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Clouds are one of the primary ways we are able to "see" our atmosphere. All air contains some amount of water in gas form; this is called water vapor. Water vapor is the result of evaporation from the oceans and other bodies of water on the surface of the Earth. But there is a limit to how much water vapor the air can hold. The exact amount depends on the temperature and pressure of the air. When the limit is reached, the water condenses to form either water droplets or ice crystals. When this occurs, the water changes from a form that is invisible to us, to a form that is visible, and we see these droplets or ice crystals as clouds.

Clouds come in a large variety of shapes and sizes and they can occur over a large range of altitudes. We do not list the many types here but recommend the web pages below. There are three "ingredients" required for any cloud to form. First, there must be water vapor available to form the droplets or ice crystals. Second, there must be temperatures cold enough, for the local pressure, to allow the water to condense into droplets or ice crystals. The third ingredient is small particles. Droplets and ice crystals do not form efficiently without some small particle on which to grow. These particles are called condensation nuclei.

For more information:


Image showing gravity waves over the ocean.
Gravity waves over the ocean (1)

(1) Photo credit: Jay Lurie. NOAA Photo Library,

PBL Navigation

> PBL Scenario
> Layers of the Atmosphere
> What is a Cloud?
> Weather and Climate
> How Clouds Form, Saturation and Nucleation
> Clouds in the Lower Atmosphere
> Clouds in the Upper Atmosphere
> How Does Climate Affect the Atmosphere?
> Glossary

Teacher's Guide

I. AIM Mission Objectives
II. Key Questions for Students
III. Problem-Solving Model
IV. Materials and Procedures


A: Sample Student Activity Sheet
B: Assessment Rubric
C: National Science Education Standards

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