Fronts are just the leading edges of much larger air masses, which can cover thousands of square miles. In an air mass, temperatures and moisture levels are similar across the entire length, breadth, and depth of this huge parcel of atmosphere. Because air masses move, creating fronts at their forward edges, they bring the weather conditions from their point of origin to other regions. So, if a large bubble of cold air slides across the Canadian border into the United States and runs into warmer air, it simply shoves the warm air aside, and so places like Ohio or Indiana will experience the same weather Ontario was experiencing twenty-four hours earlier.
Air masses like to form in source regions that feature large areas of high pressure, and meteorologists categorize them by the region where they were created. An air mass formed in a tropical area earns the designation “T,” while a polar air mass gets tagged with a capital “P.” Those forming over land get a lowercase “c” (for continental), while air masses originating over water get an “m” (for maritime). There are also arctic (A) and equatorial (E) air masses.
Those designations can be mixed and matched to nail down an air mass’s nature, and there are other tags that can be used when more detail is needed. If an air mass is moving over a warmer surface, the letter “k” is used. If the underlying surface is colder, a “w” is added to the designation. This naming system covers any kind of air mass that might form in any environment, anywhere in the world. When a meteorologist sees the letter combination “mPk,” he knows it refers to a polar air mass that originated over water and is currently moving over a warmer surface.
Maritime Tropical (mT) air masses, not surprisingly, contain a great deal of moisture. In the winter they can move northward from the Gulf of Mexico, bringing mild weather to the United States’s midsection. In the summer they cause thunderstorms to form, although they usually die out quickly.
Maritime Polar (mP) air also contains a lot of moisture, and mainly affects the Pacific coast of the United States. As they encounter the coast and the mountains farther inland, these systems give up much of their water as rain and snow. Because of the moderating effect caused by moving over water, mP masses aren’t nearly as cold as cP air.
Continental Polar (cP) and Continental Arctic (cA) air masses bring loads of cold, dry air with them, and they are responsible for the worst winter weather over the United States. Because they originate over Alaska and Canada, there is little moisture in them, and when the jet stream carries them deep into the heartland, long-standing low-temperature records can be broken.
Continental Tropical (cT) air masses, which form in Mexico and the American Southwest, bring hot, dry weather. Driven by a stable high-pressure system, cT air can move into an area like the Midwest and stay for a prolonged visit, causing severe droughts.
All of these air masses occur on a large scale and each is easily identifiable on a weather map of the continental United States. But other wind patterns occur on a much smaller scale. Swirls and eddies of all kinds constantly whirl around us, embedded in the larger air masses that regularly cruise by.
Eddies that occur on a very small scale affect a limited area such as a single block or a backyard. Many of these eddies are caused by wind running into solid objects such as trees, buildings, cars, and mailboxes that break a straight breeze into a swirling pattern called mechanical turbulence. These swirls become visible when they pick up light objects on the ground, like that big pile of leaves you just finished raking up. Local wind features like these are called microscale winds, the smallest scale of motion measured by meteorologists.
It’s strange, but low, low down—within about 0.01 millimeter off the ground—there’s almost never any wind at all, no matter how fast the breeze is blowing above. To get picked up and moved, a particle has to be taller than the 0.01 millimeter limit. As these larger particles are picked up and blown around, they can knock other bits into the air, creating dust devils and even dust storms.
All these small disturbances tend to slow down the layers of air above them too. With no obstacles to cause any friction, the air from treetop level up to around 3,300 feet is often moving much faster, sometimes twice as fast, as at the surface. The area where turbulence interacts with smooth-flowing air is called the boundary layer, or friction layer.
When the Sun heats the ground and those bubbles of warm air begin to rise, thermal turbulence adds to the general atmospheric mixing caused by mechanical turbulence, and the lower atmosphere can get fairly unstable. As the afternoon progresses, these turbulent eddies grow stronger, producing strong, gusty winds down at the surface.
When these eddie swirls form high in the atmosphere, they give airline pilots and their passengers yet another thing to worry about. Where winds suddenly change speed and direction, they produce a dangerous condition called wind shear, which can cause an aircraft to quickly gain or lose a great deal of altitude without warning. At lower altitudes, that can be deadly. Below a height of around 2,000 feet, with a commercial airliner on final approach, the pilot is forced to reduce speed and has very little time to react to violent changes in wind speed or direction.
Wind shear can now be detected with a technology called LIDAR (Light Detection and Ranging), which uses a laser instead of radar’s microwave beam. The laser reflects off the dust and other small particles in the atmosphere, and their images appear on a monitor, providing instant data on wind speed and direction.
On August 2, 1985, a Delta L-1011 was on final approach to Dallas/Fort Worth International Airport when it was hit by a violent downdraft caused by wind shear. Unable to gain altitude, the pilot lost control of the plane, which collided with several objects on the ground before crashing into a water tank near the runway. One hundred thirty-three people were killed and thirty-one injured in the crash, which brought a public outcry for more sophisticated methods of detecting wind shear. After the crash, the Federal Aviation Administration undertook a large-scale project to modernize equipment at major airports, including wind shear detectors and improved Doppler radar.
In the Dallas incident, controllers might have been alerted to the turbulence by the presence of a small thunderstorm nearby if more had been known about wind shear at the time. But another type of wind shear, clear-air turbulence, often gives no visible clue to its existence until the plane is in its grip. On December 28, 1997, United Airlines Flight 826 from Tokyo to Honolulu ran into a pocket of clear-air turbulence 5 miles up. The plane started to shake, and then unexpectedly dropped 100 feet. Oxygen masks deployed, and anyone not wearing their seat belts flew up and crashed against the ceiling of the cabin. The plane made it back to Japan for an emergency landing, but eighty-three people were injured and one passenger was killed in the incident.
To prevent this kind of accident, NASA (National Aeronautics and Space Administration) has begun a program called SCATCAT (Severe Clear-Air Turbulence Colliding with Air Traffic) that will try to find new ways of detecting turbulence before it can cause a disaster.
While microscale winds occur in a relatively small area, mesoscale systems can encompass from fifty to several hundred square miles. If you’ve ever been to the beach in the afternoon, you’ve experienced one of the more common examples of a mesoscale system known as the sea breeze, which you’ll find anywhere there’s a boundary between sea and land. Because the land heats up more quickly than the ocean, during the day the air over land areas rises and expands from the Sun’s heat, creating a weak thermal low-pressure system. The cooler air over the water begins to flow into this low, refreshing beachgoers with a cooling breeze.
At night it’s the other way around: the land cools off more rapidly than the water, producing a weak high-pressure system. Since air flows from highs toward lows, the sea breeze turns into a land breeze that flows back toward the ocean.
Mesoscale winds are known by many names according to their type, location, and season. For instance, the warm, dry wind that flows down the eastern slopes of the Rocky Mountains is called a Chinook. Strong westerly winds blow over the Rockies and fall down the eastern slopes, and the air gets compressed and heated as it descends. Because the wind loses most of its moisture on the western, windward side of the mountains, it brings dry, warmer air to the eastern valleys.
A monsoon is a wind that blows one way in the summer and the other way in winter, such as the winds found in the Indian Ocean. Like sea breezes, monsoon winds are a result of the land’s ability to warm more rapidly than the sea. During the summertime the Asian continent is heated until it’s much warmer than the ocean to its south. The resulting pressure gradient causes air to flow from the Indian Ocean up into India, where it can cause heavy rain and severe flooding. In the winter, arctic air and radiational cooling over land reverse the flow, and cold, dry air rushes south toward the sea.
Residents of the Los Angeles Basin are familiar with the hot, dry Santa Ana winds, which flow down from the high desert. When a high pressure area forms in the Great Basin east of the Sierra Nevada mountain range, it forces air downslope, causing it to compress and heat up at the rate of about 5°F for every 1,000 feet it falls. Because Santa Ana winds bring extremely dry air, they also bring a higher likelihood of wildfires in the affected areas.
The Sahara desert is home to the sirocco, another hot, dry wind caused when a low-pressure system forms in the Mediterranean south of Spain and France. A sirocco can blow dust from northern Africa all the way to Europe.
Coastal residents from Maine to the Philippines have learned to live with the storms that are part of life at the sea’s edge, but the ocean’s effects don’t stop at the shoreline. In fact the earth’s seas influence the global climate more than any other factor.