How latent heat fuels thunderstorms
To begin with, the air's humidity-its invisible water vapor-is the raw material that thunderstorms turn into rain and the balls of falling ice known as hail. More important, the air's water vapor is a fuel that powers thunderstorms. In fact, during the 2004 and 2005 hurricane seasons, you probably heard more than once that humid air supplies the energy that powers hurricanes, which are organized collections of thunderstorms. This is why hurricanes quickly begin to weaken and die when they move ashore, away from the very humid air above warm oceans.
To see how water powers thunderstorms, let's look at what happens when water changes among its solid, liquid, and gas phases.
The molecules that make up any substance are always moving, with their average speed depending on the temperature. In fact, temperature is really a measure of the average speed of a substance's molecules. When water or anything else is in its solid phase, the average speed of its molecules is the slowest-too slow to overcome the atomic forces attracting molecules to each other. The molecules are vibrating in place. This means that a solid-such as ice-keeps a definite shape no matter what kind of container it's in.
If you put ice cubes in a pan in a warm room they initially hold their shape, but begin to melt as they warm up until eventually you see a few small pieces of ice floating in water that takes the shape of the pan. As the ice warms, its molecules move faster and faster. Atomic forces still bind the water molecules to some extent, but not firmly enough to keep them in the shape of the ice cubes.
Leave the water in the pan long enough, and if the air in the room is dry, the water gradually disappears. It evaporates into water vapor, an invisible gas that's thoroughly mixed with the molecules of nitrogen, oxygen, and other gases that make up the air. If you heat the pan you speed up evaporation. Molecules of a gas are, for practical purposes, free from the cohesive forces that hold solids and liquids together; the molecules interact only when they collide.
If you stuck a thermometer into a pile of ice cubes in a pan you'd see that they don't immediately turn into water when the temperature reaches 32 degrees Fahrenheit. You'd also see that as the ice melts, the temperature of the ice/water mixture stays at 32 degrees, even if you heat the pan, until the ice all melts. In other words, the added heat doesn't increase the temperature of the ice/water mixture. Instead, it weakens the bonds holding water molecules together as ice.
If the thermometer is made to record temperatures warmer than 200 degrees F, you could leave it in the water as you bring it to a boil. You'd see that when the water reaches 212 degrees-the boiling temperature at sea level-the temperature stops rising. The added heat goes into speeding up the water's molecules until all of them are moving fast enough to become vapor.
In the 1750s and 1760s, scientists figured out what happens when ice melts and water boils, and came up with the term latent (hidden) heat. In the nineteenth century, scientists developed the kinetic theory of heat, the idea that heat is related to molecular motion. In other words, gases such as water vapor and liquids such as water contain their latent heat as molecular motion. When water vapor condenses back into water or the water freezes into ice, the latent heat is released as the substance's molecules slow down.
This is a good example of the scientific principle that energy cannot be created or destroyed-but only changed from one form to another. In other words, the latent heat in water vapor is potential energy that's released as heat when the vapor condenses into liquid water.
In fact, water undergoes six different kinds of phase changes in the atmosphere with three of them taking in heat and three giving off heat (see Figure 1). You are probably familiar with four of these changes: Water vapor condenses into liquid water and water freezes into ice. As we've seen, both of these give off heat. When ice melts into water and water evaporates into vapor, heat is taken up. You might not be as familiar with the deposition of water vapor directly into ice, which gives off heat-or the sublimation of ice directly into vapor, which takes in heat.
If you live somewhere where snow regularly falls, you're seeing sublimation every time piles of snow seem to disappear without any sign of melting. The sun's heat is turning the snow directly into water vapor. Snow crystals form via deposition, although the core of the crystal could be a frozen water droplet. Frost also forms by deposition; it's not frozen dew.
Latent heat explains how water vapor condensing into liquid water or changing directly into ice, or water droplets freezing, can supply the energy that powers thunderstorms.
When warm, humid air rises, it cools at the regular rate of 5.4 degrees Fahrenheit for each 1,000 feet it rises-no matter how warm or cold the surrounding air is. As long as the air is warmer than the surrounding air it will continue to rise. You can think of it as being like a hot air balloon. Eventually the air cools enough that the humidity begins condensing into tiny water droplets; the rising air is forming a cloud. As the water vapor condenses, it releases latent heat, which warms the surrounding air.
By the way, the term lapse rate is confusing because it's used two ways. When we talk about the dry adiabatic lapse rate we're talking about the change in temperature only of rising or sinking air. Rising air cools at this rate, and sinking air warms at the same rate no matter how warm or cold the surrounding air is. The surrounding air has a particular environmental lapse rate. You sometimes see the figure of 3.5 degrees colder for each 1,000 feet of altitude gained for the environmental lapse rate, but this is an average, and on any particular day the actual environmental lapse rate is likely to be different. In fact, on some days, the air is warmer a few thousand feet higher than at the ground. Compounding the confusion, some writers use lapse rate without saying which kind they're talking about.
The rising air is still cooling by 5.4 degrees per 1,000 feet of altitude gained, but the latent heat released by the condensing water vapor is working to warm the air. The overall effect is to slow the cooling rate to around two or three degrees per 1,000 feet. This means the temperature contrast between the rising air and the surrounding air is even greater-it's like the pilot of a hot air balloon turning up the burner.
The result is that the air rises even faster. More air flows in from the bottom of the thunderstorm to replace the rising air, which means there's even more water vapor to release its latent heat. When the air becomes cold enough for ice crystals to begin forming, the act of freezing releases even more latent heat. Even though by now the rising air is well below freezing, it's still warmer than the surrounding air.
You can see that the more latent heat is released, the faster and farther the air will rise. In general, the higher a thunderstorm's top, the more powerful it will be.
Water's phase changes that go in the other direction-those that take in heat instead of releasing it-also add to a thunderstorm's power. Falling ice crystals can melt into water drops or sublimate directly into vapor. Falling raindrops can evaporate. All of these take in latent heat, cooling the surrounding air and making it denser. In effect, air that's been cooled acts like a "cold air" balloon that's heavier than the surrounding air. It plunges toward the ground to create downbursts that blast down from the bottoms of some thunderstorms-or even rain showers with no lightning and thunder.
The amount of water vapor that's available to release latent heat is, of course, only a part of the energy that's needed to power a thunderstorm. The other major factor is the air's stability. In simple terms, this refers to the temperature profile of the air that's just sitting over a location before air begins rising and cooling. If the air near the ground is very warm and humid while the air aloft cools quicker than an average of 5.4 degrees per 1,000 feet, the air is unstable. This means that as air rises and cools at 5.4 degrees per 1,000 feet, it's going to remain warmer than the surrounding air, which means the rising air will be less dense than the surrounding air and will continue rising.
You have to rely on weather observations and forecasts to determine whether thunderstorms are likely to force you to change your plans for a flight on a warm day. But if the air is humid you know that at least one thunderstorm ingredient-the potential energy of latent heat-is available.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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