Rain, rain, go away. Come again some other day. That's how we feel sometimes when all we want to do is go outside and play.

However, rain plays an important role in our world. It replenishes the Earth with water and helps our plants grow. Without regular rainfall, our world would be a dry, dusty place that wouldn't be any fun at all.

If you see clouds roll in on a sunny day, it could mean that rain is on the way. Have you ever stared at those clouds and wondered whether they hold rain and, if so, how much?

As it turns out, that question isn't easy to answer. Why? Clouds aren't like buckets, so they don't really “hold" water like a bucket would.

Whether you realize it or not, the air around you is filled with water. Water comes in three forms: liquid (that you drink), solid (ice) and gas (water vapor in the air). The amount of water inside a cloud is no different than the amount of water in the air around it.

In the air around a cloud, water is a gas in the form of water vapor. Inside the cloud, cooler temperatures have transformed water vapor into a liquid. This liquid is in the form of millions, billions or even trillions of tiny water droplets called cloud droplets. Scientists call this process condensation.

When water vapor in the air condenses into tiny cloud droplets, those droplets become visible. What we see is a cloud. Whether that liquid water will fall to the ground as rain depends on many factors.

Cloud droplets are very tiny and weigh very little. In a cloud, they float with the wind or simply hang in the air. In addition, updrafts (winds that blow upward from the surface of the Earth) also help keep droplets suspended within a cloud.

To fall to Earth, cloud droplets have to become heavier. If they combine with other droplets or if more liquid water continues to condense out of the air, they eventually become heavy enough to form drops that fall to Earth as rain. The scientific word for rainfall is precipitation.

Once droplets grow into drops and fall to Earth as rain, it will continue to rain as long as conditions in the atmosphere keep causing water vapor to condense into liquid water and grow into drops heavy enough to fall as rain.

So how much rain can a cloud produce? This is perhaps a better question than how much rain a cloud can hold. Scientists estimate that one inch of rain falling over an area of one square mile is equal to 17.4 million gallons of water. That much water would weigh 143 million pounds!

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