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Name: Liz
Status: Student
Age: 5
Location: N/A
Country: N/A
Date: N/A 

How long does it take for water (evaporated) to get to the clouds? This was asked by my 5 y.o. I could estimate the straight up distance to the clouds based on cloud type but would not the winds (upper level, especially) blow the vapor increassing the distance. what would the rate be to plug into the r = t x d equation?

Dear Liz-

The answer to your question about the length of time it takes water vapor to reach the clouds is not as straight forward as you might think, but depends on several factors, such as the temperature of the air, how much humidity is in the air, and the mechanism that causes the air to "lift" in the first place.

In order for clouds to form, the moisture (humidity) in the air must condense. For that to happen, the air must be cooled. The way to cool the air is to "lift" it, or to cause it to rise, away from the surface of the earth.

The amount of cooling necessary to produce saturation depends on how much humidity is in the air to start with. Unsaturated air cools at a specific rate (approximately 10 degrees centigrade per 1 kilometer altitude) until it becomes saturated, and then cools at a different rate after that (about 6 degrees centigrade per 1 kilometer altitude).

Another complicating factor is that warm air can hold more moisture (humidity) than cool air can. So the same amount of moisture in a warm parcel of air would have to rise farther to become saturated than that amount in a cool parcel of air.

There are several things that can cause air to rise away from the surface of the earth. One, and probably the most common, is sunshine. As the sun heats the earth, some parcels of air become warmer than the air above them, and they begin to rise, similiar to a helium balloon rising in the sky. When it rises far enough, the water vapor condenses into visible water droplets, and we call them clouds.

Another feature that causes air to rise, is mountains. Air blowing against a mountain is forced to ascend. It cools as it ascends, and if enough moisture is present, or it ascends far enough, clouds form.

There are other conditions that cause air to rise as well. Each of these influences cause the air to ascend at different speeds. And the distance the air must ascend before reaching saturation is dependent on other factors, such as temperature and humidity. So, without knowing all these factors, it is difficult to determine the time required for air to ascend enough to create clouds.

The substance of your question is the essence of the study of meteorology. Meteorologists and weather forecasters must understand all these physical processes that cause clouds to form, and to recognise which processes are active in any given weather situation. Much is known about these processes, but there is much to be learned about the behaviour of the atmosphere also. The advent of large, high-speed computers helps us identify these processes, and to predict how the atmosphere will change with time. We call these changes "weather forecasts," and we are increasing our forecast accuracy almost every year.

Thanks for the question, Liz... Sorry I couldn't give you a simple answer.

Wendell Bechtold, Meteorologist
Forecaster, National Weather Service
Weather Forecast Office, St. Louis, MO

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