The Weather Never Sleeps

Teaching About Humidity

Water is a great drink for a pilot before flying. There's nothing in it to upset your blood-sugar level, and there's no caffeine to jangle your nerves. Best of all, unlike alcohol, it doesn't slow reactions or fog your brain. But when instructors begin talking about water in the atmosphere using terms such as dew point, relative humidity, and latent heat, the fog generally begins rolling into students' minds.

Understanding how water works in the atmosphere is necessary for understanding clouds, fog, rain, snow, hail, icing, thunderstorms, and hurricanes. The Earth has all of these because water is in the atmosphere in all three of its phases-gas, liquid, and solid. Changes among these phases are an important part of weather. Pilots need to know how water works in the atmosphere to understand the weather that will bedevil them during their flying careers.

A good place to begin is with some basic science. First, the instructor might remind students that substances can exist as solids, liquids, and vapors. At ordinary temperatures, most things are found in only one phase. Iron, for example, is a solid at room temperature. It has to be heated significantly before it becomes a liquid, much less a gas. Yet water can be a gas (called water vapor), a liquid, or a solid (ice) at ordinary temperatures. This is why it's so important to weather.

Molecules of anything are constantly moving any time the temperature is above absolute zero, which is around minus 460 degrees Fahrenheit. The higher the temperature, the faster the molecules move. No matter whether something is a solid, liquid, or gas, its molecules will be moving at different speeds, with the average speed being highest in a gas and lowest in a solid.

As the temperature of water falls below 32 degrees F, enough molecules begin moving slowly enough to lock together to form ice crystals. Above 32 degrees, but below boiling, molecules move too fast to form ice crystals but move slowly enough for molecular attraction to hold them together as a liquid. In a liquid, some molecules are moving faster than average-fast enough to break away and fly into the air as water vapor. And some vapor molecules in the air are moving slower than average-slowly enough to be captured by liquid water they might hit. If you heat water, the average speed of its molecules increases and more of them go fast enough to escape into the air as vapor. We say that they evaporate. Some vapor molecules from the air will still hit the liquid and stay there. We say that they condense.

Let's assume that we aren't heating the air and that there's no wind blowing. Under these circumstances, the molecules of evaporating water are going into the same bubble of air, making it more and more humid. Eventually, so many water molecules will be flying around in the air that as many are joining the water as are leaving it. We say that the air is saturated. If we heat the water and air, however, the water molecules in both the water and the air speed up. We again have more molecules evaporating than condensing until we reach the air's new saturation point. In other words, when warm air is saturated, it contains more water molecules than cooler air that is saturated. This is why we say that warm air can "hold" more vapor than cooler air. When the air is warmer, more water molecules are moving fast enough to remain as vapor. If we cool the air, the molecules slow down and more begin to condense.

Before going further, it's a good idea to talk about what we mean when we say that air is saturated. If air is saturated, one molecule of water vapor will condense into liquid for each molecule of liquid water that evaporates into the air. To use the "hold" terminology, the air can't hold any more water vapor. If we cool saturated air, water vapor will begin condensing because the saturation point is lower when air is cooler.

Keeping in mind that the saturation point of air increases with temperature, we can introduce the concepts of relative humidity and dew point. A good way to explain this is to talk about a straightforward measure of humidity that sounds very scientific-the mixing ratio. This is nothing more than the number of grams of water vapor in each kilogram of air. The table labeled "Mixing Ratio" shows saturation mixing ratios for a handful of temperatures. You can see that it takes nearly 10 times as much water to saturate 95-degree air as it does to saturate 32-degree air.

First, let's look at dew point. Begin by assuming that you're taking weather observations around 1 p.m. on a hot summer day. The temperature is 95 degrees. You determine that the air has 10.83 grams of water vapor per kilogram of air. (Weather observers don't directly measure the mixing ratio but can calculate it based on other measurements.) What do these measurements tell us about the humidity? The mixing ratio table shows that air with 10.83 grams of water vapor per kilogram would be saturated if it were cooled to 59 degrees. What would happen if we cooled this particular batch of air to 59 degrees? As the air cooled, the water vapor in it would begin condensing. If this happened at high altitude, a cloud would form. If it happened right above the ground, fog would form. Vapor in the layer of air right at the ground would condense on the grass. We'd say that dew was forming. In other words, the dew point (the temperature at which dew would begin forming) of air with 10.83 grams of water vapor per kilogram is 59 degrees, no matter what the temperature is when we measure it. (By the way, the dew point also depends on the air's pressure, but that can be skipped for basic weather instruction.)

Since dew point is the temperature at which the moisture in a particular mass of air will begin condensing into clouds or fog, weather observations for pilots report the dew point. If the air temperature and dew point are close together, little cooling is needed to create fog. This is why going for a late-afternoon flight when the temperature and dew point are close together can be dangerous. Fog might form as the air cools, making it hard to find a safe place to land.

Dew point can also tell you other things. When the dew point rises above 60 degrees, most people will say that the air feels humid. When it rises above 70 degrees, most people will complain about uncomfortable humidity.

Now, let's look at relative humidity. Relative humidity, by definition, is the amount of water vapor actually in the air divided by the amount of water vapor that could be in the air (at it's particular temperature), multiplied by 100. In our example of 95-degree air with 10.83 grams per kilogram of water vapor, we divide 10.83 grams by 37.25 (the amount of water that saturated 95-degree air would contain) to get .29. Multiply by 100, and the relative humidity is 29 percent. The relative humidity table shows how the relative humidity of this batch of air changes as it cools to the dew point. We see that even though the amount of water in the air stays the same, the relative humidity increases as the air cools. You can use the mixing ratio table to see how air at around 54-percent relative humidity can actually have more water vapor in it-that is be more humid-than air at 77-percent relative humidity.

On our 95-degree day, let's assume that we measure 20.44 grams of water vapor per kilogram of air. Such air has a dew point of 77 degrees. If we divide 20.44 by 37.25 (the saturation mixing ratio for 95-degree air) and then multiply by 100, we see that the relative humidity is 54.8 percent. This doesn't sound so humid. Now, let's assume that the air's temperature is 59 degrees and it has 7.76 grams of water vapor per kilogram of air. The relative humidity is 77.6 percent. In this case, the air with 54-percent relative humidity has nearly three times as much water vapor in it as the 77-percent relative-humidity air.

The fact that water changes phase as the air's temperature changes is only part of the story. Just as important is the fact that phase changes bring temperature changes. To understand why, let's return to the fact that the molecules in a gas move at the fastest speeds, those in a liquid move more slowly, and those in a solid move at the slowest speeds. The energy represented by the motions of molecules can't be created or destroyed; it can only change form. This means that when water changes phase from one with a higher speed (vapor) to one with a lower speed (liquid), some of the energy of molecular motion becomes heat that warms the surrounding air. Going the other way, when water changes from a phase with slower molecular speeds to one with higher speeds, it needs to draw heat from the air to supply the energy to speed up the molecules. The heat that is released into or taken from the air in phase changes is called latent heat.

When water vapor condenses into liquid or liquid freezes, heat is released, warming the surrounding air. This is the source of energy that powers thunderstorms and hurricanes.

As air rises, as in a thunderstorm, it cools to its dew point, causing water vapor to condense into droplets. The latent heat that's released warms up the air, making it rise even faster. As the air rises, more humid air flows in to replace it, keeping the process going. As rain falls into drier air, some of the water evaporates into water vapor. This cools the air, making it more dense (heavier). If the air cools enough, it could plunge to the ground in a downburst, a blast of air that comes down and spreads out when it hits the ground.

Students who understand water in the atmosphere are ready to learn about fog, thunderstorms, and downbursts. And, if nothing else, being able to show how air with 54-percent relative humidity can contain more water vapor than air with 77-percent relative humidity could win some bets-money to pay for a flight on a day when the water in the atmosphere is on its best behavior.