Figure G - 1: Standard atmosphere
Below 0°C, super-cooled water may coexist with ice crystals. Below -40°C, there are only ice crystals.
G.2. THUNDERSTORMS
G.2.1. FORMATION
The thunderstorm is a cumulonimbus that develops up to a stage with an anvil top. However, a cumulonimbus may have all the dangerous characteristics of a thunderstorm (i.e. lightnings, hail, turbulence).
The development of a thunderstorm results from the conjunction of two conditions:
.
A global shearing of the atmosphere, as wind speed generally increases with altitude. That wind gradient is enhanced by earth friction, but less by sea friction
.
An airmass of high humidity at lower levels. It is a fact of physics that humid air is more unstable than dry air. Note that the maximum water content of an airmass increases very rapidly with temperature.
When those two conditions are met, vertical instability develops. Thunderstorm activity is enhanced by very small changes in the conditions surrounding the system:
.
A colder airmass arriving at high altitude
.
The system passes over an area gradually heated by the sunshine
.
Some orographic effect.
Under such conditions, the development of a thunderstorm may be extremely rapid, even at a visible pace. One may have blue sky in the morning, a severe thunderstorm in the afternoon and dissipating clouds at sunset, which makes the weather forecast difficult to interpretate. A thunderstorm often develops up to the tropopause altitude, sometimes above.
Other characteristics:
.
Most thunderstorms have a life cycle associated with the duration of sun radiation.
.
Sea thunderstorms are less severe (less wind gradient and lower surface temperature).
.
Winter thunderstorms often top at altitudes lower than the summer ones (colder airmass, therefore less water, hence less instability).
G.2.2. SINGLE CELL
A single cell thunderstorm is the result of a single updraft. This kind of thunderstorms is rare. A single cell may have a life cycle above one hour, because of its high instability.
G.2.3. MULTI-CELL THUNDERSTORMS
Multi-cell thunderstorms are more common. Each thunderstorm is at different formation stage. The downdraft of one cell creates a gust front. This gust front provides the lifting mechanism for new cells. New cells will tend to form on the downwind side of existing cells.
G.2.4. SUPER CELL
The super cell is an example of diverging mechanism in atmospheric dynamics. If the airmass is very unstable up to a large altitude, vertical speeds are high. A very active convection cycle is triggered inside the cloud, which activates condensation and icing (step 1 of Figure G - 2). This puts dry air in contact with precipitations. Dry air is rapidly cooled. This super cold air is the origin of massive air falls, called downdraft and downbursts (step 2 of Figure G - 2).
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：getting to grips with surveillance(132)
