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Image of clouds.
There are many types of clouds in the troposphere. They are distinguished by their shapes and sizes and height. They can appear bright white in color to very grey. Although it is possible for cloud ice crystals to form without nucleation sites in the troposphere, this is not usually the case. Aerosols, small airborne particles or non cloud droplets, are common nucleation sites. Aerosols have sizes on the order of 0.1 to micrometers. They come from a variety of sources including wind generated dust, volcanic ejections, sea water spray and smoke. Aerosols occur in concentrations of about 1000 particles per cubic centimeter and so are widely available for cloud nucleation. Tropospheric clouds typically occur at temperatures of -10C to -50C.

For more information:

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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The AIM mission is a part of
NASA's Sun-Earth Connection Education Forum.

Responsible Official: James M. Russell III

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