How water works the weather
When you call a flight service station for a preflight weather briefing, you are really seeking information on what the water in the atmosphere will be doing along your route of flight.
Clouds, fog, rain, snow, and other kinds of ice--including the ice that can form on aircraft--are water, of course. Water's importance to your flight, however, goes far beyond the fog, clouds, or rain you might encounter, because water also supplies energy that helps power the weather.
Water is at the heart of almost all kinds of weather because it's the only natural substance that exists as a liquid, a solid, and a gas at temperatures found on the Earth's surface, and because it takes up or gives off heat when it changes among its various forms. Imagine sitting in a 70-degree room sipping an iced drink as you plan a flight. The drink is mostly water in its liquid form (juices, colas, and most other drinks are 80 to 90 percent water). The ice in the drink is water's solid form. The air you're breathing contains the gaseous form of water, known as water vapor. A substance's molecular nature and the speed at which its molecules are moving determine which combinations of temperature and pressure make it a liquid, a solid, or a gas.
Understanding how water's molecular nature accounts for its existing in all three phases (solid, liquid, and gas) at comfortable temperatures takes you deep into physics and chemistry. The important point is that water's molecular structure determines what speeds--and thus what temperatures--are needed for it to change among 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. When a substance is a solid its molecules are vibrating in place with intermolecular, electromagnetic forces holding them firmly together. In a liquid the molecules are moving fast enough to overcome, to some degree, the intermolecular forces that attract the molecules to each other. A liquid takes on the shape of the container it's in, but atomic forces are strong enough to hold the molecules together in the container as they vibrate. The molecules of a gas are moving fast enough to almost completely overcome the attractions of intermolecular forces, allowing the molecules to fly away from an open container.
To begin seeing how water affects the weather we'll look at which happens as it changes phases, beginning with the change from solid to liquid.
At ordinary air pressures, when the temperature of ice rises to 32 degrees F, it begins to melt. Above 32 degrees F, molecules are moving too fast to stay locked into ice crystals but slowly enough for molecular attraction to hold them together as a liquid. As the ice melts to fill a container with liquid, some of the liquid's molecules will be moving faster than average; fast enough to break away and fly into the air as water vapor--they evaporate. In other words, liquid water does not have to be brought to its sea-level boiling point of 212 degrees Fahrenheit for some of it to change into water vapor.
Water vapor illustrates that much more than temperature is involved in determining water's phase. Temperature, however, is a key to how much water vapor can be found in the air. To see how this works, let's imagine we can see water vapor in the air above a container of water. To keep it simple, we'll also assume that the container is closed so the air won't mix with the outside air. If water molecules were visible, we'd see that those in the air, as vapor, are 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 that some of the molecules in the liquid are going fast enough to escape, flying into the air as water vapor. At the same time we'd see that 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 we'd see that the numbers of molecules leaving and entering the water are equal.
If we heated the water and the air above it, the number of vapor molecules would increase because the average speed of the water and vapor molecules increases, which means that more molecules are moving fast enough to remain as vapor. As the temperature stabilizes at the higher value, the number of vapor molecules also stabilizes at a higher value. We say that the air is saturated with water vapor because no more vapor will enter the air unless the temperature is increased. If we cooled the air and water, some vapor molecules would become liquid and the air would become saturated with fewer vapor molecules at a lower temperature.
In other words, the amount of water vapor needed to saturate the air depends on the air's temperature. When the air cools enough to reach the saturated mixing ratio, the water vapor--the humidity--in the air will begin to condense. Such cooling can be the result of the sun setting and the Earth's heat radiating away into space. Air also cools as it rises, which can decrease the temperature until the air reaches the saturated mixing ratio. In this case, water vapor in the air will begin to condense back into the liquid phase onto tiny particles known as condensation nuclei, which can be particles such as dust, clay, sea salt, and many other natural compounds. Some kinds of air pollution also act as condensation nuclei.
When the temperature of liquid water drops below 32 degrees, 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 a refrigerator ice tray, water molecules that happen to come together as an ice crystal will supply the template, and ice rapidly fills the container.
The story is different when the water is in the form of tiny cloud droplets or even when larger raindrops cool below 32 degrees. Odds of an ice crystal forming in such tiny drops are low until the drop grows much colder. Drops that are still liquid while below 32 degrees are called super cooled. They can instantly turn into ice when they hit something, such as the wing of an airplane flying through a cloud of super-cooled drops. 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. For example, particles known as freezing nuclei help cloud drops to turn into ice. Other kinds of particles, called deposition nuclei, encourage the formation of ice crystals directly from water vapor. Ice can also turn directly into water vapor without first becoming liquid.
In brief, the phase changes that water goes through in the atmosphere are
- melting of ice into liquid
- evaporation of liquid into vapor
- sublimation of ice directly into vapor
- condensation of vapor into liquid
- freezing of liquid into ice
- deposition of vapor directly into ice
While the temperature isn't the whole story of phase changes, it is important, and each phase change involves either taking up or giving off heat. To evaporate into vapor, for example, water has to gain heat, which means evaporation carries heat away from the liquid. The most familiar example of this is the perspiration that carries heat away from our bodies and cools us. In order to condense back into liquid, water vapor has to lose heat.
The figure on p. 51 sums up the phase changes, showing which take heat away from their surroundings and which add heat to the surroundings. The heat that phase changes add or take from the surroundings is called latent heat, which means it is heat that thermometers can't measure.
The figure shows that condensation, freezing, and deposition all add heat to the surroundings. These processes begin when air rises and cools enough for condensation, freezing, or deposition to begin. The added heat slows cooling of the rising air, causing it to rise faster and farther. Such added energy is the main source of energy for showers, thunderstorms, and hurricanes.
Evaporation, melting, and sublimation all cool their surroundings, and this also is a source of added atmospheric energy. Thunderstorm downdrafts, especially the strong downdrafts known as microbursts, occur when heat added to water or water vapor cools the surrounding air. Such cooling makes the air denser, causing it to accelerate toward the surface.
That cool drink you sip while planning a flight can be a reminder to be sure to obtain a complete weather briefing before you launch. You need to learn both what forms the water you will encounter and also how much energy that water's phase changes are likely to be adding to the atmosphere.
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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