Weather

Water hassles pilots with snow, sleet, and hail in its solid form. Liquid water includes tiny drops that make up most clouds, fog, and drizzle. Bigger drops fall as rain. Water that’s still liquid even though it’s cold enough to be ice—it’s supercooled—creates airframe icing if you happen to fly into it. This is one of weather’s most dangerous hazards. Water vapor, which is invisible, doesn’t create any direct hazards, but it moves around undetected by human eyes to turn into liquid or ice.

Water helps to create Earth’s dangerous weather not only by supplying the raw materials needed to obstruct visibility, but also because our atmosphere is in just the right temperature and pressure range to allow water to exist in all three states—solid, liquid, and gas—and change back and forth among these states. Changes among these phases supply storm energy.

Water fuels storms because it either adds or takes away energy from the surrounding air as it changes among its phases.

To see how this works, we need to look closely at what’s going on when water is in each of its phases. Molecules of anything are always moving when the temperature is above absolute zero, which is minus 460 degrees Fahrenheit. The higher the temperature, the faster a substance’s molecules are moving.

In a solid, such as ordinary ice, the molecules are vibrating in place with forces that attract molecules to each other and hold them firmly together. In a liquid, the molecules are moving fast enough to overcome these attractive forces to some degree. A liquid takes on the shape of the container it’s in, but the attractive forces are strong enough to hold the molecules together in the container as they vibrate. In a gas, the molecules are moving fast enough to almost completely overcome the attractive forces, allowing them to fly away from each other unless they’re in a closed container. Gravity keeps all of them from escaping the bonds of Earth.

How water supplies storm energy.

Water supplies storm energy because it either gives off or takes up energy when it changes among its phases. Each phase change involves either taking up or giving off heat. For example, to evaporate and become vapor, water has to gain heat, which means evaporation carries heat away from the liquid or anything the water is on. The most familiar example of this is the perspiration that carries heat away from our bodies, cooling us when it’s hot.

In order to condense back into liquid, water vapor has to lose heat. All of water’s phase changes involve either a gain or a loss of heat.

In brief, here are the phase changes that water goes through and whether the change adds or takes away energy from the surroundings, including the atmosphere when the water is in the air:

• Condensation of vapor into liquid warms the air.
• Freezing of liquid into ice warms the air.
• Deposition of vapor directly into ice warms the air.
• Melting of ice into liquid cools the air.
• Evaporation of liquid into vapor cools the air.
• Sublimation of ice directly into vapor cools the air.

Water's energy powers

Thunderstorms.

Thunderstorms form when warm, humid air rises from the surface. As it rises it grows cooler, eventually becoming cold enough for the water vapor to begin condensing into cloud drops, which warms up the air. This causes the air to rise even faster and farther. As the air continues to rise, growing colder all of the way, vapor in the air begins depositing directly into ice, and water drops in the rising air freeze to form more ice crystals. The energy both processes create causes the air to rise even farther and faster, which adds to the thunderstorm’s violence.

Eventually ice crystals or water drops grow large enough to begin falling and dragging air down with them. Compression heating warms this sinking air. But, phase changes to higher-energy forms, such as raindrops evaporating into vapor and ice melting into water and sublimating into vapor, draws energy from the sinking air, which offsets the compressional heating. As the air cools it becomes denser, which makes it fall faster.

Water’s phase changes increase the speeds of both the updrafts and downdrafts in a thunderstorm. Wind shear between updrafts and downdrafts increases incredible turbulence in thunderstorms. If the sinking air cools enough it can hit the ground as a blast of wind that quickly changes the ambient wind speeds and directions. The strongest of such blasts are known as microbursts, a major hazard to aviation.

Clouds, fog, and supercooled water

More than temperature is involved in determining whether water is vapor, liquid, or ice.

Imagine we can see water vapor in the air in a closed container that keeps the water from interacting with the surroundings. If we could see them, the water molecules in the air as vapor would be moving at a wide range of speeds. The same is true of the water molecules making up the liquid in the container; they are moving at a range of speeds. But, the average speed of the vapor molecules is higher than the average speed of the liquid molecules. We would also see some of the molecules in the liquid going fast enough to escape, flying into the air as water vapor. At the same time, some of the vapor molecules are going slowly enough to become liquid in the container.

If the temperatures of the water and the air above it have been constant for a while, the numbers of molecules leaving and entering the water would be equal. If we heated the water and the air above it, the number of water molecules moving fast enough to become vapor would increase because the average speeds of both the water and vapor molecules increases. As the temperature stabilizes at the higher value, the number of vapor molecules also stabilizes at a higher value.

When this happens, meteorologists say the air is “saturated” with water vapor, because no more vapor will enter the air unless the temperature is increased. When something cools the air and water, some vapor molecules would condense back into liquid and the air becomes saturated with fewer vapor molecules at the lower temperature.

The amount of water vapor needed to saturate the air at any temperature is known as the saturated mixing ratio.

When air cools enough to reach the saturated mixing ratio, in theory the water vapor in the air will begin condensing. Such cooling can be the result of the sun setting and the Earth’s heat radiating away into space or air growing colder as it rises.

Another way of saying the air has reached the saturated mixing ratio is to say the relative humidity is 100 percent. We say condensation occurs in theory because nature isn’t quite this simple.

To begin condensing, water vapor needs to latch onto tiny particles known as condensation nuclei. These can be particles such as dust, clay, sea salt, and many other natural compounds. Some kinds of air pollution also act as condensation nuclei. In laboratory experiments with extremely clean air, scientists have shown that water vapor can reach levels higher than 100 percent relative humidity, but this apparently doesn't happen naturally.

In a similar way, when the temperature of liquid water drops below 32 degrees Fahrenheit, its molecules can begin coming together to form six-sided ice crystals, but water needs a template to form ice.

If the water is in a large enough container, such as a section of an ice tray you put in a refrigerator’s freezer compartment, water molecules that happen to come together as an ice crystal will supply the template and ice rapidly fills the container.

On the other hand, when the water is in tiny cloud drops or even raindrops that cool below 32 degrees F, the odds of an ice crystal forming in such tiny drops are low until the drop grows much colder. In other words, the drops are “supercooled.”

They can instantly turn into ice when they hit something, such as the wing of an airplane. What happens in clouds as water droplets and ice crystals form is so complex that it’s the topic of its own science, called cloud physics.