La Niña
A period of stronger-than-normal trade winds and unusually low sea-surface temperatures in the central and eastern tropical Pacific Ocean; the opposite of El Niño. See La Niña fact sheet.
A flood of mud and rocks from the slopes of a volcano. May be triggered by earthquakes, landslides, or when the rim of a caldera lake is breached. See When Rivers of Rock Flow
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.
land cover
The characteristics of a land surface as determined by its spectral signature (the unique way in which a given type of land cover reflects and absorbs light).
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.

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.

Langley Research Center (LaRC)
Oldest of NASA's field centers, LaRC is located in Hampton, Virginia, and focuses primarily on aeronautical research. Established in 1917 by the National Advisory Committee for Aeronautics, the Center currently devotes two-thirds of its programs to aeronautics, and the rest to space. LaRC researchers use more than 40 wind tunnels to study improved aircraft and spacecraft safety, performance, and efficiency. LaRC Web Site
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.
latent heat
The heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, or sublimation.
latitude (aka the geodetic latitude)
The angle between a perpendicular at a location, and the equatorial plane of the Earth.
lava dome
lava that piles up over a volcanic vent, usually in a rounded mass
leaf area index (LAI)
The area of foliage per unit area of ground. Conventionally this refers to the ratio of the area of the upper side of the leaves in a canopy projected onto a flat surface to the area of the surface under the canopy. Occasionally this has been used in reference to both sides of the leaves.
A listing that contains symbols and other information about a map.
Acronym for 'Light Detection and Ranging,' a technique for performing accurate remote measurements of atmospheric trace gas concentration over ranges of several meters to tens of kilometers. This is done by probing the absorption lines of the gases with narrow spectral laser radiation using the differential absorption lidar technique.

1. 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.

2. 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.

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.

Lightning Imaging Sensor (LIS)
A small, highly sophisticated instrument that detects and locates lightning over the tropical region of the globe. Looking down from a vantage point aboard the Tropical Rainfall Measuring Mission (TRMM) observatory, 218 miles (350 kilometers) above the Earth, the sensor is providing information that could lead to future advanced lightning sensors capable of significantly improving weather "nowcasting." The Lightning Imaging Sensor is three times more sensitive than a predecessor instrument known as the Optical Transient Detector. The LIS will study both day and night cloud-to-ground, cloud-to-cloud and intra-cloud lightning and its distribution around the globe. See LIS web site
The component of the Earth's surface comprising the rock, soil, and sediments. It is a relatively passive component of the climate system, and its physical characteristics are treated as fixed elements in the determination of climate.
Little Ice Age
A cold period that lasted from about A.D. 1550 to about A.D. 1850 in Europe, North America, and Asia. This period was marked by rapid expansion of mountain glaciers, especially in the Alps, Norway, and Alaska. There were three maxima, beginning about 1650, about 1770, and 1850, each separated by slight warming intervals.
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.
longwave radiation
The radiation emitted in the spectral wavelength greater than 4 micrometers corresponding to the radiation emitted from the Earth and atmosphere. It is sometimes referred to as 'terrestrial radiation' or 'infrared radiation,' although somewhat imprecisely.
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.

Madden-Julian Oscillation
A "wave" of disturbed tropical weather that spreads eastward around the globe on a 30-to-60-day cycle, usually beginning in the Indian Ocean. The disturbance brings periods of enhanced tropical rainfall followed by periods of suppressed rainfall, and it also influences cloudiness and sea surface temperature. Although the Madden-Julian Oscillation originates in the tropics, it influences weather in the subtropics and mid-latitudes as well.
The region surrounding a celestial body where its magnetic field controls the motions of charged particles. The Earth's magnetic field is dipolar in nature. That is, it behaves as if produced by a giant bar magnet located near the center of the planet with its north pole tilted several degrees from Earth's geographic north pole.

The Earth's magnetic field presents an obstacle to the solar wind, as a rock in a running stream of water. This obstacle is called a bow shock. The bow shock slows down, heats, and compresses the solar wind, which then flows around the rest of Earth's magnetic field. See Van Allen belts.

Marshall Space Flight Center (MSFC)
The George C. Marshall Space Flight Center, located in Huntsville, Alabama, is responsible for developing spacecraft hardware and systems, and is perhaps best known for its role in building the Saturn rockets that sent astronauts to the Moon during the Apollo program. It is NASA's primary center for space propulsion systems and plays a key role in the development of payloads to be flown on the shuttle (such as Spacelab). MSFC also manages two other NASA sites: the Michoud Assembly Facility in New Orleans where the Shuttle's external tanks are manufactured, and the Slidell Computer Complex in Slidell, Louisiana, which provides computer support to Michoud and to NASA's John C. Stennis Space Center. MSFC Web Site
mass balance
The term ‘mass balance’ is often used by glaciologists to describe the difference between all of the ice that is added to a glacier, and all of the ice the glacier loses over a period of time. Ice sheets and glaciers can lose mass due to melting, calving, evaporation, etc. They can gain mass from direct precipitation, avalanching, and windblown snow. The net result of all these outputs and inputs of ice are then the glacier’s mass balance.
mean sea level
The average height of the sea surface, based upon hourly observation of the tide height on the open coast or in adjacent waters that have free access to the sea. In the United States, it is defined as the average height of the sea surface for all stages of the tide over a nineteen year period. Mean sea level, commonly abbreviated as MSL and referred to simply as 'sea level,' serves as the reference surface for all altitudes in upper atmospheric studies.
Measurements of Pollution in the Troposphere (MOPITT)
MOPITT will fly aboard Terra as part of NASA's Earth Observing System(EOS). It is an instrument designed to enhance our knowledge of the lower atmosphere and to particularly observe how it interacts with the land and ocean biospheres. MOPITT is a scanning radiometer employing gas correlation spectroscopy to measure upwelling and reflected infrared radiance in three absorption bands of carbon monoxide and methane.
(mbps) Millions of bits per second. A unit of information transfer rate -- e.g. Ethernet can carry 10 mbps.
The upper boundary of the mesosphere where the temperature of the atmosphere reaches its lowest point.
The atmospheric layer above the stratosphere, extending from about 50 to 85 kilometers altitude. The temperature generally decreases with altitude.
the sum of all the chemical and physical processes within a living organism, including anabolism and catabolism
Information describing the content or utility of a data set. For example, the dates on which data were procured are metadata.
The former Soviet Union's series of polar orbiting weather satellites. The Meteor satellites transmit images in a system compatible with the NOAA polar-orbiting satellites.
a solid mass of mineral or rock matter that has fallen to the earth's surface from outer space without being completely vaporized in the atmosphere.
Study of the atmosphere and its phenomena.
(METEOrological SATellite) Europe's geostationary weather satellite, launched by the European Space Agency and now operated by an organization called Eumetsat.
Methane (CH4)
A hydrocarbon that is a greenhouse gas. Methane is produced through anaerobic (without oxygen) decomposition of waste in landfills, animal digestion, decomposition of animal wastes, production and distribution of natural gas and oil, coal production , and incomplete fossil fuel combustion. The atmospheric concentration of methane has been shown to be increasing at a rate of about 0.6% per year and the concentration of about 1.7 parts per million by volume (ppmv) is more than twice its preindustrial value. However, the rate of increase of methane in the atmosphere may be stabilizing.
any microorganism
One millionth of a meter, used to measure wavelengths in the electromagnetic spectrum. Also known as a "micron" or µm
Electromagnetic radiation with wavelengths between about 1000 micrometers and one meter.
middle infrared
Electromagnetic radiation between the near infrared and the thermal infrared, about 2-5 micrometers.
millibar (mb)
One thousandth of a bar, a unit of atmospheric pressure. The average atmospheric pressure at sea level is 1.01325 bars or 1013.25 mb. See pascal (Pa), atmospheric pressure.
model (noun)
A mathematical representation of a process, system, or object developed to understand its behavior or to make predictions. The representation always involves certain simplifications and assumptions.
Moderate-resolution Imaging Spectroradiometer (MODIS)
MODIS will fly aboard Terra as part of NASA's Earth Observing System (EOS). It will view the entire surface of the Earth every 1-2 days, making observations in 36 co-registered spectral bands, at moderate resolution (0.25 - 1 km), of land and ocean surface temperature, primary productivity, land surface cover, clouds, aerosols, water vapor, temperature profiles, and fires. See MODIS Web Site.
A name for seasonal winds, first applied to the winds over the Arabian Sea that blow for six months from the northeast and for six months from the southwest. The term has been extended to similar winds in other parts of the world (i.e., the prevailing west to northwest winds of summer in Europe have been called the European monsoon). The primary cause for these seasonal winds is the much greater annual variation of temperature over large land areas compared with neighboring ocean surfaces, causing an excess of pressure over the continents in winter and a deficit in summer, but other factors, such as topography of the land, also have an effect. The monsoons are strongest in the southern and eastern sides of Asia, but also occur along the coasts of tropical regions wherever the planetary circulation is not strong enough to inhibit them. The monsoon climate can be described as a long winter-spring dry season, which includes a cold season followed by a short hot season just preceding the rains; a summer and early autumn rainy season, which is generally very wet but may vary greatly from year to year; and a secondary warming immediately after the rainy season.
Montreal Protocol
An international agreement to drastically reduce CFC production, the Protocol was adopted in Montreal in 1987. It was significantly strengthened at a subsequent meeting in London in 1990 that called for a complete elimination of CFCs by the year 2000. The agreement was again amended by a Meeting of the Parties in Copenhagen in November 1992. Consumption of controlled substances--such as CFCs and halons--was greatly reduced or eliminated, and many accountability dates were moved forward, often from 1 January 2000 to 1 January 1996.
Mount Pinatubo
A volcano in the Philippine Islands that erupted in 1991. The eruption of Mount Pinatubo ejected enough particulate and sulfate aerosol matter into the atmosphere to block some of the incoming solar radiation from reaching Earth's atmosphere. This effectively cooled the planet from 1992 to 1994, masking the warming that had been occurring for most of the 1980s and 1990s.
mountain and valley breezes
System of winds that blow downhill during the night (mountain breeze) and uphill during the day (valley breeze).
Multi-angle Imaging Spectro-Radiometer (MISR)
MISR will fly aboard Terra as part of NASA's Earth Observing System. It will monitor the monthly, seasonal, and long-term trends in the amount and type of atmospheric aerosol particles, including those formed by natural sources and by human activities; the amount, types, and heights of clouds; and the distribution of land surface cover, including vegetation canopy structure. See MISR Web Site.
Multispectral Scanner (MSS)
A line-scanning instrument flown on Landsat satellites that continually scans the Earth in a 185 km. (100 nautical miles) swath. On Landsats 1, 2, 4, and 5, the MSS had four spectral bands in the visible and near-infrared with an IFOV of 80 meters. Landsat-3 had a fifth band in the thermal infrared with an IFOV of 240 meters.