Meteorological satellites are our "weather eyes in the sky". The perspective of space is unique; a satellite image can often provide views of broad-scale weather systems in their entirety. A geosynchronous satellite, high above the equator at an altitude of 23,000 miles and revolving in the same direction as the earth rotates, remains fixed over the same scene on earth. Individual views of the planet below or a rapid sequencing of such images as animation, can display surfaces and clouds associated with weather systems.
Weather satellite sensors are basically tuned to two types of radiation: visible light and infrared. Visible light views are like black-and-white television. Dark surfaces reflect little sunlight and appear dark, while clouds and snow cover are highly reflective, appearing bright. However, clouds, fog, and surface features (forests, mountain ranges, major rivers) are usually visible from space only during daylight hours.
Infrared (heat) radiation is emitted by a surface at a rate directly proportional to its temperature. Consequently, thermal infrared images can be interpreted as temperature maps of the underlying surfaces and clouds. Because heat emissions are continuous, these satellite images are available day and night. Water and ground surfaces are generally warm and appear dark while middle level clouds are cool and look gray. The highest, coldest clouds such as thunderstorm tops are bright white. These temperature ranges may be enhanced by assigning various color schemes for television and computer display.
Specially tuned infrared sensors can even detect invisible water vapor in the middle troposphere. Regions of the atmosphere with little water-vapor content appear dark on "vapor channel" images while high vapor content areas are milky white. Clouds also show as bright white in vapor images.
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Instruments on board essentially all weather satellites sense the intensity of electromagnetic radiation coming from the planet in several wavelength bands within the electromagnetic spectrum. These on-board sensors on a geosynchronous satellite make "full disk" images of a hemisphere every half hour, or in a rapid scan mode, the sensors concentrate on a selected area for more detail in shorter time intervals. The information collected by these sensors are transmitted as a series of digital signals to a receiving station on earth, where the information is processed to form a recognizable image. Computer generated map outlines are placed upon the finished images for orientation purposes. Three types of satellite images are available from the Datastreme Homepage and the following describes some of the hints that can be applied to aid in the interpretation of satellite images:
Some satellite sensors operate within the visible range of the electromagnetic spectrum, sensing the sunlight reflected back to the satellite from the earth-atmosphere system. The brightest and most white elements appearing in these visible images indicate the most reflective surfaces, where a greater intensity of sunlight is reflected back into space, such as from clouds or a fresh snow cover. Conversely, the darkest parts of the image indicate the least reflective surfaces, such as nearly black ocean surfaces where much of the sunlight penetrates the surface and is not reflected back. Land surfaces tend to appear gray.
Differences in shading of clouds usually relate to cloud thickness. Often a cloud that appears bright on a visible image is a thick cloud that scatters back most of the solar radiation that strikes the cloud.
One way to identify a visible satellite image is to look for the dark region of space on the edge (limb) of the earth's disk when observing a full-disk image. [The satellite images that you can access from the Datastreme Homepage are limited to that sector that focuses upon the continental United States.] A major limitation to visible imagery is that it is essentially limited to the illuminated (daylight) regions of the planet below the satellite.
Some of the interesting features that may appear on the visible images include:
This type of image is produced by infrared satellite sensors that detect long wave radiation emitted by the earth's surface, atmosphere and clouds. Infrared or IR represents that proportion of the electromagnetic energy spectrum that is emitted by essentially all objects at normal temperatures. In other words, the warmer the body, the more infrared radiation that surface emits. By looking at an IR image, one can detect the relative temperatures of the ocean, land, and clouds.
Objects with the coldest temperatures appear in IR imagery as the most white features, while the warmest bodies are the darkest. For example, interplanetary space beyond the limb of the planetary disk appears white on the full disk IR images because of the extremely cold temperatures of space; this feature can be used to distinguish IR imagery from visible.
The infrared sensors typically used on satellites respond to IR radiation within one of the narrow IR windows of the atmosphere. Hence, IR radiation emitted from the earth's surface in a cloud free area passes through the atmosphere and can be detected by the satellite sensor. Surface features can be detected in IR imagery by noting subtle shading contrasts resulting from differences in the surface temperatures. One example is the relative temperature difference between large water bodies and land. Warm land surfaces tend to be dark. Typically, cloud free ocean regions appear more uniform because of more uniform sea surface temperatures. On the other hand, large differences in surface temperature over continents produce images with dark regions over hot deserts and lighter regions over mountainous terrain.
Since the troposphere cools with increasing altitude, cloud tops would usually appear on IR imagery as cool bright areas while land surfaces would be seen as dark areas. Differences in IR cloud image shading often relate to subtle differences in cloud top temperature. Thus, infrared imagery can be used to help distinguish between high, middle and low clouds. Usually, fog and low clouds will be more gray because they are warm, while higher cold clouds will appear to be more white. Because fog may be at the same temperature as the surface, fog banks may be hard to discern from land areas in the IR. Towering thunderstorm clouds will appear bright and white. A milky white appearance over an otherwise cloud free region may indicate a cold air mass affecting the surface temperatures.
A large advantage of IR is that useful images can be made regardless of local darkness. The animated loops that are used on television weather shows usually represent a sequence of IR images. However, detail is lost due to lower resolution of the IR sensors, as compared with the visible sensors.
The images produced from water vapor channel sensors represent a slight modification of the traditional IR images. The wavelength bands used by this vapor channel are at a slightly different wavelength interval than those used by the IR sensors. The IR radiation that is detected by the water vapor channel sensors is in a region of strong emission (and absorption) by water vapor.
The amount of radiation in this channel depends upon the total amount of water vapor in a vertically oriented atmospheric column, especially weighted toward the mid to upper troposphere between 20,000 and 30,000 feet. Hence, the vapor images depict water vapor concentrations in the mid troposphere.
Differences in shading of this type of image typically relate to subtle differences in mid-tropospheric moisture, with white regions on the imagery representing more moisture than dark regions. Additionally, white regions probably indicate rising air, while dark regions indicate layers experiencing sinking motion. Regions that appear as light gray streaks typically contain upper tropospheric jet streams carrying large amounts of moisture.
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.