METEOROLOGY - ATMOSPHERIC PRESSURE AND WIND [Main Title]

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  • Title: METEOROLOGY - ATMOSPHERIC PRESSURE AND WIND [Main Title]
  • Film Number: AMY 244
  • Other titles:
  • Summary: A training film focusing on the variations in atmospheric pressure and the consequences for both climatic conditions and flying operations, the physiological effect on aircrew, and the performance of the essential aircraft instrument, the altimeter.
  • Description: Reel one: Film opens with scenes of aircrew within a decompression chamber, simulating the effect of low pressure encountered at high altitude. As the pressure decreases the activity of the aircrew become increasingly slow and lethargic, indicating the requirement for special precautions at high altitude. Air pressure at the earth’s surface is due to the weight of air above it, and thus the density is greatest at ground level and decreases steadily with height. “The aneroid barometer”: Air pressure exerts a force in every direction. An evacuated, metallic capsule will tend to deform with variation in air pressure but is not easily seen, so a system of levers is arranged to magnify the deformations to move a pointer over a scale, the aneroid barometer, or move a pen over a rotating drum, the aneroid barograph. Both instruments are calibrated in millibars. “The mercury barometer”: This is more accurate than the aneroid barometer. A column of mercury is contained within an evacuated glass tube immersed vertically in a reservoir of mercury open to the atmosphere. The height of the mercury column is balanced by the atmospheric pressure and the resultant height variations, determined by a micrometer scale, is a measurement of the ambient air pressure. A millibar is the unit of pressure adopted in meteorological practice, a mercury column of 30 inches is balanced by an atmospheric pressure of 1016 millibars. Mercury barometers of the type shown are used by meteorological stations for obtaining altimeter settings. “The altimeter”: The decrease in pressure with height is observed on an aneroid barometer as it moves upwards and if calibrated in feet, is called an altimeter. The altimeter of an aircraft on the ground should agree with the height of the ground at mean sea level. It will not do so unless an adjustment is made to the sub-scale of the altimeter. The setting for the sub-scale, known as the QNH (barometric pressure reduced to sea level), is provided by the meteorological office, and is set by the pilot prior to take off. “Isobars”: An isobaric surface is one where the pressure is the same everywhere. Lines which join surfaces of equal pressure are called isobars. A series of Isobar charts display the character of low and high areas of pressure, a col, a ridge, secondary depression, cold and warm fronts. The climatic conditions associated with these features is explained. “Altimeter correction for pressure variation”: Knowledge by the pilot of the pressure distribution along the flight path is essential so that the altimeter may be reset as the flight progresses. The pilot sets the altimeter to the station of departure height and flies at (say) 3,000 ft. The flight progresses through a depression (an area of increasingly low pressure) and follows a downward slope (defined by an isobar) to arrive at the destination station where the true height will be less than the indicated height. The danger is obvious, particularly in thick cloud, so the pilot must set his altimeter to the QNH setting of the destination station. The pilot must take care he is fully aware of horizontal variations of pressure as his flight progresses, monitoring the altimeter and applying corrections as necessary. Reel two: “Wind”: Camera cuts to an isobaric map depicting an extensive depression. The distribution of the pressure changes are represented on the map by the arrangement of the isobars. The rate of pressure change largely dictates the wind speed, from light wind to gales, blowing almost parallel to the isobars, not at 90 degrees as might be expected, because of other forces that come into action when the air starts moving. “The Geostrophic force”: Moving air is subject to a force due to the rotation of the earth. This force acts on air moving at right angles to it’s path and is known as the geostrophic effect. In the northern hemisphere this deflection is such that the air moves in a counter clockwise direction around a depression. “The cyclostrophic force”: Air moving on a curved path is subject to centrifugal force which also acts at right angles to the path of the air. This is known as the cyclostrophic effect. The combined effect of these forces is for air to flow counter clockwise in the northern hemisphere. The same forces cause winds to flow clockwise around an area of high pressure. “The frictional force”: The flow of the wind is retarded by friction at the surface of the ground, and the effect is to disturb the balance of forces. Within a depression surface winds blow inwards at about 300 to an isobar. Depressions are a familiar feature at temperate latitudes, over the equator the pressure distribution is indefinite and in the southern hemisphere the relationship between wind and pressure gradient is reversed. “Change in pressure and wind”: Continuous changes in pressure are recorded on a barograph, in principal the same as an aneroid barometer, but incorporates multiple capsules mounted in series to magnify the movement. Film shows a large synoptic map incorporating isobaric lines, cold and hot fronts, and a variety of annotations representing climatic observations from weather stations. A deep depression over the north Atlantic is approaching England. The meteorological officer is seen with an aircrew discussing the features represented on the map, and the climatic implications that may affect the choice of route for their flight plan, and in particular to avoid unexpected changes in the wind and atmospheric pressure.
  • Alternative Title:
  • Colour: B&W
  • Digitised: Yes
  • Object_Number: AMY 244
  • Sound: Sound
  • Access Conditions: Attribution: © IWM
  • Featured Period: 1946-1975
  • Production Date: 1939
  • Production Country: GB
  • Production Details: ROYAL AIR FORCE (Production sponsor) Science Films (Production company)
  • Personalities, Units and Organisations:
  • Keywords:
  • Physical Characteristics: Colour format: B&W Sound format: Sound Soundtrack language: English Title language: English
  • Technical Details: Format: 35mm Number of items/reels/tapes: 2 Length: 1418ft
  • HD Media:Yes
  • Link to IWM Collections page:

    AMY 244

  • Related IWM Collections Objects:

    AMY 243 (METEOROLOGY - ATMOSPHERIC TEMPERATURE [Main Title]) AMY 215 (METEOROLOGY - FOG [Main Title])
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