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9.17 Clouds and Precipitation

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Cloud Formation and Dispersal

There are many different conditions that trigger the formation cloud and when the trigger mechanism is removed the cloud disperses.

Convection: While the sun heats a surface of land, warm air is released as a thermal into the atmosphere. If there is enough water vapour in the thermal as it rises, adiabatic expansion takes place and a cumulus cloud forms when it passes through dew point.

Turbulence: When wind blows over an undulating landmass air ascends and descends. Condensation occurs on the up currents and evaporates in the down currents. This process continually produces cloud in the up currents and dissipates them in the down currents. The type of cloud often produced is stratocumulus through the day.

Orographic Uplift: When air moves up over a mountain it cools adiabatically. The type of cloud formed will be an orographic stratiform cloud or an orographic cumuliform cloud, depending on the environmental stability. Clear ice is possible in this type of cloud.

Fronts and Air Masses: The atmosphere consists of air masses which are different notably in temperature and moisture content. In general the air masses do not mix but are separated by relatively narrow transition zones called fronts. Fronts are lines where 2 air masses are converging. Meteorologists define a front as, “the moving leading edge of warm or cold air”.

A cold front is part of a system along which cold air is advancing; a warm front is that part of a system along which cold air is retreating. The fronts are frequently characterized by clouds and produce some of the stormiest weather. The type and extent of the clouds depend on the air mass characteristics. Some fronts have few clouds when the air is very dry.

Lenticular Clouds: are lens shaped and indicating there is mountain wave activity. Mountain waves form above and downwind of topographic barriers when strong winds blow with a significant vector component perpendicular to the barrier in a stable environment. A pilot’s task is to anticipate mountain wave development, assess its strength, and determine the possibility of clear air turbulence, strong winds, wind shear, and turbulence near the surface.

Rotors and low-level turbulent zones associated with mountain waves can be as violent as any found in the atmosphere. They have been implicated in several aircraft crashes, yet have not received the same attention as other aspects of mountain wave systems. Rotors have been difficult to understand in part because their intense turbulence can make aircraft measurements dangerous.