METEOROLOGY - ATMOSPHERIC TEMPERATURE [Main Title]

A very technical training film aimed principally at aircrew, which explains the fundamental physical properties of the atmosphere and the implications for aircraft flight. Excellent cartoons and animated diagrams comprise most of the film, linked by an authoritative narrative. The large volume of scientific and numerical data presented is not easily summarised without losing the necessary detail, so principal points only are identified here.

Reel one: Changes of temperature in the atmosphere are usually associated with changes of weather and are therefore important to air crew as well as meteorologists. Temperature changes aloft affect aircraft performance also and the accuracy of certain instruments such as the pressure altimeter. To measure air temperatures, pressure and relative humidity up to 8,000 ft. above the ground surface the radio sonde is used. Cut to staff releasing and tracking a sonde. A radio in the sonde transmits the numerical data back to the station. The varying height of the sonde is calculated from the three recorded parameters. Normally temperature decreases with increasing height. This is called the Lapse rate. An increase in temperature with increasing height is called Inversion. No change in temperature with increasing height is called the Isothermal layer. The average lapse rate in the United Kingdom is 2oC per 1,000 ft. up to a height of about six miles. This layer, ground to six miles is called the troposphere. Above this layer the temperature barely changes to a height of about 20 miles. This almost isothermal region is called the stratosphere. The boundary between the stratosphere and the troposphere is called the tropopause. The height of the tropopause varies from day to day. It has an average over the equator of eleven miles, decreasing less than five miles at the poles. This is caused by heat and the main source of heat is the sun, a process known as radiation.

"Radiation": Solar radiation takes the form of electromagnetic waves of various lengths ranging from invisible light (ultra violet), to visible light, to the longer infra red rays. The latter are also invisible and are known as radiant heat. Energy from the sun is barely absorbed by the atmosphere, only when it is absorbed by the ground surface is it converted to heat. The phenomenon known as the greenhouse effect (where the greenhouse interior air is warmer than the outside air) is described in detail. The analogy is made with the presence of water vapour in the atmosphere, which will govern the amount of solar radiation absorbed by the ground surface. Other factors influencing the ground's absorption are considered: ground type, ground cover, sea/water surfaces, angle of incidence of the sun, climatic conditions and seasonal variations.

Reel two: "The transfer of heat": The temperature of the air is dependent upon the transfer of heat from the ground, by conduction and/or convection.

"Conduction": Conduction takes place when energy is passed from one molecule to the next. A diagram depicts warm air above cold ground. Air is a poor conductor of heat, so only the bottom layers become cool, the layers above remain warm. For cold air above warm ground heat may be transferred by a second process called convection and involves the bodily transfer of heated masses of air.

"Convection": A heavier fluid over a lighter fluid will sink because of gravity. Air is a fluid and changes in density (by heating or cooling) will cause vertical displacement. Compressed air will rise in temperature when compressed, conversely, when compressed air expands its temperature will fall. Temperature changes caused by compression or decompression are called adiabatic. Air cools adiabatically as it rises and expands. The rate of cooling with height is called the adiabatic lapse rate. For dry air this rate is 30 C / 1000 ft. For wet, saturated air this is halved because condensation occurs and latent heat is released as a result.

"Stability": The extent to which air currents can rise depends upon the stability of the atmosphere. Stability is a measure of resistance to change. In the atmosphere, stability is a condition where displaced air at any level is forced to return to it's original level. This occurs when there is no change in temperature with increasing height in the lowest layer of the atmosphere, known as an isothermal lapse rate. Stable conditions occur in the atmosphere when the actual lapse rate is less than the adiabatic lapse rate and often accompanied by haze layers such as strato cumulus.

"Instability": When the actual lapse rate is greater than the adiabatic lapse rate atmospheric instability occurs, generating heat clouds, expanding cumulus and thunderstorms. Violent vertical movements are associated with unstable conditions. The film shows the behaviour of small balloons released under stable and unstable conditions of the atmosphere, the narrative incorporates numerical data to clarify the scientific principles involved. Knowledge of the temperature structure of the atmosphere obtained from the meteorological office will enable aircrew to select a suitable flying level with regard to clouds and bumpiness.

35mm