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

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

• 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