The visible satellite imagery is essentially a snapshot of what the satellite sees, unlike Infrared (IR) satellite imagery, which depicts the temperature of the clouds. As the sun approaches midday over a given area, clouds will appear as bright white, as opposed to gray at sunrise and sunset. This is due to more sunlight being reflected as the sun moves overhead. Bodies of water, including lakes and rivers, absorb more sunlight and appear as black, with landmasses displaying as dark gray. It is also important to note that Visible imagery is only updated between sunrise and sunset, when each satellite's 'flashbulb' (a.k.a. the sun) is available.
A weather satellite
is a type of satellite that is primarily used to monitor the weather
of the Earth. These meteorological satellites, however, see more than clouds
and cloud systems
. City lights, fires, effects of pollution, auroras, sand and dust storms, snow cover
, ice mapping, boundaries of ocean
currents, energy flows, etc., are other types of environmental information collected using weather satellites.
Weather satellite images helped in monitoring the volcanic ash cloud from Mount St. Helens and activity from other volcanoes such as Mount Etna. Smoke from fires in the western United States such as Colorado and Utah have also been monitored.
Other environmental satellites can detect changes in the Earth's vegetation, sea
color, and ice fields. For example, the 2002 oil spill off the northwest coast of Spain was watched carefully by the European ENVISAT, which, though not a weather satellite, flies an instrument (ASAR) which can see changes in the sea surface
El Niño and its effects on weather are monitored daily from satellite images. The Antarctic ozone hole is mapped from weather satellite data. Collectively, weather satellites flown by the U.S., Europe, India, China, Russia, and Japan provide nearly continuous observations
for a global weather