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Words begining with "L"
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
- 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 fulfillinq
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.
grc (Glenn Research Center)
See NASA Centers.
Light
- Form of radiant energy that acts upon the retina of the eye,
optic nerve, etc., making sight possible. This energy is
transmitted at a velocity of about 186,000 miles per second by
wavelike or vibrational motion.
- A form of radiant energy similar to this, but not acting on
the normal retina, such as ultraviolet and infrared radiation
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 thunderstrom.
Limb Viewing (Occultation)
The process of viewing the atmosphere at a tangent to the Earth's
surface. The viewing signal, from a star or another satellite, is
occulted or obscured by the intervening atmosphere. The absorption of
light from the sun or star provides information on the properties of
the atmosphere at different heights. Limb viewing instruments can
also sense infrared or microwave-emitted radiation from the
atmosphere.
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 10 rasied to the 2nd power is
100.
Longitude
The angular distance from the Greenwich meridian (0 degrees),
along the equator. This can be measured either east or west to the
180th meridian (180 degrees) or 0 degrees to 360 degrees W.
Loss of Signal (LOS)
The inability to receive a satellite signal because the
satellite's orbital path has taken it below the antenna's horizon.
This term is relevant to all satellites except geostationary.
Low
A logic state corresponding to a binary "O". 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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