Weather Glossary: L's

1. The measure of the time between the center of mass of precipitation to the center of mass of runoff (on the hydrograph); basin lag is a function of not only basin characteristics, but also of storm intensity and movement. Some hydrologic texts define lag from the center of mass of rainfall to the hydrograph peak.
2. The time it takes a flood wave to move downstream.

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. The global average rate of temperature change with height in the atmosphere is 6.5°C/km.

The adiabatic lapse rate (or dry adiabatic lapse rate) is the normal rate of change (9.8°C/km) for a dry parcel of air that is moved up or down and cools or warms as the pressure changes. The wet (moist) adiabatic lapse rate (4.9°C/km) is the rate at which saturated air cools as it ascends.

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. Also, see entrance region and right rear quadrant.

It is a stability index used to determine thunderstorm potential. The LI is calculated by taking a representative low-level air parcel and lifting it adiabatically to 500 mb. The algebraic difference between this air parcel and the sounding temperature at 500 mb (around 18,000 feet/5,500 meters) denotes the LI.

Since the LI accounts for moisture below 850 mb, it provides more reliable stability information than the Showalter Index (SWI). The greater negative values of LI indicate energy available for parcel ascent.

A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate. A meso-high pressure area is usually present behind the accelerating thunderstorms.

A meso-low pressure area is usually present at the crest of the wave. 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.

• Visual Greenness Maps - These maps portray vegetation greenness compared to a very green reference such as an alfalfa field or a golf course. The resulting image is similar to what you would expect to see from the air. Normally dry areas will never show as green as normally wetter areas.
• Relative Greenness Maps - These maps portray how green the vegetation is compared to how green it has been historically (since 1989). Because each pixel is normalized to its own historical range, all areas (dry to wet) can appear fully green at some time during the growing season.
• Departure from Average Greenness Maps - These maps portray how green each pixel is compared to its average greenness for the current week of the year.
• Live Moisture Maps - These maps portray experimental live vegetation moisture with values ranging from 50 to 250 percent of dry weight.

This National Climatic Data Center (NCDC) publication is produced monthly and annually for some 270 United States cities and its territories. The LCD summarizes temperature, relative humidity, precipitation, cloudiness, wind speed and direction observation.

Most monthly publications also contain the 3-hourly weather observations for that month and an hourly summary of precipitation. Annual LCD publications contain a summary of the past calendar year as well as historical averages and extremes.

The LCD contains 3-hourly, daily, and monthly values. The annual issue contains the year in review plus normals, means and extremes.

Reservoirs used for recreational use or storage of irrigation, municipal or industrial water. Because water is impounded on a "permanent" basis, the design of these dams is more complex than for tailings or flood control detention dams.

A long-term storage dam may include an impermeable core surrounded by shell material, have many types of drains and filters, outlet works, with gates and valves, seepage collection boxes, and possibly several spillways. The capacity of the spillway is dependant upon the downstream hazard potential.

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).

It often forms at 1-1.5 km under the exit region (the place just ahead of a speed maximum) of an upper-level jet (ULJ) streak. It has a strong isallobaric component due to the pressure rise/fall pattern under the right exit/left exit region of the jet streak, respectively. It is usually at a significant angle with respect to the ULJ and has a strong southerly component.

This enhances the warm, moist air ahead of an upper level front-jet system. This is common over the Plains states at night during the warm season (spring and summer). Another type of a LLJ is called a nocturnal jet.

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.