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Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

L

Lake Breeze - An onshore wind at the lake shore that results due to differences in heating during the day. The land warms quicker than the water, so an area of low pressure forms. The water remains cool, where high pressure develops. The resulting wind flow from high to low pressure creates the lake breeze. Similar to a sea breeze.

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.

Land Breeze - The opposite of a sea or lake breeze. At nighttime the land cools faster than the water. High pressure develops over the cooler land and low pressure develops over the warmer water. As the air flows from high to low pressure, the land breeze commences.

Landspout - [Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.

Lapse Rate - The rate at which a variable in the atmosphere, usually temperature decreases with height. A parcel of dry air will lose temperature at the rate of 5.5 °F for every 1000 feet of ascent. The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.

Large-scale - See synoptic-scale.

Latent Heat - The amount of heat that is lost or gained when a substance undergoes a phase change, such as condensation, evaporation or sublimation.

Left Front Quadrant (or Left Exit Region) - The area downstream from and to the left of an upper-level jet max (as would be viewed looking along the direction of flow). Upward motion and severe thunderstorm potential sometimes are increased in this area relative to the wind speed maximum. See also entrance region, right rear quadrant.

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. See also right mover.

Lenticular Clouds - The official meteorological name for flying saucer clouds is a real mouthful - "altocumulus standing lenticularus". The flying saucer or lenticular clouds are excellent signs of strong winds aloft and can foretell changes in our weather. Lenticular clouds are a good indicator of a developing "chinook" wind event along the Front Range. The jet stream winds bend and squeeze their way past the mountains, but then the bottom drops out from under them on the plains. This causes the air pressure to fall, creating a low pressure area just east of the Rockies (meteorologists call this a "lee-side" low. The falling pressure causes a slight cooling of the air aloft and that cooling causes moisture to condense and form the clouds. This is the reason the lenticular clouds seem to hang over the plains just east of the mountains instead of moving on to the east. The winds aloft basically form a "standing wave" in the atmosphere (again, think of the way water looks just below a big rosk in a mountain stream). The "wave" will park right over the metro area at times, meaning the mountains are bathed in sunlight, as are the plains east of Denver - but we are stuck under a large cloud right over the city.

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 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. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.

Lightning - Nature's own fireworks display is a source of beauty, wonder and danger! The most common form of damaging weather, lightning occurs in over 2,000 storms around the world at any given time.
Lightning is caused by static electricity created by the friction of raindrops and ice crystals being thrown about by strong thunderstorm winds. When the static electricity grows large enough, it creates a giant spark that contains great amounts of electricity - about 150,000 amps during the fraction of a second the flash occurs. Lightning may jump up, down or sideways. When we see lightning strike, it appears to go from the cloud to the ground. Actually it comes from both the ground and the cloud. The opposite charges begin to move toward each other, when they meet, the bright flash occurs.
Lightning bolts appear to be very wide, but it's really just an illusion. A careful study of lightning strike areas show most bolts to be no wider than a pencil. Lightning is very hot. A typical bolt may have a temperature of 50,000¼F. This extreme heat causes the surrounding air to rapidly heat up and expand, creating the shock wave we hear as thunder.
You can tell how far away a lightning bolt is by counting the number of seconds between the time you see the bolt and hear the thunder and divide that number by five. For example 15 seconds, divided by five, equals 3 miles.

Loaded Gun (Sounding) - [Slang], a sounding characterized by extreme instability but containing a cap, such that explosive thunderstorm development can be expected if the cap can be weakened or the air below it heated sufficiently to overcome it.

Longwave Trough - A trough in the prevailing westerly flow aloft which is characterized by large length and (usually) long duration. Generally, there are no more than about five longwave troughs around the Northern Hemisphere at any given time. Their position and intensity govern general weather patterns (e.g., hot/cold, wet/dry) over periods of days, weeks, or months. Smaller disturbances (e.g., shortwave troughs) typically move more rapidly through the broader flow of a longwave trough, producing weather changes over shorter time periods (a day or less).

Low-level Jet - 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).
The term also may be used to describe a narrow zone of strong winds above the boundary layer, but in this sense the more proper term would be low-level jet stream.

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 often exhibit a striking visual appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation. They are capable of producing tornadoes and very large hail. Radar identification often is difficult relative to other types of supercells, so visual reports are very important. 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.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society