How Warm Moist Air is Lifted to Form Clouds

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How Warm Moist Air is Lifted to Form Clouds

By Moira Whitehouse PhD

Previously we said that this lifting action for the water cycle was provided by convection.

Convection

Mountain lifting

Air Mass movement

Clouds form when warm moist air is lifted and cooled.

Here we will look at all three ways that air is lifted and cooled to form clouds:

We really need to understand two things for this “cloud making” lesson to make sense.

Second, the temperature of the air determines how much water vapor (molecules of water in the gaseous form) the air can hold. Warmer air holds more water.

First, the temperature of the air goes down as the elevation gets higher. Roughly, for every 1000 feet we go up the temperature gets about 2 degrees F cooler.

Because it is heavier, gravity pulls harder on cold air which then forces the lighter warm air up and out of its way.

In cold air the molecules are closer together and the air is therefore, more dense and heavier than warm air.

As a result, the warmer air is pushed up (lifted) into cooler zones above the surface.

1. Convection

Following is a simple demonstration of convection where a candle is used to heat the air and a piece of burning rope is used to show the air’s movement.

Notice that air over the candle rises and is replaced by cool air coming down the other chimney.

Note: There is a short video on the next slide after the still shot (Quicktime may be required to view the video).

Quick time

In our world, the Sun’s heat replaces the candle, heating the ground which heats the

warm moist air allowing it to

be pushed up by the cooler air. As the warm air goes up

it gets cooled. Cool air cannot hold as much water vapor as warm air.

As the warm moist air is

cooled, the vapor changes to tiny droplets of liquid, the stuff of clouds.

Once again, the vapor changes into tiny droplets; exactly what is needed to make a cloud.

2. Mountain liftingWhen warm moist air meets a mountain, it is pushed upward into cooler temperatures by the slope of the mountain itself. As this warm, moist air moves up the mountain, it is cooled and cannot hold as much water vapor.

As wind or air mass movement pushes this warm moist air toward the mountain, the

slope of the mountain

forces it up into higher

cooler air. In this cooler environment, the air just can’t hold as much water vapor, and clouds are formed as the vapor changes to liquid droplets.

3. Air Mass movementClouds formed because of air mass lifting are pretty much like clouds formed by mountains, except that the lifting is forced by another mass of air instead of the mountain.

Air mass lifting can occur with both warm and cold front movement but it is always the warm air that gets lifted.

When a cool air mass moves into a mass of warm moist air, the mass of warm air is quickly forced up and thunderclouds develop. Thunderstorms often occur.

When a warm air mass moves into a mass of cold air, it slides over top of the cold air, is cooled and stratus clouds form. A gentler longer rain often follows.

In order for these clouds to form, water vapor had be lifted up into the colder

atmosphere.

Can you remember the three ways they might have been lifted?