Although there’s usually plenty of water vapor in the atmosphere, it could never condense without the presence of tiny particles—called condensation nuclei—because of the high surface tension of each vapor droplet. Condensation nuclei are so small that a volume of air the size of your index finger contains anywhere from 1,000 to 150,000 of them, but they make the perfect seed for a cloud droplet. Some of these specks, such as salt particles, bond easily with vapor and are called hygroscopic, or water seeking. Ever notice how difficult it is to get salt out of a shaker when the air is humid? Those salt particles love their moisture. On the other hand, other atmospheric bits are hydrophobic, or water repelling, like particles from petroleum by-products, and resist binding with water vapor even when the humidity is more than 100 percent.
So now you know a cloud’s dirty little secret. Put condensation nuclei and water vapor together, and voilà—instant cloud, right? Well, as usual, there’s a bit more to it than that. You also have to have air that’s (a) rising; (b) expanding; and (c) cooling.
If you’ve ever watched a pot of spaghetti cooking, you’ve probably noticed that it seems to circulate in the pot even if you don’t stir it. Through a process called convection, the hot water carries the spaghetti toward the surface. When it cools slightly, more hot water rises to take its place, circulating the noodles over and over.
With cloud formation, the Sun heats the earth’s surface, causing it to radiate warmth. Any area that heats more rapidly than its surroundings, such as deserts or large areas of asphalt or concrete, can create a bubble of warm air that rises into the sky, mixing with the cooler, drier air around it. When this happens, the warm air expands and cools, and if this process continues, the air bubble will begin to fall back toward the surface again, just like spaghetti circulating in the pot. But if more warm air arrives from underneath, it will keep growing until it reaches the saturation point and condenses, making a fluffy little cumulus cloud.
When the cloud gets big enough to cast a sizable shadow, it starts to cut off its own heat engine as the ground below it cools. This throws a monkey wrench into the whole convection process, and the cloud begins to show ragged edges as the wind moves it along, causing it to eventually dissipate. But now the ground is free to heat up again, and soon another bubble floats skyward, ready to make yet another cumulus cloud. That’s why you’ll often see one cloud after another form around the same spot on a sunny afternoon.
Of course, when the atmosphere is unstable, even more interesting things can happen. When meteorologists use the word stable, they’re talking about the atmosphere being in balance. Air that’s in a state of balance, or equilibrium, holds true to Newton’s First Law of Motion: when it’s at rest, it tends to remain at rest, and so it resists any upward or downward movement. In other words, it doesn’t like to be pushed around. So if an air mass encounters surrounding air that’s cooler or warmer, and quickly adapts to that temperature, the air mass is said to be stable.
On the other hand, the atmosphere becomes unstable when there’s a big difference in temperature between the upper and lower layers, or between warm and cold air masses. Generally speaking, a rising air mass will become unstable. Because warm air rises, instability usually results from the warming of surface air. If air at ground level is warm and moist and upper levels are cold and dry, a process called convective instability can occur, causing a rapid, often violent, cloud growth that can produce severe thunderstorms and tornadoes quicker than you can say, “Run for the basement!”
Let’s take a closer look at a cumulus cloud as it grows up to become a towering cumulonimbus. We’ve discussed how cumulus clouds form and dissipate in a stable environment, but when the air above is cooler than the layers below, more and more heat is released inside the cloud as it rises and its vapor condenses. Rain droplets and ice particles begin to form and are churned and swirled by the turbulence from the rising air. Strong updrafts form in the cloud’s core, causing it to grow even faster. The rain and ice particles surge upward, getting larger and larger as they merge with other specks of moisture, creating a swirling mass of rain and ice within the cloud. And even with all this activity, no rain is falling yet, because the cloud is putting all its energy into the growth stage.
There are nearly 1,800 thunderstorms occurring worldwide at any moment, although most last an average of only thirty minutes. Out of the 100,000 or so storms that occur each year in the United States, only about 10 percent are classified as severe, but even small storms can create heavy rain and dangerous lightning.
In the next phase, called the mature stage, the raindrops and ice crystals get too large to be supported by the updraft and so they start to fall. This creates downdrafts within the cloud, and a pitched battle between falling and rising air begins. With updrafts still raging at speeds of up to 6,000 feet per minute, the severe turbulence causes a tremendous amount of friction in the cloud, and jagged lightning bolts begin to stab outward and downward as the storm mushrooms up toward the stratosphere. As the rain-cooled downdrafts reach the ground, they spread out horizontally into a gust front. Rain and hail begin to hammer cars, trees, buildings, and anything else unlucky enough to be caught in the storm’s path. The monster cloud’s top reaches the jet stream, and strong winds begin to pull it into a long anvil shape.
As the gust front spreads out underneath the storm, it cuts off the cloud’s supply of warm air. Eventually, the storm’s downward-moving air currents gain the upper hand, and the cloud’s growth slows and finally stops. Soon the internal updrafts cease completely, and downdrafts are all that’s left, carrying the rest of the cloud’s moisture to the ground as rain, often for several more hours.
If thunderstorms are the 300-pound gorilla of weather, supercells are the King Kongs. Although fewer than one in eighty thunderstorms develop into supercells, the ones that do are extremely dangerous and can be unpredictable. Supercells are the storms that most often produce tornadoes, making them the targets of storm chasers during springtime on the Great Plains.
Supercells feed off wind shear, which is the effect caused by winds blowing in different directions and speeds at different atmospheric levels. Wind shear actually tilts the storm, causing the cooler air descending inside to be pushed completely out of the cloud. Warm moist air is still free to surge in, however, and without the cooler air to act as a stabilizer, the storm’s consumption of warm air becomes a feeding frenzy, creating a strong, rotating updraft within the storm called a mesocyclone—the first stage of a tornado.
Because of the strong vertical wind shear inside a developing supercell (where updrafts can reach speeds of 150 miles an hour!), the updrafts and downdrafts can actually wrap around each other, creating an extremely volatile environment. These violent currents can keep hail suspended for so long that it can reach the size of grapefruit or larger before finally escaping the storm and plummeting to earth.
The National Weather Service gives supercells special attention, using radar to peer deep into their cores to catch early signs of developing tornadoes, which cause a characteristic “hook echo.” When a severe thunderstorm or tornado warning is given for your area, believe it and take cover as soon as possible.