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3 Answers

Reason for why a clear sky is needed for radiation fog

Asked by: 858 views Weather

I understand the reason for calm winds and high humidity for the formation of radiation fog, but how does a clear sky play in its formation?

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3 Answers

  1. Best Answer


    Mark Kolber on Jul 27, 2016

    Non-technical answer to guide your inquiry. And I bet you already know this if you think about it. Part of it is just definitional.

    The cooling that takes place at night is referred to a “radiative cooling.” It’s actually akin to the same process that makes you cold on a cool day – your body heat escapes and can’t keep you warm.

    Continuing the analogy, cloud cover provides a “blanket” that helps keeps the ground level warmth generated during the day from escaping as quickly. So radiation cooling is slowed considerably and the temperature is prevented from cooling to the dew point.

    When the cloud cover isn’t there – that cool, clear night – the ground heat escapes, the ground temperature cools to the dew point and and radiation fog is produced.

    That doesn’t mean fog won’t form on a cloudy day in the daytime, but it wouldn’t be called “radiation” fog.

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  2. Drew on Jul 27, 2016

    Thank you, Mark.

    I thought that the formation of clouds would actually aid in the formation of radiation fog, as it would increase the cooling rate of the ground by blocking the sunlight. My eyes have been opened.

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  3. Mark Kolber on Jul 28, 2016

    Drew, if you read the description of radiation fog in AC 00-6A, “Aviation Weather” you will see that radiation fog

    “…forms almost exclusively at night or near daybreak.”

    Keep in mind that when the aviation weather handbooks talk about the different types of fog, they are usually talking about how fog is formed. Once formed, however, I would expect dissipation to be delayed on a cloudy day for exactly the reason you say – the sum might bot be strong enough to “burn it off.”

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