Welcome to You Ask Andy

Angella Alsobrooks, age 11, of Matthews, North Carolina, for her question:

Exactly how is rain made?

Obviously, the main ingredient is plain water, which comes splashing down from the clouds in small , large  or medium size drops. The mystery lies in how all this water gets up in the air, especially when we learn that even a mini rain cloud weighs 100 tons. Rainmaking is an enormous, complicated project. It involves the beaming sun, the filmy atmosphere and the solid earth, with its vast expanses of land and sea.

If our scientists knew this secret down to the last detail they could perform wonders. They could coax the clouds to shed showers on the thirsty deserts, end dreadful droughts and maybe tame deluging floods. At present they know more or less how raindrops are created in the weathery atmosphere. But they do not know enough to control nature's rainmaking on a global basis.

Weather scientists are meteorologists, and they refer to the rain as a form of precipitation  a word which means something that comes tumbling down. Other forms of precipitation are snow, sleet and hail. In the tricky formation of raindrops, one or all of these other forms of precipitation may play a part. For example, a raindrop that finally splashes on the ground may have started as an ice crystal or spent a brief spell as a feathery snowflake.

In general, all types of precipitation are formed from gaseous water vapor, which the sun evaporated from the moist surface of the earth.

Every hour, countless tons of    this invisible vapor rise to several miles above the ground. Chilled by the cooler air aloft, the gaseous vapor becomes liquid droplets of misty cloud moisture. This is the material used to make rain and all other forms of precipitation.

The droplets in a misty cloud are too small to be seen one by one, and each is surrounded by plenty of space. Thousands of these moist midgets must merge to form one raindrop  and the problem is how to get them together. Meteorologists now know that a successful rain cloud must contain tiny solid particles called nuclei. A nucleus may be a fine fragment of salt drawn up from the sea, a speck of dust or a tiny ice crystal formed in the chilly air aloft.

From the ground, the average cloud looks like a peaceful mountain of mist, calm and serene. Actually, it is a billowing mass where breezes blow up and down and in all directions. As the nuclei are whisked around, they bash into the cloud droplets and gather films and more films of liquid water. Soon the growing drops get too big and too heavy to float aloft in the air  and down dashes the rain.

Usually, a growing raindrop starts on high and falls several miles through the breezy cloud. On the way it is whirled through warmer and cooler pockets, damper and drier pockets. Sometimes it first forms a snowflake or an ice pellet  and melts to form a raindrop in a low layer of warm air just above the ground.

 

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