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MODIS (or Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra's orbit around the Earth is timed so that it passes from north to south across the equator in the morning, while Aqua passes south to north over the equator in the afternoon. Terra MODIS and Aqua MODIS are viewing the entire Earth's surface every 1 to 2 days, acquiring data in 36 spectral bands, or groups of wavelengths (see MODIS Technical Specifications). This map shows the temperature of Earth's lands during the nighttime. Temperature is a measure of how warm or cold an object is. During the day, the Sun's rays warm Earth's lands. At night, the lands typically cool off. Landscapes cool off at night because they release their warmth to air above while they are no longer receiving sunlight. Scientists can measure the temperature of Earth's lands from space using instruments carried on satellites. Scientists want to know the land's temperature for many important reasons. For example, in places where it is too hot or too cold food crops may die. Temperature also influences weather and climate patterns. So, mapping the temperature of Earth's lands helps scientists to better understand our world. The colors on these maps represent temperature patterns of the top millimeter (or skin) of the land surface including bare land, snow or ice cover, urban areas, and cropland or forest canopy as observed by MODIS in clear-sky conditions for the time period indicated. Yellow shows the warmest temperatures (up to 45°C) and light blue shows the coldest temperatures (down to -25°C). Black means "no data."