The Weather Never Sleeps: Water in the Atmosphere

Any young person who is now learning to fly could possibly pilot an airplane on Mars in 20 or 30 years. It's a long shot, but possible.

Today a few people at both NASA and the European Space Agency are looking into manned airplane flights over Mars, maybe in the 2030s. If you ever have the opportunity to fly on Mars you can look forward to preflight weather briefings that would be a lot easier to understand than those needed on Earth.

This is because with hardly any water in that planet's atmosphere, Mars has no clouds or fog to restrict visibility and ceilings. The planet also lacks hurricanes, rain, snow, hail, violent thunderstorms that stretch miles into the sky, and icing. The weather on Mars does include wind that kicks up dust storms.

For now, however, a future Mars pilot has to gain skill and experience on Earth, which requires learning enough about water in the atmosphere to understand Earthly weather briefings.

Water is a major part of Earth's weather because our atmosphere's temperatures allow water to exist in all three phases - solid, liquid, and gas - and change back and forth among these states.

Despite what you might have learned in school, temperature is not the only factor determining whether water is solid, liquid, or gas. Temperature is really an indirect measure of the average speed of the molecules of the object being measured. 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 the molecules are moving.

When liquid water is warmer than 32 degrees F (0 degrees Celsius) its molecules are moving too fast to lock into ice crystals but slow enough for molecular attraction to hold them together as a liquid. In the liquid some molecules are always 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 going slower than average, slowly enough to be "captured" by any liquid water that they might hit. The warmer the water and the air, the more water molecules will be going fast enough to stay in the air as water vapor.

Most of the problems that water in the atmosphere causes for pilots occur when water vapor condenses into water drops to cause fog, clouds, rain, and thunderstorms. Clouds form when rising air becomes cold enough for the water vapor in the air to begin condensing into drops. The most common kind of fog forms when the ground cools at night, cooling the air right above the ground enough for its water vapor to condense. Other kinds of fog form when the wind pushes humid air over cold ground or water, cooling the air.

You often hear that warm air can "hold" more water vapor than cool air. Describing the air as holding water vapor, however, can give misleading ideas of what happens. For example, as air warms and expands, it does not make room for more water vapor molecules. And air does not hold water vapor in the way a basket holds apples

For a clearer picture of the relationship between temperature and water vapor in the air, imagine a closed container about half full of (liquid) water while the other half contains air with no visible water vapor in it. To us the water looks perfectly still, although some of its molecules are moving fast enough to escape into the air as invisible water vapor. As more vapor molecules join the air, we say it is becoming more humid. Eventually, depending on the air's temperature, some of the vapor molecules slow down enough to rejoin the liquid; they are condensing. Eventually, just as many water molecules will be condensing as evaporating. When this happens we say the air is saturated because it can't become more humid.

If we heat the container of water and air, the liquid and vapor molecules speed up, and we'd again have more water evaporating than condensing, until the water and air reach the air's new saturation point at the higher temperature. In other words, when the air is warmer more water molecules will be moving fast enough to remain as vapor. If we cool the air, the molecules slow down and some begin to condense, to form clouds or fog. If the cooling air is right at the ground, water will condense on the grass and other surfaces.

The temperature at which the rising air will be cool enough for condensation to begin is called the dew point, even though the condensation might be forming cloud drops a few hundred feet above the ground. The dew point depends on how much water vapor is in the air.

A common scientific measure of water vapor is the mixing ratio, which refers to the number of grams of water vapor in a kilogram of air. The mixing ratio at which the air will be saturated for a particular temperature is called the saturated mixing ratio for that temperature. Figure 1 (above right), which shows the saturated mixing ratio for air at different temperatures, will help you understand the concept of dew point, which is the most useful humidity measurement for pilots.

To see how this works, assume that a weather observer found at 1 p.m. on a summer day that the air's temperature was 90 degrees F and that the mixing ratio was 11.03 grams of water vapor per kilogram of air. (Observers don't directly measure the mixing ratio or report it, but use instruments that directly measure the dew point.) A glance at the table shows us that air with an 11.03 mixing ratio would be saturated at a temperature of 60 degrees F, which would be reported as the dew point. A pilot seeing a weather observation saying the dew point is 60 degrees would know that if the temperature cooled to 60 degrees F at the surface, fog would begin to form.

Weather reports for the general public use relative humidity, which really isn't a very useful figure. Relative humidity is how much water vapor is actually in the air compared with how much could be in the air at the current temperature. In the case of a 90-degree F temperature and 60-degree dew point, we would divide the 11.03 actual mixing ratio by the 30.95 saturated mixing ratio, which gives you 0.356. Multiply by 100 to get a relative humidity of 35.6 percent. If the same air cooled to 80 degrees F, we'd divide the 60-degree saturated mixing ratio, which stays the same as the air cools, by the 80-degrees saturated mixing ratio for a relative humidity of 49.7 percent. When the air cools to the dew point temperature, it becomes saturated and the relative humidity is 100 percent.

In other words, even though the actual amount of water vapor in the air stays the same, the relative humidity increases as the air becomes cooler. This doesn't help a pilot. Dew point, on the other hand, is useful. For instance, if you're going for an early evening flight and the temperature is 60 degrees F and the dew point is 50 degrees, you should be alert for fog forming over the airport if the temperature drops to 50 degrees after sundown. If you ever fly on Mars, this is one scenario you wouldn't have to plan for.

Jack Williams, a freelance science writer specializing in weather and climate, is an instrument-rated private pilot. The latest of his six books is The AMS Weather Book: The Ultimate Guide to America's Weather. He answers questions about weather on his Web site.