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Lessons and Modules

Introduction to Clouds

How Climate Researchers Classify Clouds

Classifying clouds into different cloud types is the first step in analyzing clouds. Recently scientists have suggested an alternative method of classifyng clouds into the traditional groups. This new method classifies clouds based upon how optically thick they are and how high they are in the sky.

The following chart shows how this method classifies clouds based on their optical thickness and cloud top pressure:

Modified ISCCP Cloud Classification

Modified version of the cloud classification table provided by the International Satellite Cloud Climatology Project (ISCCP).

Cloud Height

To indicate cloud height, climate researchers measure the atmospheric pressure pushing down on the top of the cloud. The lower the cloud is in the sky, the greater the atmospheric pressure on top of the cloud.

High cloud top pressure --> Low altitude clouds

Low cloud top pressure --> High altitude clouds

Optical Thickness

The optical thickness of a cloud is a measure of not only the physical thickness of the cloud but also of the amount and phase of water in the cloud. A cloud can be physically thin, but contain water at a high enough density to produce a high optical thickness.

Clouds that allow most of the sunlight hitting them to pass through them are considered optically thin. Clouds that reflect most of the sunlight hitting them are considered to be optically thick.

You are now ready to proceed to the next section, Method, to propose an investigative procedure for studying clouds.