Radiation inversions occur at night when the ground and air lose more heat through infrared radiation than they receive from the sun. This causes the air closest to the ground to cool more than the air above, forming an inversion layer. Ideal conditions for strong radiation inversions are calm winds, long nights, dry air, and clear skies. They commonly result in below-freezing temperatures near the ground and temperatures 5°C warmer just a few meters above. The coldest temperatures are usually observed at sunrise as cold air settles into low-lying areas overnight.

1. NIGHTTIME COOLING
• As the sun lowers, its energy is spread over a larger area
• In late afternoon or early evening, the earth’s surface and air above begin
to lose more energy than they receive
• The ground and air above cool by radiating infrared energy, a process called
radiational cooling
• The land is much better radiator than air
• Surface air transfers some energy to the ground by conduction
• The measured increase in air temperature just above the ground which is
formed mainly through radiational cooling of the surface is known as a
radiation inversion
• As radiation inversions occur on most clear, calm nights, they are also
called nocturnal inversions

3. Radiation Inversions
• A strong radiation inversion occurs when the air near the ground is
much colder than the air higher up
• Ideal conditions for a strong inversion;
• calm air
• long night
• fairly dry air and cloud-free sky
• Windless night
• Long night
• Clear sky and dry air

5. • On winter nights in middle latitudes, it is common to experience
below-freezing temperatures near the ground and air 5°C warmer at
your waist
• Top of the inversion is usually not more than 100 m above the
ground
• A surface that is wet or covered with vegetation can add water vapor
to the air
• Lowest temperature on any given day is usually observed around
sunrise
• Cold, heavy surface air slowly drains downhill during the night and
eventually settles in low-lying basins and valleys