Satellite Imagery

Pictures taken from space have many uses--to determine where there is snow on the ground, to monitor the amount of sea ice on shipping routes, and even to detect smoke from forest fires. They're particularly valuable, though, for forecasting the weather. Two different types of weather satellites provide forecasters with real-time photos of the Earth and the clouds forming above it--the geostationary satellite and the polar-orbiting satellite.

Infrared image from a Geostationary Operational Environmental Satellite

There are 2 GOES satellites (Geostationary Operational Environmental Satellites) monitoring the weather over North America and the oceans around it--one over the western and one over the eastern part of the continent. Their name includes the word "stationary" because they remain in one spot relative to the Earth. GOES satellites travel at an altitude of about 36 000 km above the Earth's equator. Their speed is carefully calculated so that they finish one complete circle or orbit every 24 hours, the same length of time it takes the Earth to rotate on its axis. That's why GOES satellites appear to hold still, hovering over the same part of the Earth. A sequence of pictures from this type of satellite will show forecasters how weather systems are moving and developing.

Satellite picture from polar-orbiter

Polar-orbiting environmental satellites (POES) are much closer to the Earth and can provide more detailed images of smaller areas. They circle the Earth at a height of about 850 km in a north-south orbit, passing close to both poles. It takes these satellites about 100 minutes to travel around the world, sending back pictures of a swath about 3000 km wide. Because the Earth continues to rotate while the POES satellite is orbiting above it, each orbit covers a path about 2 time zones west of the previous pass. In addition to sending back cloud pictures, this type of satellite carries UV sensors to monitor ozone levels.

These satellites have several sensors to produce different kinds of pictures. Two common types are visible and infrared. One set of sensors uses visible light (just like your eyes do) to take pictures much like the ones you would get from an ordinary camera. Other sensors use infrared light, and interpret everything in terms of temperature. Warmer objects are darker, and cooler ones are lighter in this type of photo. Because temperature normally decreases with height in the lower atmosphere, higher-topped clouds would usually be colder and show up as whiter. One big advantage of infrared pictures is that they work the same overnight as in the daytime. Sensors using visible light can't send back pictures during the dark hours overnight when there's no light from the sun. Some of these pictures are also posted to our public Web site.

Just as they can with radar pictures, forecasters can loop satellite photos together to see how quickly clouds are popping up, how cold (and therefore how high) their tops are, and where they're moving. This literally gives them "eyes in the sky".

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