lake A body of fresh or salt water entirely surrounded by land.
land breeze A nocturnal coastal breeze that blows from land to sea. In the evening the water may be warmer than the land, causing pressure differences. The land breeze is the flow of air from land to sea equalizing these pressure differences. See sea breeze.
Landsat Land Remote-Sensing Satellite, operated by the U.S. Earth Observation Satellite Company (EOSAT). Commercialized under the Land Remote-Sensing Commercialization Act of 1984, Landsat is a series of satellites (formerly called ERTS) designed to gather data on the Earth's resources in a regular and systematic manner. Objectives of the mission are: land use inventory, geological/mineralogical exploration, crop and forestry assessment, and cartography. Landsat has a spatial resolution of 28.5 meters.
Restructured Federal agency responsibilities for the Landsat program are effective for the acquisition and operation of Landsat 7. New operating policy specifies that NOAA will be responsible for satellites after they are placed in orbit, NASA will be responsible for the development and launch of Landsat 7, and that the U.S. government will provide unenhanced data to users at no cost beyond the cost of fulfilling their data request.
landsats (aka Earth resources satellites) Any land remote-sensing satellites. Includes the U.S. Landsat system and the French SPOT.
LaRC (Langley Research Center) See NASA Centers.
laser (light amplification by stimulated emission of radiation) Active instrument that produces discretely coherent pulses of light (light waves with no phase differences, or with predictable phases differences, are said to be coherent).
laser ranging The use of lasers to measure distances.
latitude (aka the geodetic latitude) The angle between a perpendicular at a location, and the equatorial plane of the Earth.
legend A listing that contains symbols and other information about a map.
LeRC LeRC (Lewis Research Center) See NASACenters.
Interplay between light rays and the atmosphere cause us to see the sky as blue, and can result in such phenomena as glows, halos, arcs, flashes, and streamers.
lightning A discharge of atmospheric electricity accompanied by a vivid flash of light. During thunderstorms, static electricity builds up within the clouds. A positive charge builds in the upper part of the cloud, while a large negative charge builds in the lower portion. When the difference between the positive and negative charges becomes great, the electrical charge jumps from one area to another, creating a lightning bolt. Most lightning bolts strike from one cloud to another, but they also can strike the ground. These bolts occur when positive charges build up on the ground. A negative charge called the "faintly luminous streamer" or "leader" flows from the cloud toward the ground. Then a positively charged leader, called the return stroke, leaves the ground and runs into the cloud. What is seen as a lightning bolt is actually a series of downward-striking leaders and upward-striking return strokes, all taking place in less than a second.
Lightning bolts can heat the air to temperatures hotter than the surface of the sun. This burst of heat makes the air around the bolt expand explosively, producing the sound we hear as thunder. Since light travels a million times faster than sound, we see lightning bolts before we hear their thunderclaps. By counting the seconds between a flash of lightning and the thunderclap and dividing by five, we can determine the approximate number of miles to the lightning stroke. See thunderstorm
line-of-apsides line-of-apsides (aka major-axis of the ellipse) The straight line drawn from the perigee (point of orbit closest to Earth) to the apogee (point of orbit farthest from Earth) is the line-of-apsides.
line-of-nodes The line created by the intersection of the equatorial plane and the orbital plane.
line-of-sight Area within which visible contact can be made. For example, NOAA polar-orbiting satellites continuously transmit the APT signal. Radio reception of the APT signal is possible only when the satellite is above the horizon of a particular location (not obstructed by the Earth's surface), with a line-of-sight contact with the satellite.
logarithm Exponent of the power to which it is necessary to raise a fixed number (the base) to produce the given number. For example, the logarithm of 100 (base 10) is 2 because 102 equals 100.
longitude The angular distance from the Greenwich meridian (0 degree), along the equator. This can be measured either east or west to the 180th meridian (180 degrees) or 0 degree to 360 degrees W.
low A logic state corresponding to a binary R0S. Satellite imagery is displayed on a computer monitor by a combination of highs and lows. See high
low or low-pressure system A horizontal area where the atmospheric pressure is less than it is in adjacent areas. Since air always moves from areas of high pressure to areas of low pressure, air from these adjacent areas of higher pressure will move toward the low pressure area to equalize the pressure. This inflow of air toward the low will be affected by the Earth's rotation (see Coriolis force) and will cause the air to spiral inward in a counterclockwise direction in the northern hemisphere. The air eventually rises near the center of the low, causing cloudiness and precipitation.
The air in a low rotates in a counterclockwise direction in the Northern Hemisphere, and in a clockwise direction in the Southern Hemisphere. Low-pressure cells are called cyclones.
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