Weather Matters With Matthews (11/21/21): What is a nocturnal inversion, and what makes it useful to forecasters?

Weather Wisdom

Hello and welcome to another episode of “Weather Matters With Matthews.” Today I will talk about a weather feature that typically happens in the morning and when the temperature just above the surface increases with height rather than decreases. This feature is called a nocturnal inversion, or a radiation inversion.

On a clear and calm night, a nocturnal inversion forms when the ground cools after the sun sets. This happens because the ground emits more solar radiation than it absorbs. In other words, the temperature of the ground lowers because it lets loose radiation it received from the sun during the day. This emitted radiation keeps the air above the surface warm. Therefore, the temperature increases for a little while as you go up in altitude.

As forecasters, a nocturnal inversion can help us indicate if fog or frost will form at or near the surface. The inversion acts like a lid, which traps rising pockets of air near the surface. When these pockets of air drop down to a temperature below the dew point temperature on a clear and calm night, condensation occurs, and fog will form in between the surface and the top of the inversion. Frost can also be determined from a nocturnal inversion, much like the frost seen this past weekend. The surface temperature can be below freezing even if the temperature on your weather app says it is above freezing. Therefore, frost can likely form despite the air temperature recorded.
To recap, a nocturnal inversion forms when the ground cools overnight. The ground emits radiation which keeps the air close to the ground warmer than the surface. The presence of a nocturnal inversion can help forecasters determine if fog or frost will form overnight.

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