Severe Weather

The Weather Satellite

A very valuable tool utilized to forecast the weather is the satellite. Satellites provide extremely valuable cloud photographs of areas where there are no ground-based observations. Because water covers over 70% of the earth's surface, there are vast regions where few surface cloud observations are made. Before weather satellites were used, severe storms, such as hurricanes and typhoons, often went undetected until they moved dangerously near inhabited areas.

The weather satellite is a cloud observing platform in earth orbit. Satellites enable scientists to obtain an bird's-eye view of clouds on a global scale. There are two primary types of weather satellites in use for viewing clouds.

Geostationary satellites orbit the equator at the same rate the earth spins and remain at nearly 36,000 km (22,300 mi) above a fixed spot on earth's surface. This positioning allows continuous monitoring of a specific region.

Generally, the lower the cloud, the warmer its top. Warm objects emit more infrared energy than cold objects. Thus, an infrared satellite picture can distinguish warm, low (gray) clouds from cold, high (white) clouds.

Polar orbiting satellites closelly parallel the earth's meridian lines. These satellites pass over the north and south polar regions at each revolution. As the earth rotates beneath the satellite, each pass monitors an area to the west of the previous pass. Eventually, the satellite covers the entire earth. Polar-orbiting satellites have the advantage of photographing clouds directly beneath them.

Both types of satellites use radiometers that detect emitted radiation.

The forecaster can obtain information on cloud thickness and height from satellite photographs. Visible photographs show the sunlight reflected from a cloud's upper surface. Because thick clouds have a higher reflectivity than thin clouds, they appear brighter on a visible satellite photograph. However, high, middle, or low clouds have just about the same reflectivity, so it is difficult to distinguish among simply by using visible light photographs.

Infrared cloud pictures can make this distintion. Since warm objects radiate more energy than cold objects, high temperature regions can be made to appear darker on an infrared photograph. Because the tops of low clouds are warmer than those of high clouds, cloud observations made in the infrared ccan distinguish between warm low clouds (dark) and cold high clouds (light).

Below are examples of these different types:

Full disk Infrared Image (10/07/2002) Full disk Visible Image (10/07/2002)

Arizona Infrared (4km) (10/07/2002) Arizona visible (1km) (10/07/2002)

Current Images and Animations

Arizona, 1km, visible (current) Arizona, 4km, infrared, color enhanced (current)

Arizona, 1km, visible (animation) Arizona, 3km, infrared (animation)

Full disk Infrared Image (10/07/2002)	Full disk Visible Image (10/07/2002)

Arizona Infrared (4km) (10/07/2002)	Arizona visible (1km) (10/07/2002)

Current Images and Animations
Arizona, 1km, visible (current)	Arizona, 4km, infrared, color enhanced (current)
Arizona, 1km, visible (animation)	Arizona, 3km, infrared (animation)

http://www.atmo.arizona.edu/students/classlinks/fall01/atmo336/lectures/sec2/satellite.html

Last modified: Mon Feb 21 16:26:03 MST 2005