Why don’t all clouds rain?

There is always moisture in the air in either a water vapor or liquid form. This moisture is invisible in the water vapor form. Moisture in the air becomes visible in the liquid form.

Question Asked by student at Preston Jr. High

Answered by Travis Wyatt Meteorologist National Weather Service

Clouds form when air rises and cools, causing water vapor to change to liquid form. The same thing happens when you go outside on a cold day and air exhaled from your warm lungs hits the colder air and you “can see your breath”. Similarly, if you fill a glass with ice water, the glass will gradually get wet on the outside as water vapor in the air around the glass is cooled.

The air can hold only a certain amount of moisture and once it gets “full” the liquid water falls out as rain. If we visualize a glass, as you fill the glass with water it holds all of it until the water reaches the top and then overflows.

Finally, the rain that falls out of a cloud travels through the air as it heads toward the ground. If the air is dry below the cloud, then the rain may evaporate (return to its vapor form) and not reach the ground. Likewise, if the air is moist below the cloud, then the rain will likely reach the ground.

Hot Air Expands and Rises

Air is made up of molecules that are constantly in motion. As air warms up, the molecules start to vibrate and bump into each other, increasing the space around each molecule. Because each molecule uses more space for motion, the air expands and becomes less dense (lighter). In other words, the same number of air molecules occupy a larger space or the same sized space with increased air pressure.

The opposite effect happens when air cools. As the temperature drops, molecules move more slowly, taking up less room. The amount of space the air takes up shrinks, or reduces the air pressure.

Did you ever notice that when you open your freezer, you could feel the cold air rush downward towards the floor? Cold air, because the molecules are closer together, is heavier than warm air. Warm air, because the molecules are further apart, is lighter than cold air.

Materials: Plastic drink bottle, balloon, 2 buckets, hot water, cold water, ice

  1. Place the mouth of the balloon over the empty bottle. It should be a tight fit.
  2. Fill one bucket half way with cold water and add some ice cubes to make it really cold.
  3. Fill the other bucket half way with hot water. Be careful. You may need an adult’s help with this.
  4. Hold the bottle by the neck and push it down into the hot water so that the water comes up to between half way and three quarters way up the bottle. Keep the bottle upright and make sure the balloon stays out of the water.
  5. Leave the bottle in the hot water for a few minutes. Watch the balloon. Do you see anything happening?
  6. Take the bottle out of the hot water and place it the same way in the cold water. What happens now?
    Even though we can’t see the air in the bottle, the bottle is actually full of air molecules. The air molecules are affected by temperature – the hotter they are the more space they need, so when you heat up the air in the bottle it expands and when you cool it back down it contracts.

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