Glossary of Meteorological Terms (L)

L

LAI

Abbreviation for leaf area index. The ratio of total projected leaf area per unit ground area, widely used to characterize canopy light conditions.

Lambert

A unit of luminance (photometric brightness). One lambert is the luminance of a surface that emits or reflects one lumen per square centimeter. The lambert honors the German physicist Johann Lambert (1728-1777), who showed that the illuminance of a surface is inversely proportional to the square of the distance from the light source.

Laminar

Smooth, non-turbulent. Often used to describe cloud formations which appear to be shaped by a smooth flow of air traveling in parallel layers or sheets.

Langley

A unit of energy per unit area commonly employed in radiation theory. Equal to one gram-calorie per square centimeter.

Lapse line

A curve showing the variation of temperature with height in the free air. See lapse rate.

Lapse rate

The decrease of an atmospheric variable with height, the variable being temperature, unless otherwise specified.

Large calorie

See calorie.

Laurence

A common type of terrestrial scintillation; shimmering over a hot surface (such as a roadway) on a quiet, cloudless, summer day.

Leeward

Facing away from the wind.

Lee wave

A wave disturbance in airflow due to some barrier in the flow, i.e. a hill or mountain.

LIDAR

Light Detecting And Ranging. A technique used to detect atmospheric constituents or related parameters such as atmospheric extinction coefficient. Light is produced in a modulated source and the resulting backscattered or reflected light is analyzed to quantify some property of the atmosphere.

Light

Visible radiation (about 0.4 to 0.7 microns in wavelength) considered in terms of its luminous efficiency, that is, evaluated in proportion to its ability to stimulate the sense of sight.

Light air

Wind with a speed between 1 and 3 knots (1 and 3 mph); Beaufort scale number 1.

Light breeze

Wind with a speed between 4 and 6 knots (4 and 7 mph); Beaufort scale number 2.

Linearity

The maximum deviation of any points from a straight line drawn as a "best fit" through the calibration points of an instrument with a linear response curve. Usually expressed as a percentage of full-scale range.

Linke-scale

A type of cyanometer, an instrument used to measure the blueness of the sky. The Linke-scale is simply a set of eight cards of different standardized shades of blue. They are evenly numbered 2 to 26. The odd numbers are used by the observer if he or she judges the sky color to lie between any of the given shades.

Liquid thermometer

Thermometer in which the difference in the rates of expansion with temperature of a liquid and its receptacle is used as a measure of the temperature. The liquid used may be ethyl alcohol, toluene, petroleum, or mercury.

Lithometeor

The general term for dry atmospheric suspensoids, including dust, haze, smoke, and sand. Compare to hydrometeor.

Lithosphere

The outer, solid portion of the earth; the crust of the earth.

Livingstone sphere

An clay atmometer consisting of a hollow ceramic sphere through which evaporation occurs. Evaporation is measured by the loss of water from the reservoir which feeds the sphere.

Lizard balloon

A balloon having a detachable tail which is released when the balloon has undergone a predetermined expansion. It thus serves to measure approximately the density of the atmosphere at the point of release.

Local visual distance

The meteorological visual range, which can be estimated from the average extinction coefficient using the Koschmieder equation.

Local winds

Winds which, over a small area, differ from those which would be appropriate to the general pressure distribution.

Long-wave radiation

Radiation with wavelengths greater than 4 microns. (In meteorology, same as infrared radiation.)

Lo-reference signal

The audio-frequency signal transmitted by the Diamond-Hinman radiosonde when the baroswitch pen passes each fifth contact of the commutator up to a number determined by the design of the commutator. It then signals every contact except the fifth, which is transmitted as a hi-reference signal.

Low

An area of low barometric pressure, with its attendant system of winds. Also called a depression or cyclone.

Low level wind shear

A local variation in the wind direction or speed. This condition can present danger to aircraft, especially at landing, when a sudden shift from headwind to tailwind can cause a rapid loss of airspeed and lift.

Lucimeter

Instrument for measuring the mean intensity of global solar radiation (direct and diffuse) near the earth’s surface in a specified time interval.

Lull

A momentary decrease in the speed of the wind.

Lumen

A unit of luminous flux. The lumen is equal to the luminous flux radiated into a unit solid angle (steradian) from a small source having a luminous intensity of one candle. An ideal source possessing an intensity of one candle in every direction would radiate a total of 4 pi lumens. "Lumen" is a Latin word for light.

Luminance

A measure of the intrinsic luminous intensity emitted by a source in a given direction. Luminance is a measure only of light. The comparable term for electromagnetic radiation in general is radiance.

Luminescence

Any emission of light at temperatures below that required for incandescence.

Luminous flux

The flux of visible radiation, so weighted as to account for the manner in which the response of the human eye varies with the wavelength of radiation. The basic unit for luminous flux is the lumen.

Luminous intensity

The intensity (flux per unit solid angle) of visible radiation weighted to take into account the variable response of the human eye as a function of the wavelength of light. Usually expressed in candles.

Lux

A photometric unit of illuminance or illumination equal to one lumen per square meter.

Lysimeter

A type of evaporation gauge consisting of a tank or pan of soil placed in a field so that the soil, water, thermal, and vegetative properties in the tank duplicate as closely as possible the properties of the surrounding area.