered peaks of the Rockies after they passed Independence Rock and started trudging up along the Sweetwater River.
What fear and trepidation they must have felt. But they were lucky. Lying before them was South Pass, discovered by fur trappers in 1812, a broad, open saddle of land with prairie and sagebrush, sort of a natural opening across the Continental Divide. South Pass is located in southwestern Wyoming at an elevation of seventy-four hundred feet. The Wind River Range is to the north, and the Antelope Hills are to the south.
Over a quarter of a million pioneers crossed the Rocky Mountains through South Pass. Today, Wyoming Highway 28 follows the Oregon Trail through South Pass. Wagon ruts are clearly visible at several places.
The father of modern chemistry, Antoine Lavoisier, gave us the first definitive answer some two hundred years ago. He stated that oceans are the “rinsings of the Earth.” He meant that salts are washed from the land into the ocean.
The rocks on land contain calcium carbonate (limestone), magnesium sulfate (Epsom salts), and sodium chloride (table salt). The process of weathering breaks down the minerals in these rocks and salts, and they dissolve in the water as rivers and streams wash the salts from the land into the ocean. We can see evidence of this interaction of water and stone at most any cemetery. Many early headstones were made of marble. After a hundred years of wind and rain, the inscriptions are hard to read.
The salt in the ocean also comes from volcanic activity. While there is hardly any sulfur and chlorine in rocks, volcanoes spew these elements into the atmosphere, and they end up falling in the world’s oceans. Sulfur and chlorine add to the saltiness of the oceans.
Since weathering and volcanic eruptions continually happen, it might seem that the oceans should become more and more salty. But salt is constantly being removed by clams and other shellfish that use calcium carbonate to build their shells. So the salinity of the oceans has remained fairly consistent for a long time.
The Dead Sea, on the border of Israel and Jordan, is surrounded by the lowest land on Earth, and it’s also one of the world’s saltiest seas. It has a salinity of 31.5 percent, almost nine times that of the ocean. The Dead Sea has no outlet. The minerals that flow into the Dead Sea stay there for centuries. The majority of freshwater bodies have rivers flowing out of them that dispose of dissolved minerals. Not so for the Dead Sea.
Mono Lake, near Yosemite National Park in California, has a salinity of about 10 percent. Diverting of water to Los Angeles caused the high salt content, leading to loss of water due to evaporation that soon exceeded freshwater inflow from streams. The lake continued to get smaller and saltier until 1994, when the drawdown was finally halted.
The Great Salt Lake in Utah is the remains of a pluvial, or rainwater-filled, lake that covered much of Utah in prehistoric times (see question 60). Today, three rivers deposit their sediment in the lake; they leave behind more than a million tons of minerals each year. The lake has no outlet, so water disappears by evaporation only. When water evaporates, the minerals are left behind. No need to worry about drowning in the Great Salt Lake; people float in it, because the water is denser than the human body.
All the planets are round because of gravity. Gravity is the force exerted inward toward the center of the planet so that all parts of the surface are pulled evenly toward the center. The result is a sphere, or ball. When the Earth and other planets were forming, gravity gathered all the gas and dust into bigger and bigger clumps. Collisions made the material hot and molten, and gravity pulled it all inward as much as possible to make a sphere. Later, all the molten material cooled and hardened in these spherical forms.
Planets are not perfect spheres. Any spinning mass wants to throw that mass to the outside, as far away from the center of rotation as possible. Witness the shape of a lasso, or lariat. A planet’s rotation causes it to bulge out more at the equator. The Earth is close to being a sphere, but not quite. The bulge from its spinning makes it twenty-six miles farther from one point to the opposite point on the equator than it is from pole to pole.
Jupiter is flattened, or oblate, by about 7 percent. It is a greater distance around the equator of Jupiter than it is around the poles. Jupiter is a huge planet, with about 320 times the mass of Earth. It spins once on its axis in about ten hours. You can look at Jupiter through a small telescope and see that it is not spherical. And you can use a NASA photo of Jupiter to measure the difference between a polar diameter and equator diameter.
Mountains and valleys, on planets that have them, are smaller deviations from a perfect sphere. The highest mountains on Earth are about six miles high, and gravity prevents them from getting much higher. Mountains that were fifty or one hundred miles tall would be crushed by their own weight.
Early on in our planet’s history, there were millions of volcanoes, with steam belching out of their mouths. Steam is a form of water, which has two hydrogen atoms and one oxygen atom. These volcanoes also emitted carbon dioxide and ammonia. Carbon dioxide has one carbon atom and two oxygen atoms. Ammonia has one nitrogen atom and three hydrogen atoms. The elements of these gases provided the building blocks of the air we breathe today.
Carbon dioxide levels in the atmosphere dropped because most of the carbon dioxide dissolved into the oceans and simple bacteria took in carbon dioxide, along with sunlight. These bacteria produced oxygen as a waste product, causing oxygen to build up in the atmosphere. Sunlight broke apart the ammonia molecules, separating them into nitrogen and hydrogen. Hydrogen is the least dense of all the elements, so it drifted off into space. A hydrogen atom has a large amount of kinetic energy, the energy of motion. It escapes the Earth’s gravitational pull and heads for outer space. Hydrogen atoms can exceed the Earth’s escape velocity of 25,000 mph. The nitrogen from the ammonia remained and eventually became the predominant gas in the planet’s atmosphere.
These days, plants and animals thrive in a delicate balance. Animals take in oxygen and give off carbon dioxide. Plants take in carbon dioxide and emit oxygen. Life depends on this delicate equilibrium of 78 percent nitrogen, 21 percent oxygen, and 1 percent is carbon dioxide and the inert gases of argon, krypton, neon, xenon, and helium. These gases were present in minute amounts after the formation of the solar system, some 5 billion years ago.
Our Earth has oxygen, and that’s what we humans need to sustain life. Earth’s atmosphere protects us from dangerous radiation, especially potentially deadly ultraviolet radiation. The atmosphere also keeps us warm by holding heat close to the surface of the Earth and not letting it escape and provides us with rain that nourishes and irrigates the planet.
Some other planets in our solar system have atmospheres, too, but they can’t support life. Mercury, like the Moon, has no atmosphere. Venus is cloaked in dense carbon dioxide gas. We humans would have a hard time finding oxygen to breathe. Mars has an atmosphere that has one-hundredth the density of our own, very close to a vacuum. The large planets Jupiter, Saturn, Uranus, and Neptune have atmospheres of hydrogen, helium, methane, and ammonia.
It does appear that Earth is the only planet in our solar system that is comfortable to live on.
Very tall skyscrapers must be built on bedrock. The chief obstacle to building upward is the downward pull of gravity. Every time one adds more floors on top, the total force on the bottom layers increases. You could build upward almost indefinitely, but the bottom floors would eventually have to be so massive and so thick that there wouldn’t be any living space left inside.
Each vertical column in a skyscraper sits on a spread footing: the column sits on a cast-iron plate, which sits on top of a grillage. The grillage is a spread-out stack of horizontal steel beams that sits on a thick concrete pad, which sits on bedrock. Bedrock is the hard, consolidated, intact material that can support a massive weight.
New York City’s Manhattan sits on the hardened remains of molten lava that flowed down the Hudson Valley centuries ago. The skyline of Manhattan traces out the subterranean mountain range of that lava flow, including a fold in the lava toward the southern end of the island.
Lower Manhattan has skyscrapers like the new One World Trade Center and the Woolworth Building. There are no tall buildings stretching more than a mile north from there. In midtown Manhattan, we once again see tall buildings: the Empire State Building, Rockefeller Center, the UN Building, and the Chrysler Building. Tall buildings are built on the two underground mountains where the lava flow is close to the surface.
The tallest building in the world at the present time is the Burj Khalifa, at 2,722 feet. Located in downtown Dubai in the United Arab Emirates, the South Korean–constructed building opened on January 4, 2010.
Rain is the result of the continuous water cycle. Water evaporates from lakes, oceans, rivers, and wet ground. To “evaporate” means to go from a liquid state to a vapor state. As the warm moisture-laden air rises, it expands because the air it is rising into is less dense (see question 58). That expansion lowers its temperature, because air molecules don’t collide with each other as frequently as they do in warmer air. The cooler air cannot hold as much moisture in vapor form as warm air can hold. The relative humidity shoots up and eventually reaches 100 percent. Relative humidity is the percentage of the moisture in the air compared to the maximum amount of moisture the air can hold.
The temperature at which the air has cooled enough to be completely saturated is termed the dew-point temperature. The cool air can no longer hold water vapor and gives it up; it condenses (goes from vapor to liquid state) on microscopic particles such as pollen, dust, smoke, and oxygen molecules in the atmosphere, forming a cloud (see question 89). These tiny water droplets in the clouds fuse together, or coalesce, to form larger water droplets. Air turbulence and winds move these droplets around, making them bigger as they collide. Soon they become heavy enough to overcome rising air currents, and they fall as raindrops.
In special conditions, such as a cumulonimbus thunderstorm, the raindrops are driven upward by rising air currents to where the temperature is below the freezing point. Tiny ice balls fall and gather more raindrops. The ice balls may ride up and down the air currents, and each time they do, they gather more water on their surfaces. Each time they rise, they freeze. Finally, they’re heavy enough to fall all the way to the ground. We call this hail. Next time it hails, go outside and gather a few big hailstones and take them in the house. Use a parry knife and cut one in half and examine the cross-section. On a marble-size hailstone, you can see and count the layers of ice, much like determining the age of a tree by counting the tree rings. The rings of ice will tell you how many times the hailstone rode up and down in the storm clouds.
The driest habitable place on Earth is Arica, in the Atacama Desert of Chile, where the annual rainfall is .03 inches. The cactuses take moisture from the fog.
The wettest place on Earth is claimed by both Llora, Colombia, in South America and Cherrapunji, in northeast India. Llora receives 40 feet (yes, feet) of rain per year.
The heaviest rainfall recorded was in Holt, Missouri, where on June 22, 1947, twelve inches of rain fell in forty-five minutes. On July 3, 1976, ten inches of rain fell in four hours over Big Thompson Canyon, Colorado, which is on the way to Estes Park and Rocky Mountain National Park. Flash flooding killed 144 people, mostly campers.
Despite the damage it can sometimes cause, rain is a blessing. It is the primary source of freshwater the world over, and it warms and irrigates our good Earth. Rain makes life and all human activity possible.
For hardness, diamond is the standard by which all other materials are judged (see question 63). Mention diamonds and most of us think engagement rings, anniversary rings, and “a girl’s best friend.” But those in industry value diamonds for their use as cutting tools, abrasives, and wear-resistant protective coatings.
Diamonds are a form of carbon, which is one of the most common elements in the world and one of the four essentials for the existence of life, the others being water, food, and oxygen. We humans are more than 18 percent carbon, and the air we breathe contains traces of carbon. Diamond formation takes place about one hundred miles below the surface of the Earth, where the tremendous heat and pressure change carbon into diamonds. The majority of the diamonds we see today were formed billions of years ago and brought to the surface of the Earth by magma eruptions. Most of the huge and more famous diamonds were found in South Africa.
The Mohs Scale is used to determine the hardness of solids, especially minerals. It is named after the German mineralogist Friedrich Mohs. The scale reads as follows, from softest to hardest:
As this scale indicates, no other material can scratch diamond, making it the hardest natural material on Earth. However, recently a patent was taken out for a synthetic compound of carbon and nitrogen that is said to rival diamond in hardness. It will serve as an inexpensive substitute for diamonds used in industrial applications. This new super-hard material could be used to cut steel, which diamond can’t do because it burns when it gets hot. Also, this new synthetic “diamond” might be used to coat metals such as gears and bearings and make them last a lot longer. There are two kinds of these synthetic diamonds, one going by the name of high-pressure high-temperature (HPHT) diamonds and the other by chemical vapor deposition (CVD) diamonds.
The deepest place in the oceans is the Marianas Trench in the Pacific Ocean. In 1960, the US Navy sent the Trieste, a mini-submersible, or bathyscaphe, named for the city in Italy where it was built, down to the bottom. A bathyscaphe is a hollow metal ball-shaped diving vessel that can hold a small crew and is used to explore ocean depths. Unlike earlier bathyscaphes, the Trieste was not tethered to the mother ship above it.
The two-man Trieste crew consisted of Jacques Piccard, son of the vessel’s designer, Auguste Piccard, and US Navy Lieutenant Don Walsh. They went down roughly thirty-six thousand feet and rested their vessel on the bottom of the ocean floor. It took them four hours to descend to the bottom of the deepest place on Earth. More than seven miles of ocean were above them. The Trieste used a tank filled with gasoline, which is lighter than water, and lead pellets, which are heavier than water, as ballast to control buoyancy. The pressure of the water at the lowest point was sixteen thousand pounds per square inch. In comparison, the air pressure in a car tire is a tad over thirty pounds per square inch.
The Deepsea Challenger is a modern, small, high-tech submarine that carries a one-person crew. On March 26, 2012, Canadian film director James Cameron piloted his craft to one of the deepest parts of the Marianas Trench, a recorded depth of 35,756 feet. This depression, the Challenger Deep, was named in honor of the British Royal Navy survey ship HMS Challenger, whose crew made the soundings in its 1872-to-1876 voyage. Cameron spent three hours exploring the ocean bottom in his solo dive.
Cameron stated that he saw no fish at the bottom of the ocean, only small amphipods, which are shrimplike bottom feeders. Rolex was a major sponsor of the vessel and voyage. They wrapped a Rolex watch on the vessel’s robotic arm. It functioned normally throughout the dive. Very good advertising material!
Two phenomena are operating at the same time to keep Earth in its orbit. First, the gravitational pull of the Sun is pulling the Earth toward it. Second, the forward motion (inertia) of the Earth is guiding the planet in a straight line. It is the competing action of these two “forces” (inertia is technically not a force, but it’s responsible for the planet’s trajectory because the Earth “wants” to go in a straight line, at a tangent to its orbit) that makes the Earth go in a smooth, and nearly circular, orbit around the Sun.
If only one of these “forces” were acting, it would be a disaster for the Earth. If our world were somehow made stationary in its path, the Sun’s gravity would pull it in and burn it to a cinder. On the other hand, if the Sun’s gravity somehow magically disappeared, Earth would fly off tangent to its orbit and be lost forever.
It is much like swinging a ball on the end of an elastic string around your head. If the string broke, the ball would go flying off in a straight line. The string is much like the gravitational pull between the Earth and the Sun. If the ball suddenly stopped moving forward, the elastic string would pull the ball toward your hand.
Anyone who has ventured out in the country and gazed up into the night sky has asked the question, “Are we alone in the universe?” It is a difficult question, for it is one for which we have no data or proof. Earth is the only place in the universe on which we are certain that life exists.
Now, we have to define what we mean by life. There is general agreement that life forms have the following four characteristics:
The educated guess of most astronomers and cosmologists is that the universe is teeming with life. They argue from the laws of probability, and their hypothesis goes something like this: Earth is one little planet orbiting one little ordinary star. The universe is filled with billions of galaxies, with billions of solar systems and billions of planets going around them. Why should Earth be the only planet among those billions on billions that has life?
There is another case to be made for extraterrestrial life. Life on Earth depends on just a few basic molecules. The elements that make up those molecules are common to all stars. The laws of science apply to the entire universe. Given sufficient time, life must have originated elsewhere in the cosmos.
An opposing view maintains that intelligent life on Earth is a result of a whole series of extremely fortunate accidents—geological, astronomical, chemical, and biological—and therefore life anywhere else in the universe is unlikely.
There is yet a third view, the creationist point of view, that claims that life was created, resulting from the actions of a supreme being. This is a religious point of view that says God created humans out of nothing and that any changes in life forms are divinely directed.
Will we ever find out? If in some distant era we hear the faint but unmistakable radio chatter of an advanced civilization, it will be a profound moment in the history of humanity, for then we will know that we are not alone. If we never hear anything, that will also be profound. It will mean we are unique and there is nothing like us in the universe!
This can be a controversial topic, considering that some people, such as creationists, consider the Earth to be only a few thousand years old. But the age of the Earth is firmly rooted in solid scientific logic and empirical evidence. The Earth and the solar system were formed between 4.53 and 4.58 billion years ago. Our Milky Way Galaxy is roughly 13 billion years old.
What’s the evidence? The ages of Moon and Earth rocks are found by measuring the decay of long-lived radioactive isotopes. Radioactive uranium-235 and uranium-238 undergo a change or transmutation (decay) into other elements, eventually ending up as stable lead. The more plentiful U-238 yields Pb-206. Pb-207 is the end result of the rarer U-235. All the lead that we have on Earth came from uranium.
Let’s say you have a rock that has a high ratio of uranium to lead—in other words, a lot of uranium and not much lead. This means you have a very young rock, because it has not existed long enough for much of the uranium to change to lead.
Now let’s pretend you’re holding a rock that has a low uranium-to-lead ratio: a lot of lead, but not much uranium. This means you’ve got yourself a very old rock, one that has lasted a sufficient time for most of the uranium to change to lead. Determining the age of the Earth is harder than finding the age of the Moon. Earth rocks have been recycled and destroyed by plate tectonics, uplifting, heating, and cooling. It’s hard to find rocks on Earth that are in their original, unaltered state, but rocks exceeding 3.5 billion years of age have been found on every continent.
The Moon is another story. Not many changes have occurred on the Moon in billions of years. While the oldest rock found on Earth has been dated at 4.3 billion years, the oldest Moon rock brought back by the astronauts was shown to be between 4.4 and 4.5 billion years old.
The best estimate of the age of the Earth comes not from dating individual rocks but instead from considering the Earth, the Moon, and meteorites as part of the same system, in which the ratio of uranium to lead can give a quite precise age. Thousands of meteorites, of over seven different types, have been radiometric dated. They show an average age of 4.56 billion years. Taken together with the ages of the oldest Earth and Moon rocks ever found, we arrive at our estimate of 4.53 to 4.58 billion years for the age of the Earth.
Dirt is the thin layer of soil that covers our planet. In most places, it is just a few feet thick, because nearly all of the Earth is a big, hard, solid rock, with an inner liquid core.
Dirt is mostly made of bits and pieces of this rock, which is broken down into smaller and smaller pieces because of weathering and microorganisms breaking down plant matter. Moisture, temperature, wind, rain, freezing, and thawing are all part of the weathering process. Over hundreds of years, rocks break down into tiny grains, and these small grains, mixed with plant and animal matter—decayed roots, leaves, dead bugs and worms, and other organic matter thrown in, along with water and air—is what we call dirt or soil.
The type of rock determines the alkalinity and texture of the soil. Limestone produces soils that are fertile, neutral (not base or acid), and finely textured. We have a lot of limestone-based soils where I’m from in Wisconsin. Soft shale rock yields a heavy clay soil. Sandstone becomes a coarse, sandy soil. Granite gives a sandy loam and acidic soil.
Dirt that is dark and black has a lot of old plants in it. The dark soils of southern Minnesota are some of the richest on Earth. Dirt that is light-colored contains a lot of silicate, or sand. Sandy soils drain quickly and tend to need a lot of water to grow productive crops, which is why you often see irrigation systems on land with sandy soil.
Clay soils are composed of extremely fine minerals and flat particles that pack together tightly. Clay soils tend to be reddish, harden when dry, and drain poorly. They tend to feel sticky when wet. The southern states of Georgia and Alabama are examples of areas with clay-based soils.
On a lighter note, dirt is what you track in on your mother’s floor. Soil is what plants grow in. Dirt is what you get on your uniform sliding into second base. Soil is vital to the crops that feed people. Every state has selected a state soil, twenty of which have been established by their states’ legislatures. In my home state of Wisconsin, the 1983 legislature named Antigo silt loam as the official state soil. It is a well-drained soil suited for forests, dairy, and potatoes.
We are all familiar with gravity. We know how it behaves. Hold a ball in your hand and let go; sure enough, it drops straight downward. And we all have an innate fear of gravity. Get on top of a tall ladder or tall building, look down, and we instinctively know what gravity will do if we lose our balance.
Gravity makes falling objects go faster and faster. In other words, objects accelerate when they fall. In the English system of measurement, a falling object goes thirty-two feet per second faster after each second. That works out to about 22 mph faster each second. In the metric system, it’s about ten meters per second faster than the previous second. These numbers are called “the acceleration due to gravity.”
The force, or pull, of gravity depends on the mass of the Earth and of the object being pulled. That force of gravity on Earth is known as the weight of the object. The more mass you and I have, the more we weigh.
Gravity gets weaker the farther you get from the surface of the Earth. Like many forces in nature, it obeys the inverse square law. Twice as far from the Earth, the pull of gravity is the inverse of two squared, or one- fourth. Three times greater distance means the inverse of three squared, so gravity’s pull is one-ninth.
Isaac Newton, beginning with experiments in 1667, was the first to get a mathematical handle on gravity. He figured the same gravity that caused an apple to fall from a tree was the force that made the moon “fall” around Earth.
Albert Einstein, in the early twentieth century, redefined gravity as a sort of geometry of space and time. The presence of mass causes a curvature of space-time. So objects follow a curved path, and their movement along this path we call “acceleration.”
Still, gravity is a great mystery. Why should any two objects in the universe attract each other? One goal of science today is to combine several forces, namely gravity, electromagnetism, and two nuclear forces, into a single unified theory that answers such questions. For now, the simplest and best explanation of why the Earth has gravity is that the Earth has mass and gravity is a property of mass.
Wind is caused by a difference in pressure from one area to another area on the surface of the Earth. Air naturally moves from high to low pressure, and when it does so, it is called wind.
Generally, we can say that the cause of the wind is the uneven heating of the Earth’s surface by the Sun. The Earth’s surface is made of different land and water areas, and these varying surfaces absorb and reflect the Sun’s rays unevenly. Warm air rising yields a lower pressure on the Earth, because the air is not pressing down on the Earth’s surface, while descending cooler air produces a higher pressure.
But there are many other factors affecting wind direction. For example, the Earth is spinning, so air in the Northern Hemisphere is deflected to the right by what is known as the Coriolis force. This causes the air, or wind, to flow clockwise around a high-pressure system and counter-clockwise around a low-pressure system. The closer these low- and high-pressure systems are together, the stronger the “pressure gradient,” and the stronger the winds. Vegetation also plays a role in how much sunlight is reflected or absorbed by the surface of the Earth. Furthermore, snow cover reflects a large amount of radiation back into space. As the air cools, it sinks and causes a pressure increase.
And wind can get even more complex. Some parts of the Earth, near the equator, receive direct sunlight all year long and have a consistently warmer climate. Other parts of the Earth, near the polar regions, receive indirect rays, so the climate is colder. As the warm air from the tropics rises, colder air moves in to take the place of the rising warmer air. This movement of air also causes the wind to blow. It’s a dynamic, complex mechanism, which is why weather forecasting is not quite a precise science.
Today we see windmills, used to make electricity, in operation in all parts of the United States, but especially along our coasts. Coastal regions tend to have fairly strong winds blowing in from ocean to land during the day and out from land to ocean during the night. The cause of this phenomenon is that land heats up and cools down faster than water, again creating a pressure gradient.