Weather Glossary - J K L


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Jet Max (or Speed Max, Jet Streak) - a point or area of relative maximum wind speeds within a jet stream

Jet Streak - a local wind speed maximum within a jet stream

Jet Stream - relatively strong winds concentrated within a narrow current in the atmosphere, normally referring to horizontal, high-altitude winds

Joule - the derived unit of energy or work equal to 1 Newton-meter. Named for James Prescott Joule (1818-1889), a British physicist who studied heat.



Kelvin Temperature Scale (K) - an absolute temperature scale in which a change of 1 Kelvin equals a change of 1 degree Celsius; 0K is the lowest temperature on the Kelvin scale.

Kinetic energy - energy that a body has as a result of its motion. Mathematically, it is defined as one-half the product of a body's mass and the square of its speed.

Kirchhoff's law - at a given temperature, a good absorber of a given wavelength is also a good emitter of that wavelength

Klystron - the electronic component of the NEXRAD transmitter that generates the coherent radiation necessary for the determination of Doppler velocities. The WSR-88D typically transmits its radiation using a power of 750,000 watts. Other commercial Doppler radars sometimes use a Magnetron to generate the transmitted radiation. Magnetrons typically generate less steady patterns of radiation than do Klystrons.

Knot - the unit of speed in the nautical system; one nautical mile per hour; it is equal to 1.1508 statute miles per hour or 0.5144 meters per second



Lake Breeze - a wind blowing from the surface of a large lake onto the shores during the afternoon

Land Breeze - a breeze from land to sea, caused by the temperature difference when the sea surface is warmer than the adjacent land

Lapse Rate - the rate of change of temperature with height

Latent Heat - the heat released or absorbed by a substance during a phase change

Lee Side - the side of an object (e.g., mountain) that is sheltered from the wind

Left Mover - a thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm

LEWP (Line Echo Wave Pattern) - a bulge in a thunderstorm line producing a wave-shaped "kink" in the line. The potential for strong outflow and damaging straight-line winds increases near the bulge, which often resembles a bow echo. Severe weather potential also is increased with storms near the crest of a LEWP.

Lifted Index (or LI) - a common measure of atmospheric instability; its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that computed temperature to the actual temperature at that level. Negative values indicate instability; the more negative, the more unstable the air is and the stronger the updrafts are likely to be with any developing thunderstorms.

Lifting Condensation Level (LCL) - the level in the atmosphere at which an unsaturated air parcel lifted dry adiabatically would become saturated. See convective condensation level (CCL).

Lightning - any visible electrical discharge produced by thunderstorms

Liquid - one of the basic three phases of matter; characterized by free movement of the constituent molecules among themselves but without the tendency to separate

Liquid Water Equivalent - the amount of water estimated from a reflectivity signature on a radar. The liquid water equivalent is computed using an empirical mathematical equation.

Local Winds - winds which blow over a small area at a speed or in a direction generally different from those which are appropriate to the general pressure distribution of the region

Longwave Radiation - in meteorology, radiation having a wavelength greater than that of visible light

Low - in meteorology, an region of low pressure; see also cyclone

Low-Level Jet (abbreviated LLJ) - a region of relatively strong winds in the lower part of the atmosphere. Specifically, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).

LP Storm (or LP Supercell) - Low -Precipitation storm (or Low -Precipitation supercell); a supercell thunderstorm characterized by a relative lack of visible precipitation; visually similar to a classic supercell, except without the heavy precipitation core. LP storms almost always occur on or near the dry line, and thus are sometimes referred to as dry line storms.

LSR (Local Storm Report) - a product issued by local NWS offices to inform users of reports of severe and/or significant weather-related events

Luminance - a measure of the brightness of a surface; See radiance


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