How to make a thunderstorm
If you've ever climbed out of an outdoor swimming pool on a 95-degree day in a dry place, such as Las Vegas, you probably started feeling chilly after a couple of minutes and grabbed a towel to dry off. What's going on to make you chilly on a hot day? If you had a couple of sensitive thermometers, they would show you that while your skin is cooling, the air next to you is not growing hotter.
Where is your body's heat going?
The answer goes back to the 1750s and 1760s, when scientists made careful measurements showing that when water in a pot reached a temperature of 212 degrees Fahrenheit (at sea level) it began boiling, but the water temperature didn't rise above 212 degrees even though the fire under the pot stayed just as hot. They also discovered that when ice is heated to 32 degrees, it doesn't all melt, but neither does it warm up.
The scientists realized that boiling water needs energy that thermometers don't measure to turn into water vapor (steam). In a similar way, ice needs extra energy to turn into water. They called this mystery energy latent (hidden) heat because thermometers don't measure it as they do the heat we feel.
You feel chilly after climbing from the pool because your body is supplying the latent heat needed to evaporate water from your body. To see how this works, we need to turn to the theory of heat that nineteenth-century scientists worked out long after the discovery of latent heat. In brief, they determined that the molecules of any substance at any temperature are always moving. The warmer the substance, the faster its molecules are moving.
We'll focus on how this affects water, but the basic principle applies to any substance. Water is unusual in that it's the only natural substance that is found in all three phases of matter--solid, liquid, and gas--at ordinary temperatures and atmospheric pressures. A glass of ice water illustrates this. The glass contains both solid (ice) and liquid water (generally when we say water we mean liquid water unless we're talking about its other phases). The air above the water in the glass contains water vapor, an invisible gas that's mixed with the air.
When water is frozen into ice, the average speed of its molecules is the slowest--too slow to overcome the atomic forces attracting molecules to each other. But the molecules are vibrating in their places as parts of ice crystals. This means that ice, like any solid, 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.
The energy that melts the ice comes from the surrounding air and water, slightly cooling them. If you leave the water in the pan after all of the ice melts, the water gradually disappears as it evaporates into water vapor. 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. They fill all parts of any container they are in. You can speed up evaporation by heating the pan, but you don't have to bring the water to a boil to evaporate it.
The air's temperature and how much water vapor is already in it determine how fast water evaporates. They are important because the air's temperature determines how much water vapor it can hold.
Now we can see why you feel chilly after climbing out of an outdoor pool into hot, dry air. The water on your body quickly evaporates because your body is supplying some of the energy needed to accelerate the molecules of liquid water to a speed that sends them into the air as water vapor. On the other hand, if the air is very humid, water such as perspiration doesn't evaporate as quickly--if at all.
Evaporation, the change from liquid water into water vapor, is one of six phase changes that water undergoes, and all of them play a role in weather. The figure on the previous page sums up these changes and shows whether latent heat is taken from the surroundings or added as the change occurs.
Changes to a more-energetic phase, such as evaporation from liquid to vapor, take heat from the surroundings, while changes to a less-energetic phase, such as condensation from vapor to liquid, add heat to the surroundings.
U.S. meteorologists referred to both of the phase changes between vapor and ice as sublimation until the 1970s, and some texts still use that term for both. Today, however, meteorologists generally call the change directly from vapor to ice deposition, while sublimation is used for the change from ice to vapor. A common example of sublimation is snow that disappears without any sign of melting, such as puddles. This happens when solar heat turns snow directly into water vapor. Snow crystals form via deposition, although many begin life as frozen water drops. Frost also forms by deposition--it is not dew that formed as vapor condensed into water then later froze.
If two or three hours after climbing from the pool you see a cumulus cloud growing into the thunderstorm (this happens on an average of a dozen days a year in Las Vegas), you could say, "I contributed to that." The water that evaporated from your body could have contributed a tiny share of the water vapor that condensed into the water droplets and ice crystals that make up the cloud. However, most of the water vapor creating a thunderstorm over Las Vegas would have evaporated from Lake Mead, the Colorado River, other sources in the area such as swimming pools--or came over the mountains after evaporating from the Pacific Ocean.
As we saw above, when water vapor condenses, or turns directly into ice, it releases the latent heat it gained when it originally evaporated. To see how this latent heat contributes to thunderstorms, let's look at how cumulus clouds form and sometimes grow into thunderstorms.
When warm, humid air rises, it cools at the regular rate of 5.4 degrees Fahrenheit for each 1,000 feet it rises. It always cools at this rate; the temperature of the surrounding air doesn't affect the cooling rate. As long as the air is warmer than the surrounding air it keeps on rising--much like a hot air balloon that rises because the air inside the envelope is warmer than the surrounding air.
Eventually, the air becomes cool enough that the water vapor in it begins to condense 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.
Once condensation begins, 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 now 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 than it would be without the addition of latent heat. What's happening is somewhat like the pilot of a hot air balloon turning up the burner to cause the balloon to ascend.
As air rises into a thunderstorm, air flows in near the ground to replace the rising air. As long as air continues rising into the thunderstorm, more air flows in from the bottom of the thunderstorm to replace the rising air, supplying more water vapor to release its latent heat. When the air becomes cold enough for ice crystals to begin forming, freezing releases additional latent heat.
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A thunderstorm reaches its mature stage when rain begins to fall and downdrafts as well as updrafts form. When this happens, the thunderstorm gains energy by taking heat from its surroundings through the phase changes.
Falling ice crystals melt into water drops or sublimate directly into vapor. Falling raindrops evaporate, especially if dry air is coming into the thunderstorm from the sides. Since these phase changes take in latent heat, they cool the air, making it denser. This cooled air plunges, possibly hitting the ground as a deadly downburst with winds that can exceed 100 mph.
Thunderstorms, especially powerful ones, need more than a good supply of humid air. Without this essential fuel, however, they won't thrive. While thunderstorms do occur during the winter--some even bring snow--the warm weather of spring and summer bring more frequent and larger thunderstorms, which are the biggest danger to pilots during what is otherwise a pleasant time of year. Warm weather is thunderstorm weather because warm, humid air supplies the most water vapor for thunderstorm fuel.
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.
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