On warm days where temperatures reach well into the 70s during the day, how are temperatures able to drop so quickly into the 40s at times overnight? It happens from a combination of different factors, and occurs because of a process called radiational cooling.

Radiational cooling is a process that occurs on calm, clear nights that causes temperatures to drop very rapidly from warm to cold in a short period of time. 

 In short, this occurs when more longwave radiation is able to escape from the earth’s surface, without enough shortwave radiation coming in to balance out the deficit. 

First, let’s break down how energy is transferred between the sun and the earth:

Photo courtesy: britannica.com

There are 2 main types of energy transferred between them:

  • Longwave radiation reflected back down to the surface and reabsorbed
  • Shortwave radiation directly absorbed by the earth

The sun emits this shortwave radiation in the form of UV rays and visible light, which is then absorbed by the earth’s surface. Since the earth is significantly cooler than the sun, some of this radiation is reflected back from the surface in the form of longwave radiation, or infrared rays. 

How does radiational cooling occur?

There are 3 key factors that help this process:

  • High pressure

This creates sinking air (clear skies) and calm weather conditions.

  • Clear Skies

Clear skies allow heat to escape the earth’s surface more rapidly, while clouds help to trap the heat that’s absorbed and reflected off the earth’s surface.

  • Calm Winds

This prevents the mixing of warmer air aloft down to the surface.

In situations where the surface is rapidly cooling and winds are calm at the surface, this will create a layer above this rapidly cooled air that’s much warmer called the inversion layer. This is a layer in the atmosphere where temperatures suddenly increase with height instead of decrease. When winds are present and are allowed to mix down to the surface, it allows the warmer air above to mix throughout the different layers of the atmosphere which reduces the cooling effect it has on the air. This is why calm winds are crucial to keeping things cool at the surface.

See the explanation graphic below:

Heat isn’t something that just disappears, but is something that is transferred from object to object. When the heat is lost from the ground at night, it has to escape somewhere. Without clouds overhead at night, this heat is free to escape from the surface which allows temperatures to drop significantly.

Photo courtesy: climatekids.nasa.gov

Alternatively, when clouds are overhead at night, they act more like a blanket to keep the warmth in. So while this is true of clouds present during the night, it acts the opposite during the day. Have you ever noticed that clouds keep things cooler during the daytime?

Why do clouds keep us cooler during the day, but warmer during the night?

Yes, clouds can both help keep the heat in and keep things cool. It all depends on the time of day and the type of clouds in place. 

On a cloudy day, clouds act to block the light being emitted from the sun, so not as much heat can be absorbed by the surface. Clouds will act similarly at night, except instead of trapping heat from getting to the earth’s surface, they trap in the heat that’s already been absorbed by the earth. 

Always think of the clouds as a giant blanket wrapped around the earth!

Photo courtesy: climatekids.nasa.gov

The thickness of the clouds have an effect on heating as well. Thick, overcast skies will act to keep things cool during the day and warmer during the night, but having thin, wispy clouds present on a mostly sunny day will actually help warm the earth more than cool.

So, while the absence of clouds during the night lets heat escape the surface, when the sun is shining during the day the absence of clouds helps more heat to be absorbed by the surface. 

That might have been a mouthful, so take this as a rule of thumb:

The best time of the year for this to occur is autumn, or fall since this is the time of year where we get more intrusions of cooler air from the dipping jet stream. This can also occur during the winter, and sometimes even more so when snow is covering the ground since it’s able to lose heat much faster than the ground.