Cloud fraction map showing average global cloud cover. Colours range from dark blue (no clouds) through light blue (some clouds) to white (frequent clouds). Clouds tend to form where air rises as part of atmospheric circulation patterns. As the air rises it cools and can hold less water, causing water vapour to condense into cloud particles. This happens most frequently in a arrow strip near the equator, and in two wider strips in the mid-latitudes. Data obtained between July 2002 and April 2015 by the MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA's Aqua satellite.