Satellite Meteorology

Observations from instruments flown on satellites are extensively used in weather analysis. There are two types of orbits that weather satellites fly in:

Satellites in a sunsynchronous orbit circle the earth from pole-to-pole. A satellite in this orbit views all regions of the earth in a single day and are good for global weather studies. Sunsynchronous orbits are also referred to as polar orbits as the polar regions are frequently viewed, for example 14 times a day. Here is a recent composite view of the south polar region. In the tropical regions the satellite flys by a particular region of the earth twice a day -- for example once in the afternoon and once in the evening. If you draw a line from the satellite to the center of the earth, were the line hits the surface of the earth is the sub-satellite point. The groundtrack of the satellite connects consecutive sub-satellite points and describes the path of the satellite with reference to the earth's surface. NOAA (National Oceanic and Atmospheric Administration) currently maintains two polar orbiting satellites. The NOAA-14 views the United States at approximately 2 p.m. and 2 a.m. local time and the NOAA-12 views regions of the US around 10 a.m. and 10 p.m. local time.
Geostationary Satellite orbits the earth as fast as the earth spins and therefore appear to hang over a single point over the earth. An instrument in a geostationary satellite does not view the entire earth and has a poor view of the polar regions. The advantage of a satellite in the geostationary orbit is that an instrument as a continuous view of the mid-latitude and tropical regions. Satellites in this orbit are good for weather studies as you can track the movement of the storms. The satellite loops displayed on your favorite weather channel are observations from a satellite in a geostationary satellite. There are currently two geostationary satellites viewing the US. GOES-8 (Geostationary Operational Environment Satellite) has a good view of the east coast and the GOES-9 which has a better view of the west coast. Other satellites in geostationary orbit include the METEOSAT (METEOrological SATellite) which views the eastern Atlantic Ocean, Africa and Europe and the GMS (Geostationary Meteorological Satellite) which has a good view of Asia, Australia and the west Pacific Ocean

Flown on-board these satellites are instruments that measure electromagnetic energy that is either reflected or emitted by our planet. These instruments are called radiometers. There are two common types of radiometers used in satellite meteorology. One type measures the amount of visible light from the sun reflected back to space by the earth's surface or by clouds. The second measures the amount of radiation emitted by these entities.

Interpreting Satellite Images.

The radiometers flown on satellites are not cameras. Moving Mirrors make the radiometer view different regions of the earth. The instrument begins by looking off in one direction and then scans across a scene making observations as it proceeds. In this manner the radiometer scans a scene line-by-line. Data is recorded in a two-dimensional format (line and element) which, when displayed on a computer monitor or television screen, makes up an image. The smallest part of this image is the pixel -- which stands for "picture element." The radiation coming from each pixel is presented in an image format in terms of a grayscale. An image consists of 256 gray shades, ranging from 0 for pure black to 255 for pure white.

Visible Imagery.

Dark areas in a visible image represent geographic regions were small amounts of visible light from the sun are reflected back to space. Whiter regions are objects that are highly reflective.

The oceans are usually dark while snow and thick clouds are bright.

Don't be fooled by visible imagery.

Latest GOES-8 visible image.

Latest GOES-9 visible image.

Infrared Imagery.

All objects emit electromagnetic radiation. The infrared radiometers on satellites measure radiation with wavelengths of 11 microns. For easy interpretation, the radiant energy measured by infrared radiometers is converted to a temperature.

Cold objects are white and hot surfaces appear black.

Time sequences of infrared images are animated and shown on television news stations.

Don't be fooled by infrared imagery.

Latest GOES-8 IR image.

Latest GOES-9 IR image.

Combined Visible and Infrared Imagery.

The best way to interpret satellite images to view visible and infrared imagery together.

Example 1

Water Vapor Imagery

Meteorologists make extensive use of observations from radiometers that measure radiation with wavelengths of 6.7 microns. These images are referred to as water vapor images as the radiometers are measuring radiation emitted by atmospheric water vapor. Unlike the 11 micron radiometers, the water vapor radiometers measure radiation that is coming from the 200 to 600 mb layer of the atmosphere. The grayscale represents the relative humidity of the midtroposphere.

Moist regions of the midtroposphere are gray, and dark areas in the image represent dry regions.

Satellite loops of water vapor images give an indication of the circulation of the midtroposphere.

Example 1

Latest GOES-8 water vapor image.

Latest GOES-9 water vapor image.

Global Cloud Cover

By compositing all the geostationary observations into one image, you can view the cloud cover of the world! Latest Composite (updated every 3 hours) from SSEC. Movie loop