AIR MASSES AND FRONTS

AIR MASSES AND FRONTS

"fronts,” the colder air mass projecting un­der the warmer air mass in the form of a wedge. This condition is termed a "stationary front" if the boundary is not moving.

Front-is a boundary of air mass.

Warm Front. –warm air replaced cold air warm front has high humidity. Here appear drizzle,thunderstorm, rain

Cold Front. –cold air replaced warm air. Here temperatures are cool. This front is usually fast-moving.

Occluded front

This is a condition in which an air mass is trapped between two colder air masses and forced aloft to higher levels until it finally spreads out and lose jts identity. As far as the pilot is concerned, the weather in any occlusion is a combination о warm front and cold front conditions.

Thunderstorms are accompanied by thunder lightning, heavy rain showers and sometimes hat squalls and tornadoes.

Thunderstorms are associated with cu­mulonimbus clouds and there may be several thunderstorm “cells” within one cloud. They constitute a severe hazard to the pilot, espe­cially in light aircraft.

• severe windshear, causing flight path deviations and handling problems, loss of air­speed and possibly structural damage;

• severe turbulence, causing loss of control and structural damage;

• severe icing possibly with formation of the very dangerous clear ice from large su- ^ percooled water drops;

• damage from hail to the airframe and windows;

• reduced visibility;

• damage from lightning strikes, includ­ing electrical damage;

~ interference to radio communications arid radio navigation instruments.

The atmosphere always contains a certain amount of foreign matter — smoke, dust, salt particles, and particularly moisture in the form of invisible water vapor. The amount of moisture that can be present in the atmosphere depends upon the tempera­ture of the air.

Relative Humidity

dewpoint is the tem­perature to which air must be cooled to become saturated.When temperature reaches the dewpoint, water vapor can no longer re­main invisible, but is forced to condense, be­coming visible on the ground as dew or frost, appearing in the air as fog or clouds, or falling to the earth as rain, snow, or hail.

Fog and Mist. When the air near the ground is four or five degrees above the dew- point, the water vapor condenses and becomes visible as fog / mist. The difference between mist and fog is: mist exists if the visibility exceeds 1 km, fog exists if it falls below 1 km. It is usual for mist to precede fog at a place and to follow fog as it disperses.

Ceiling A ceding is defined as the height above the surface up to the base of the lowest layer of clouds or obscuring phenomena that hide more than half of the sky, and is reported as bro ken or overcast

Visibility Closely related to ceiling and cloud cover is "visibility” — the greatest hori­zontal distance at which prominent objects can be distinguished with the naked eye. Precipitation

Rain that falls from the base of clouds but

finally reaches the ground and includes:

-rain consisting of liquid water drops;

-drizzle consisting of fine water drop

lets;

-snow consisting of branched and star­shaped ice crystals;

-hail consisting of small balls of ice;

-freezing tain or drizzle which freezes when contacts a cold surface.

evaporates before reaching the ground (hence is not really precipitation) is called virga.