Geostationary Satellite Images
Geostationary satellite images are the mainstay in meteorology world-wide. The main reason is their view of the earth 'sees' the weather at the same time.
There are five geostationary is use around the world at any one time. Because of world-wide agreements these five are positioned in such a way to assist all countries.
The United States operates two meteorological satellites in geostationary orbit, one over the equator at 75°W with a view of the East Coast and the other over the equator at 135°W for the West Coast view. The Japan Meteorological Agency operates one over the Western Pacific Ocean. EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) operates a satellite over the Indian Ocean and another one over Western Africa.
With the satellites positioned over the equator, they view the North and South Hemisphere at an angle so you can get a sense of the vertical development of the clouds. Also taller clouds will cast shadows onto lower ones so visible imagery is an excellent tool for locating developing thunderstorms.
GOES-E and GOES-W
However, when the local television meteorologist shows the satellite image is appears like the image to the left making it appear the satellite is located over the U.S.
To achieve this effect, each pixel from each satellite is remapped onto the coordinate system of the view desired. The result is usually a seamless composite view of North America.
The current series of GOES satellite only image the earth in black & white. Any colorization seen in current images are added to the image and are not a true color view. However, the next generation GOES satellites will be able to produce images in true color.
At their core, geostationary weather satellites provide three types of images; visible, infrared and water vapor.
Visible imagery is just like the name suggests; an image of the earth in visible light. This is a similar manner to that of a person taking a picture with a camera.
The satellite detects sunlight reflected from objects within the viewfinder. In the case of the satellite, the objects are the upper surfaces of clouds. Thick clouds do a much better job of reflecting light and therefore appear brighter in visible photos.
View the current visible composite view of North America.
There are two obvious problems with visible imagery. First, visible images of the earth are only available during the daylight. Second, the clouds are all the same color of white so we cannot distinguish clouds of different heights.
To combat this problem, the infrared (IR) sensor was devised. It senses radiant (heat) energy given off by the clouds. Warmer (lower in the atmosphere) clouds give off more energy than cold (higher level) clouds. The infrared sensor measures heat and produces images based upon the cloud temperature.
Infrared images can are available day and night. Since we can determine cloud temperature we often assign colors to these different temperatures to help isolate and identify cloud heights.
Compare the backgrounds in the visible and infrared images. The dark background in the visible image is due to space being black in color. But in the infrared image the background is white. This is due to the fact that space is cold which is represented as white.
View the current infrared composite view of North America.
Water Vapor Images
Even when there is not a cloud in the sky, water vapor is present. Water vapor is water in the gaseous state and therefore invisible. The GOES satellites can detect this gas and provides us with an image of the distribution of water vapor in the atmosphere.
However, due to absorption of energy by the atmosphere this view only "sees" the top third of the troposphere. While the low level moisture is hidden from the satellite sensor, the upper level moist/dry areas is plainly observable. Moist areas show up as white and dry areas as black.
This image is particularly useful in summer. Many times, thunderstorms in the Northern Plains of the U.S. appear to form from seemingly innocuous weather systems. But a look to the water vapor often shows a stream of water vapor extending from the eastern Pacific Ocean into those storms that is not seen in either visible or infrared images.