Temperature Inversion

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   CFIsince1990 on Jul 19, 2016

   A temperature inversion is when the air at a higher altitude is warmer than the air at a lower altitude (temperature increased with height above the ground). This is opposite (or inverted) of what is “normal”. Normally, in a standard model atmosphere, temperature decreases with height at a standard lapse rate of 2 degrees Celsius per 1000 feet (or 3 1/2 per 1000′). This lapse rate is due to the fact that the earth’s surface absorbs the sun’s energy and heats up. The air that is in contact with the ground then heats by conduction (much like holding a piece of metal over a flame when the heat conducts up through the metal to your hand), causing the air very near the surface to heat up. However, air is a poor conductor of heat, so this layer is very shallow. As different surfaces of the earth heat up at different rates, that air close to the surface also heats up at different temperatures, creating uneven heating of the air and different densities of air at the surface.

   For instance, the air over a hot spot such as a large parking lot with new black asphalt, creates warm, less dense air very near the surface as opposed to a grassy area or body of water (which would be cooler, thus cooler, more dense air just above the grassy area or water). This uneven heating creates thermals as the cooler, more dense, air moves into the area of warmer, less dense air (as it attempts to equal things out) and buoys or forces the warmer, less dense air upward. As that warm air rises, it expands and cools, That, coupled with the fact that as you increase your height, you are getting further away from the heat source (the earth’s surface). This is analogous to being cooler as you get further from the fireplace. Given all this, that is why the air is normally cooler as you increase your height.

   A temperature inversion, however, means the air is warmer as you go higher, as if the atmosphere is inverted from its normal lapse rate. This can be caused by cold, clear nights with little wind. On these nights, the earth radiates it heat well away into the atmosphere, if there are no clouds to help hold that radiation in and send it back down to the surface. This causes a dramatic cooling of the surface, which in turn, cools the air in contact with the surface, causing a shallow cold layer, with a warm layer above (thus a temperature inversion). If sufficient moisture is in the air and the air can be cooled down to its dewpoint, radiation fog can form, Calm winds help create this inversion, because if the wind is more than a few knots, the wind would help to mix the warm air and cold air (and help keep the radiation fog from forming).

   Sometimes air that is sinking, such as under a high pressure system, will compress and warm as it sinks, causing warmer air aloft. Other causes could be ahead of a warm front, when warm air meets cold air and rides up over the cold air in a gentle slope.

   Inversions are stable air, with no turbulence as the cold, heavier air below does not want to rise and the warm, lighter air above does not tend to sink. However, around the boundary/transition zone, their could be some turbulence as the air may be flowing in different directions and speeds.

   Sorry for the length ….

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