Atmospheric Cooling
Nocturnal Radiation Cooling
The troposphere is heated from the earth’s surface by terrestrial radiation and conduction. As a result, the temperature normally decreases with altitude up to the tropopause. After sunset, the earth’s surface continues to radiate heat and its temperature begins to drop. Air however, unlike the earth’s surface is not a good conductor of heat, therefore the atmosphere does not cool appreciably by radiation after sunset. Only the layer of air near the earth’s surface cools due to conduction. The radiation cooling that occurs at night will rarely affect more than the first 5,000 feet of the atmosphere. Except for deep arctic nights, the temperature above this layer remains virtually unchanged.
Nocturnal Radiation Cooling
A temperature increase with altitude is called an inversion. If the inversion was caused by nighttime cooling it is defined as a Nocturnal Inversion. The top of an inversion is clearly marked by a sharp layer of haze and dissipates as the sun rises.
Wind Effect
Windy nights and turbulence mix the lower few thousand feet of the atmosphere and distributes the cooler air aloft. Therefore, the temperature of the surface layer does not drop as much and the inversion is much weaker.
Cloud Effect
Clouds, low-level clouds in particular, absorb terrestrial radiation and re-radiate it back towards earth. For this reason, cloudy nights are generally warmer than clear nights.
Topographical Effect
Cold air is denser than warm air and therefore it tends to flow into low-lying areas such as valleys. If the air is trapped in these valleys nocturnal inversions can become very strong. Also, if air is forced to rise along the side of a hill or mountain it will expand and as a result cool. The mechanism in which air is forced up sloping terrain is called “Orographic Lift”.
