For now, however, earthbound pilots not only have to cope with the problems caused by water in the atmosphere, but they also need to understand at least a little about how water works in the atmosphere-including what meteorologists mean when they use terms such as dew point or relative humidity.
Water helps to create Earth's dangerous weather because our atmosphere is in just the right temperature range to allow water to exist in all three states - solid, liquid, and gas - and change back and forth among these states.
In its solid state water hassles pilots with snow, sleet, and hail. Liquid water includes the tiny drops that make up most clouds and fog, along with rain and drizzle. Liquid water that's cold enough to be ice - at or below 32 degrees Fahrenheit or zero degrees Celsius - creates airframe icing, one of aviation's biggest 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.
How fast water molecules are moving determine which state water is in. Molecules of anything are always moving when the temperature is above absolute zero, which is minus 460 degrees Fahrenheit. Generally, the higher the temperature, the faster the molecules are moving. When the temperature of water is below 32 degrees F, enough molecules are moving slowly enough to lock together to form ice crystals.
Above 32 degrees F, molecules are moving too fast to lock into ice crystals but slowly enough for molecular attraction to hold them together as a liquid. But in a 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 are going slower than average, slowly enough to be "captured" by any liquid water that they might hit.
If you heat the water, the average speed of its molecules increases and more end up going fast enough to escape into the air as vapor. We say they evaporate. Some vapor molecules in the air will still be hitting the liquid and staying there-they are condensing.
For a simplified 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 water vapor in it. When we look at it, the water seems to be just sitting there. However, some of its molecules are moving fast enough to fly into the air - they are evaporating into invisible water vapor. As more and more vapor molecules join the air, we say it is becoming more humid. As the air becomes humid, 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.
Now, let's heat the container of water and air. As we do this, the liquid and vapor molecules speed up, and we'd again have more water evaporating than condensing, until they reach the air's new saturation point at the higher temperature.
This is why we say warm air can hold more vapor than cooler air. 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 more begin to condense, maybe to form clouds or fog.
Over the years scientists have carefully measured the amount of water vapor in saturated air at various temperatures and air pressures. One common scientific measure of humidity is the mixing ratio, which is the grams of water vapor in a kilogram of air.
The table above shows the mixing ratios of air that is saturated at sea-level air pressure and various temperatures. We see that 95-degree air can "hold" more than three times as much water vapor as 59-degree air. These numbers make it easy to understand what meteorologists mean when they talk about the dew point and the relative humidity.
To see how this works, assume that a weather observer found at 1 p.m. on a summer's day that the air's temperature was 95 degrees F and that the mixing ratio was 10.83 grams of water vapor per kilogram of air. (Observers don't directly measure the mixing ratio or report it, but assuming that they do makes humidity easier to understand.) A glance at the table shows us air with that mixing ratio would be saturated at a temperature of 59 degrees.
What happens when the air becomes saturated either from adding more vapor or by cooling the air? Water vapor begins condensing to form clouds if it's aloft, to form fog a few feet above the surface, and to create dew right at the surface. In this case, if this particular batch of air cooled to 59 degrees, dew would begin forming on the grass around the weather station. That's why the ob-server would report the dew point as 59 degrees F.
Dew point is really the more important measure of humidity because it tells us the most. A pilot seeing a weather observation saying the dew point is 59 degrees would know that if the temperature cooled to 59 degrees at the surface fog would begin forming.
And while weather reports for the public usually give the relative humidity instead of the dew point, the dew point is really a better measure of how the air is going to feel. For most people the humidity feels comfortable when the dew point is below 60 degrees. Dew points between 60 and 70 will make the air feel humid to most people, and when the dew point is above 70 degrees, most people will tell you that the humidity is uncomfortable.
While pilots don't use relative humidity in making weather decisions, it's a good idea to understand how it works. Relative humidity compares the amount of water vapor actually in the air at a particular time to the amount of vapor the air would have it in if it were saturated.
The table below shows how this works. At the time of the observation, the air could hold 37.25 grams per kilogram of water vapor, but only 10.83 grams per kilogram are in the air. Divide 10.83 by 37.25 and multiply by 100 and you get the relative humidity as a percentage. The table shows how the relative humidity increases as the air cools even though the amount of water vapor in the air stays the same.
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