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LAN

Abbreviation used in aviation work to stand for "over-land".

Lapse rate (of temperature)

The decrease of temperature with height in the atmosphere. Confusingly, the opposite case, an increase in temperature with height, is known as a negative lapse rate.

Latent heat

The amount of energy needed to accomplish a phase change. Latent heat of fusion is the amount of energy required to melt ice, and at 0°C is 3.34 * 105J kg-1 (or about 80 cal/g). The latent heat of vaporisation is the amount of energy needed to evaporate liquid water. It is equivalent to 2.50 * 106 J kg-1 (or about 600 cal/g) at 0°C. The latent heat of sublimation is the energy needed to carry out a change from solid (ice) to gas (vapour). It is the sum of the latent heats of fusion and vaporisation, i.e. 2.83 * 106 J kg-1 (or about 680 cal/g) at 0°C. When water freezes, condenses or changes from a gas to a solid, 80 cal/g, 600 cal/g and 680 cal/g are released to the environment respectively. The processes are all reversible.

Left exit

Lies on the cold side of the jet axis, in the region of marked deceleration of flow. A preferred region for cyclonic development.

Lenticular clouds

These form within the crest(s) of orographic (or lee) wave-trains, over and downwind of hills / mountains / islands, provided of course that the air is humid enough. The clouds are formed because air cools as it is forced to rise and if condensation takes place, lens-shaped clouds are observed with clear space in between the elements. The cloud forms within the upwind leg of each wave-crest and dissipates (evaporates) on the downwind leg: the air is therefore flowing through the cloud, with the cloud itself staying quasi-stationary; change in the cloud requires an alteration in the windflow or temperature / humidity environment. Sometimes, under very special circumstances, a 'pile of plates' is observed, where lenticular clouds are stacked vertically. The most common form of wave-forced cloud is perhaps Altocumulus lenticularis (Ac len), but lenticular cloud forms are found at all levels. Standing wave motion can also lead to a previously uniform sheet of cloud developing a lenticular appearance, and on occasion, complete dispersal. (See also MTW).

Loaded gun scenario

On a day of instability through a great depth of the troposphere, and high values of CAPE (q.v.), rising surface temperatures will at some point ensure that convection parcels leave the surface, the condensation level will be reached, cloud will grow (given sufficient moisture), and a heavy shower, or even a thunderstorm will result. It sometimes happens though that although the atmosphere is markedly unstable above, say, 2 km a 'lid' opposing surface-based convection exists at or below this level, due often to a layer of warm/dry air that has become entrained in the airflow from some source. This means that surface temperatures must become very high to overcome this 'lid', often requiring additional triggers, such as low-level convergence or release of medium level potential instability by a mid-level trough, thus lifting the whole column and releasing the 'pent-up' energy in a sudden burst... and the 'loaded gun' will be 'fired', perhaps leading to a severe storm/supercell event. (see 'Spanish plume').

LYR

Layer(s) (as used in cloud forecasting in aviation products).