- Title: METEOROLOGY - FOG [Main Title]
- Film Number: AMY 215
- Other titles:
- Summary: A training film focusing on the physical properties of the atmospheric and ground level environments required for the formation of fog. Because fog is one of the most dangerous factors when flying an aircraft, aircrew should have a basic appreciation of the laws of physics that govern the formation of fog. Much of the film is devoted to animated diagrams and a lucid narrative.
- Description: Reel one: The essential requirements for the formation of fog are cooling, moisture and turbulence. These factors require explanation before considering the actual formation of fog. Fog is one of the most dangerous factors when flying an aircraft; fog is cloud at ground level and the inherent dangers are obvious. Air temperature falls steadily with increase in height, at a variable rate known as the Lapse rate. The variation is due to the movement of air masses over Europe. Warm air expands as it rises, the atmospheric pressure decreases, heat is lost, and will continue to rise until it’s temperature is the same as the surrounding air. “Inversion”: A temperature inversion occurs when the normal rate of affairs is inverted, the temperature increases with height. This happens mainly at night as ground temperatures fall whilst the air above remains at its daytime value. “Moisture”: The atmosphere comprises 80% nitrogen and 20% oxygen plus variable amounts of water vapour called humidity. Cold air at high latitudes contains little water vapour, warm air at low latitudes contains more vapour. At saturation point the hot air will contain more water than the cold air. The temperature at which air is completely saturated is called the dew point. “Cooling”: Cooling of air below the dew point will lead to condensation. The dew point is a critical temperature, the greater the humidity the higher the dew point. Reel two: The state of the air in terms of humidity is measured with a hygrometer. This instrument comprises two identical thermometers, side by side with one bulb covered in a water saturated muslin bag, known as the wet and dry bulbs. The dew point is when the humidity is 100% at any given temperature, then the wet and dry thermometers will be identical. Hot air masses cause the temperature to rise, water to evaporate and lower the wet bulb reading. The greater the difference between the wet/dry temperatures, the drier the air. The humidity value is obtained by referring the wet/dry bulb readings to hygrometric tables. “Formation of fog”: The Meteorological office has published a series of standardised tables that define fog density in terms of the visibility. Thus, for example, dense fog is defined as 50 yards. These tables are to be used by RAF observers. “Radiation fog”: The commonest conditions under which fog occurs are those associated with anti-cyclones. Near the centre of an anti-cyclone calm, no wind conditions prevail. “Night”: The ground is cooled at night because clear skies allow a large amount of heat to be radiated (from the ground). Radiant heat is not absorbed by the air through which it passes. Air at height is not cooled to the same extent as air in contact with the ground. The amount of cooling is measured by a grass minimum thermometer. As the ground/grass temperature falls, it may pass through the dew point, and under these conditions condensation occurs. If restricted to the air just above the ground, condensation takes the form of dew. On calm nights the cooling effect continues upwards to form a significant layer, and can be thick enough to form a fog. A cooling surface, moisture and sufficient air movement are the requirements for fog. “Turbulence”: Turbulence, the movement of air, is one of the three essential factors for the formation of fog. Radiation fog is common in valleys. Turbulence carries the cooling effect of the ground upwards, the cooled air becomes more dense and falls, the cycle is repeated until the air in the valley is cooled below its dew point and fog develops. Radiation can occur from the top layer of fog. The cooling caused by this additional radiation may cause the fog to thicken and deepen. Inversion restricts the air movement to a shallow layer near the ground. “After sunrise”: Radiation from the sun penetrates the fog but may not be sufficient to destroy the inversion under which the fog persists. Fog will evaporate when the temperature exceeds that of the dew point. Air turbulence (wind) will disperse the fog more rapidly than the sun alone. Reel three: “Smoke fog”: Smoke fog is created under similar conditions to that just described, and is prevalent in industrial areas. An essential feature of smoke fog is an inversion, frequently marked by a layer of cloud (typically 1,000 to 2,000 ft.). As the smoke reaches the inversion it is prevented from rising higher and spreads laterally forming a cloud of minute solid particles, which will thicken, and progressively reduce the ambient light levels. “Sea fog”: Sea fog, like land fog, is produced by condensation at a cold surface. Moist warm air arriving from the low latitudes flows over a colder sea surface. The lower levels of the warm air are cooled below their dew point and condensation – or sea fog thus occur. “Hill fog”: Hill fog occurs when warm, saturated air approaching an elevated land surface (e.g. a hill) is forced by the wind to rise over it. Being forced to rise the air mass will expand and cool to a temperature below it’s dew point, and condensation occurs, forming a cloud. If this takes place at a level lower than the hill tops the clouds thus formed are hill fog. The film shows examples of types of cloud types forming over hills as the wind blows them along. The film ends with pictorial summary of the essential requirements for the formation of fog.
- Access Conditions:
- Featured Period: 1919-1939
- Production Date: 1939
- Production Country: GB
- Production Details: ROYAL AIR FORCE (Production sponsor) Science Films (Production company) Goodliffe, Frank A (Production individual)
- Personalities, Units and Organisations:
- Physical Characteristics: Colour format: B&W Sound format: Sound
- Technical Details: Format: 35mm Number of items/reels/tapes: 3 Length: 2081ft
- Link to IWM Collections page:
Related IWM Collections Objects:
AMY 244 (METEOROLOGY - ATMOSPHERIC PRESSURE AND WIND [Main Title])