Certain types of weather, boundaries called frontal zones or "fronts", to move along the earth's surface, to slide up over the wedge of colder air, to increase moisture content, to become satu-rated, to be caused by upslope motion or cooling of the ground after sunset, thin wisps of cirrus clouds, in advance of the point on the ground which marks the position of the front, to act like a snow plow, to form cloud types that depend on the stability of the warm air, a steeper frontal surface, with gusty and turbulent surface winds, several thunderstorm “cells” within one cloud, to constitute a severe ha-zard to the pilot, obvious dangers from thunderstorm, damage from lightning strikes, loss of control and structural damage, sudden expansion of the air in its path, interference to radio communications, severe icing possibly with formation of the very dangerous clear ice.
B: Read Text “Air Masses and Fronts” and pay your attention to the
1 CB means hail, moderate or severe icing and / or turbulence
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Way they are used in the context.
Text. AIR MASSES AND FRONTS
Air masses can be associated with cer-tain types of weather. When two air masses meet, they will not mix readily unless their temperatures, pressures, and relative hu-midities are very similar. Instead, they set up boundaries called frontal zones, or "fronts," the colder air mass projecting un-der the warmer air mass in the form of a wedge. This condition is termed a "stationary front" if the boundary is not moving.
Usually, however, the boundary moves along the earth's surface, and as one air mass withdraws from a given area it is replaced by another air mass. This action creates a mov-ing front. If warmer air is replacing colder air, the front is called "warm;" if colder air is replacing warmer air, the front is called "cold."
Warm Front. When a warm front moves forward, the warm air slides up over the wedge of colder air lying ahead of it.
Warm air usually has high humidity. As this warm air is lifted, its temperature is lowered. As the lifting process continues, condensation occurs, low nimbostratus and stratus clouds form and drizzle or rain de-velops. The rain falls through the colder air below, increasing its moisture content so that it also becomes saturated. Any reduction of temperature in the colder air, which might be caused by upslope motion or cooling of the ground after sunset, may result in extensive fog.
As the warm air progresses up the slope, with constantly falling temperature, clouds appear at increasing heights in the form of altostratus and cirrostratus, if the warm air is stable. If the warm air is unstable, cumulonimbus clouds and altocumulus clouds will form and frequently produce thunders-torms. Finally, the air is forced up near the stratosphere, and in the freezing tempera-tures at that level, the condensation ap-pears as thin wisps of cirrus clouds.
The upslope movement is very gradual, ris-ing about 1,000 ft. very 20 miles. Thus, the cirrus clouds, forming at perhaps 25,000 ft. altitude, may appear as far as 500 miles in advance of the point on the ground which marks the position of the front.
Cold Front. When the cold front moves forward, it acts like a snow plow, sliding under the warmer air and forcing it aloft. This causes the warm air to cool suddenly and form cloud types that depend on the stability of the warm air.
Fast-Moving Cold Fronts. In fast-moving cold fronts, friction retards the front near the ground, which brings about a steeper frontal surface. This steep frontal surface results in a narrower band of weather concentrated along the forward edge of the front. If the warm air is stable, an overcast sky may occur for some distance ahead of the front, accompanied by general rain. If the warm air is conditionally unstable, scattered thunderstorms and show-ers may form in the warm air. At times an al-most continuous line of thunderstorms may form along the front or ahead of it. These lines of thunderstorms (squall lines) contain some of the most turbulent weather expe-rienced by pilots.
Behind the fast-moving cold front there is usually rapid clearing, with gusty and turbu-lent surface winds, and colder temperatures.
Occluded front
One other form of front with which the pi-lot has to be aware of is the “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 loses its identity. As far as the pilot is concerned, the weather in any occlusion is a combination of warm front and cold front conditions. Thunderstorms are accompanied by thunder, lightning, heavy rain showers and sometimes hail, squalls and tornadoes.
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Thunderstorms are associated with cu-mulonimbus clouds and there may be several thunderstorm “cells” within one cloud. They constitute a severe hazard to the pilot, espe-cially in light aircraft.
Dangers to aviation do not exist just in-side or under the cloud, but for quite some distance around it. Some obvious dangers from thunderstorms include:
- severe windshear, causing flight path deviations and handling problems, loss of air-speed 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, includ-ing electrical damage;
- interference to radio communications and 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 tempera-ture of the air.
Relative Humidity "Humidity" is commonly referred to as the apparent damp-ness in the air. Relative humidity is a ratio of the amount of moisture present in any given volume of air to the amount of moisture the air could hold in that volume of air at prevail-ing temperature and pressure.
For the pilot, the relationship discussed un-der relative humidity is expressed in a slightly different way — as "temperature and dew-point." In other words, dewpoint is the tem-perature to which air must be cooled to become saturated. Dewpoint is of tremendous signific-ance to the pilot because it represents a critical condition of the air. When temperature reaches the dewpoint, water vapor can no longer re-main invisible, but is forced to condense, be-coming 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. There are many types of fog, varying in degree of intensity and classified accord-ing to the particular phenomena which cause them. One type, "ground fog," which frequently forms at night in low places, is limited to a few meters in height, and is usually dissipated by the heat of the sun shortly after sunrise. Other types, which can form any time conditions are favora-ble, may extend to greater heights and persist for days or even weeks. Along seacoasts fog often forms over the ocean and is blown inland. All fogs produce low visibilities and therefore consti-tute a serious hazard to aircraft. The differencebetween mist and fog is: mist exists if the visibilityexceeds 1 km, fog exists if it falls below 1 km. It is usual for mist to precede fog at a place and to fol-low fog as it disperses.
When flying near clouds, pilots should remember that the cloud is a storm factory. The lightning that accompanies such a storm is prob-ably due to the breakup of raindrops. This pro-duces static electricity that discharges as lightning, thus causing sudden expansion of the air in its path, resulting in thunder.
Within the cloud and directly beneath it are updrafts and downdrafts; in the rear portion is a strong downdraft which becomes a wind blowing away from the cloud.
Ceiling A ceiling is defined as the height above the surface of 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. A ceiling is also defined as the vertical visibility into a surface based obscuration that hides all of the sky. A layer of clouds or ob-scuring phenomena classified as thin does not constitute a ceiling. The ceiling is unlimited if the sky is cloudless.
Visibility Closely related to ceiling and cloud cover is "visibility" – the greatest hori-zontal distance at which prominent objects can be distinguished with the naked eye. Visibility, like ceiling, is included in hourly weather re-ports and in aviation forecasts.
Precipitation In addition to possible damage by hail and the danger of icing, preci-pitation may be accompanied by low ceilings, and in heavy precipitation visibilities may sud-denly be reduced to zero.
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Precipitation refers to falling water that finally reaches the ground and includes:
- rain consisting of liquid water drops;
- drizzle consisting of fine water drop-lets;
- snow consisting of branched and star-shaped ice crystals;
- hail consisting of small balls of ice;
- freezing rain or drizzle which freezes when contacts a cold surface.
Rain that falls from the base of clouds but evaporates before reaching the ground (hence is not really precipitation) is called virga.
It should be obvious that aircraft which may have accumulated snow while on the ground should never be flown until all traces of snow have been removed, including the hard crust that frequently adheres to the surfaces. An aircraft which has been exposed to rain followed by freezing temperatures should be carefully cleared of ice and checked before takeoff to make certain that the controls operate freely.