A weather satellite is an orbiting platform upon which instruments are placed for the observation weather systems and the measurement of atmospheric properties (weather elements) from space. Operational weather satellites represent a significant portion of all artificial satellites that have been placed into earth orbit since 1960. The now familiar "satellite pictures" seen on television weather-casts are really computer generated images obtained from satellite radiometers. These instruments are placed on board a satellite to measure the radiance, or the amount of radiation emitted from the earth's surface, clouds and atmosphere in a selected portion of the electromagnetic spectrum. This digital information is transmitted by radio to an earth receiving station where the data are collected. There, the data are processed by computers to form recognizable images, with the finished picture being constructed from this digital data. Geographical and political boundaries can be placed on these images for navigation purposes. These images are then distributed to the user.
Weather satellites can be classified according to their particular orbital configuration, which essentially determines their surveillance characteristics. The altitude of the satellite above the surface and the period of orbital revolution are related. Generally, the higher the orbit above the earth, the longer is the time that the satellite needs to revolve once about the earth. Two basic types of satellites are presently used for weather surveillance: the polar orbiting and geosynchronous satellites.
The geosynchronous satellites move in a prograde orbit 35,000 km (22,300 mi) over the equator. In this orbit, the satellites move in the same direction as the earth's rotation and maintain the correct speed that matches the rotation of the earth. To the earth-fixed observer, the satellite appears to remain "stationary" over a fixed point on the equator. Sensors on board these satellites take images of the full disk every half hour. In a rapid scan mode, the sensor can be pointed at selected areas and zoomed in for more detail on command from the surface. Essentially all the images appearing on television weather broadcasts are from geosynchronous satellites, such as the GOES (Geostationary Operational Environmental Satellite) or the Meteosat . The sensors on board the satellites can take images of the hemisphere every half hour, be pointed at selected areas and zoom in for more detail on command from the surface.
The polar orbiting satellites circle the earth in a polar or near polar orbit and take images sequentially as the satellite orbits over the planet. These satellites make images of most of the earth's surface twice a day as the earth rotates within the orbit of the satellite. Images from these satellites are mainly used for special purposes, such as for surveillance of snow and ice sheet coverage in polar regions. Because of the viewing geometry, the images from this class of satellite need to be assembled as a mosaic. These images may be distorted and they suffer from time delays.
Essentially all weather satellites have on board radiance sensors (called radiometers) which make measurements of the upwelling radiation in a series of discrete spectral intervals of the electromagnetic spectrum. The sensor is designated according to the wavelength band utilized:
The visible sensors detect the visible portion of the sunlight reflected back to space from the clouds, the earth's surface and the atmosphere. The spectral range of these sensors typically lies within a wavelength band between 0.55 and 0.99 micrometers, or within the visible atmospheric window. The amount of short wave radiation detected by the sensor is a function of the reflectivity of the earth-atmosphere system. The resulting visible satellite image is essentially the same as a black and white photograph, with regions appearing white being more reflective than darker areas.
The on-board infrared sensors respond to the upwelling infrared radiation emitted by clouds, the earth's surface and the atmosphere. The amount of infrared radiation and its spectral quality (the distribution of energy as a function of wavelength) depend upon the fourth power of the absolute temperature of the radiating object. Hence, the temperature of any object can be measured remotely by these infrared sensors.
Most of the infrared sensors detect radiation within the wavelength region of 10.3 to 12.6 micrometers. This wavelength band is within a reasonably transparent "window" region of the atmosphere. Hence, in cloud free regions, the long wave radiation welling up from the radiating earth's surface can be detected by the satellite sensor.
While technically an infrared image, the water vapor image is often considered as a third channel. The sensor responsible for the water vapor channel measures infrared radiation near the 6.7 µm water vapor absorption band. A set of water vapor channel radiometers senses the infrared radiation emitted by water vapor located in the mid to upper troposphere, ranging from 5000 to 9000 m (15,000 to 25,000 ft).
Some satellites have a scanning microwave radiometer that senses the microwave (far infrared) radiation emitted by the earth with wavelengths in the vicinity of 1.5 cm. In essence, a microwave detector can penetrate clouds and distinguish between ground and ice or snow surfaces. However, this sensor offers poor spatial resolution.
Initially, weather satellites were limited to producing images of the earth-atmosphere system. In this image mode, the satellite sensors scan across segments of the earth's surface and atmosphere collecting radiance data to produce the familiar satellite image. Subsequently, a method was developed that would permit the instruments onboard the satellite to probe the atmosphere. In the sounder mode, the sensors essentially make vertical soundings of the atmosphere by detecting the thermal radiation emitted from various levels of the atmosphere over a particular point. These upwelling radiance fluxes depend upon the absorption and emission properties of the atmosphere at different wavelengths, which depend on the atmospheric temperature and moisture. These radiance fluxes are converted to temperatures at various levels by complex computer algorithms, to produce a vertical temperature profile of the atmosphere. These satellite-derived soundings provide greater spatial and temporal resolution than the current RAOB network. However, the vertical resolution of the temperature profiles is not as great as that provided by conventional radiosondes. The newer geosynchronous satellites have the option of switching between Image and Sounder modes. Eventually, the satellites will have separate imaging and sounding instruments operating simultaneously.
In all cases, the information sent from the satellite to a receiving station on earth is as a series of numeric codes. This digital information is assembled by computers at the receiving station on earth in a series of scan lines to form a picture. At this time, computer enhancements may be made to the product to highlight certain subtle features on the image. These enhancements are done on the infrared images where the temperature differences may be small, but meteorologically important.
Weather satellites provide an indirect measure of wind speed and direction. An animated sequence of satellite images of identifiable clouds at various assumed levels are used as tracers of the wind motion at that level.
The data that are sent from the satellites to the ground stations are processed to provide usable images. This operation involves a sequence that includes data acquisition, processing and display. One such system is McIDAS - Man Computer Interactive Data Acquisition System developed at the Space Science and Engineering Center of the University of Wisconsin-Madison.
The satellite images provide important information about the earth-atmosphere system. A survey of how to interpret some of these satellite images can be found in the Satellite Image Interpretation section . Real-time weather satellite images are available from numerous sites on the Internet.
Last revision 10 June 1996© Copyright, 1996 Edward J. Hopkins, Ph.D. firstname.lastname@example.org
Return to satellite imagery interpretation
Master Links Page / Current Weather Page /ATM OCN 100 Home Page /AOS Dept. Home Page