Weather satellite pictures, or more correctly, satellite images, have been available to the public for more than 25 years. These images show cloud distributions over a large area, taken from a top-view perspective. When a sequence of these images is placed together in a loop, the motions of the clouds become apparent.
Most of the familiar satellite images appearing on television, in newspapers and on the Internet are obtained from one of the geosynchronous satellites , such as the GOES (for Geostationary Operational Environmental Satellite). These satellites orbit the earth at an altitude (23,000 miles) that allows them to appear to remain "stationary" over a fixed equatorial point. Images from the lower altitude polar-orbiting satellites are often used for research purposes and for monitoring the polar ice caps.
Instruments (called radiometers) on board essentially all weather satellites sense the intensity of electromagnetic radiation coming from the planet at several wavelength bands within the electromagnetic spectrum. In the regular scan mode, the sensors on board the geosynchronous satellites make "full disk" images of a hemisphere every half hour. In a rapid scan mode, these instruments zoom in on a selected area for more detail.. 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 navigation purposes.
The particular type of available imagery depends upon the region of the spectrum where the sensor is designed to respond. Three types of images are available from weather satellites on a nearly real-time, operational basis.
Those sensors on board the satellite that operate within the visible range of the electromagnetic spectrum sense sunlight reflected back to the satellite from the various components of the earth-atmosphere system, including cloud tops and the earth's surface. The processed image from these visible sensors is essentially a black and white photograph of the earth. The brightest and most white elements appearing in these images indicate the most reflective surfaces, where a greater intensity of sunlight is being reflected back into space 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.
Visible satellite images obtained from geosynchronous satellites typically have less than a 1 kilometer resolution. That is, a landmark that has a horizontal dimension of 1 kilometer could be detected on a visible satellite image. These images are used by meteorologists to locate and identify cloud masses. From these cloud masses, some information as to the circulation regime can be inferred. Differences in the 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. A major limitation to visible imagery is that usable images are essentially limited to the illuminated (daylight) regions of the planet below the satellite. Consequently, this type of imagery is not always used on television weathercasts.
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 a surface emits and a distant sensor detects. 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 satellite 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 IR images because of the extremely cold temperatures of space. This feature can be used to distinguish IR imagery from visible images.
The infrared sensors typically used on satellites respond to IR radiation within one of the narrow IR windows in the atmosphere. Hence, IR radiation within this window that is emitted from the earth's surface in a cloud free area will pass through the atmosphere and can be detected by the satellite sensor. The resolution of IR imagery from geosynchronous satellite is on the order of 8 kilometers. 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. Cold air masses also appear as whitish features. A milky appearance over an otherwise cloud free region may indicate an exceptionally cold air mass.
A large advantage of IR is that useful images can 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.
Infrared satellite images can be enhanced by computer programs to highlight the coldest, and hence, the highest cloud types. Enhanced satellite images are characterized by cloud tops that may contain various shadings or false colors. These colors are related to temperature. A color bar that appears with the image indicates the temperature that corresponds with the particular temperature. This enhancement helps identify especially strong thunderstorms that have extremely cold tops because of their heights.
The images produced from water vapor channel sensors represent a slight modification of the traditional IR images. The wavelength bands used in the water channel are at a slightly different wavelength interval than those used by the more traditional IR sensors described above. The IR radiation intercepted by the water vapor channel sensors is in a region of strong emission (and absorption) by water vapor. Sensor resolution is on the order of 16 kilometers.
The amount of radiation emitted in this channel depends upon an integrated amount of water vapor in a vertically oriented atmospheric column, especially weighted toward the mid to upper troposphere (3 to 7 miles altitude). Hence, the water vapor images depict water vapor concentrations. 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. The striations can also indicate the strong wind shears along the edges of the jet cores, where vertical motions may be occurring.
Last revision 10 June 1996© Copyright, 1996 Edward J. Hopkins, Ph.D. firstname.lastname@example.org
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