Usually, mist and low cloud are the things which annoy us most. For meteorologists, there is nothing more frustrating than forecasting blue skies and sunshine but only to be greeted in the morning with low cloud. That said, we know that it will burn off and at a fast rate at this time of the year too. But the process behind it is pretty interesting.
Over the last few days, you will have noticed that much of Central and Eastern England has seen the repeating process of low cloud developing overnight and then during the morning this low cloud quickly burning back to the coastal fringes. It's a bit of an unsung hero effect in the weather and the weather people on the telly usually tell us that the cloud will "Burn off back to the coasts" and we take that as read, but the physics behind this is far more exciting than those six words.
The low cloud forms because cooler, moist air comes in from the North Sea throughout the day, but it's only overnight where the land temperatures drop to near the dew point of this air that the cloud and mist form. If there is light winds then there is a risk of mist and fog, but in a breezy wind, the friction near the surface means that low cloud is far more likely due to the very near surface air temperature being a little higher.
The crucial time is 6am to 11am. This is when the sun heats the land at its fastest rate of the day. The sun then gets to work on the top of the cloud and begins warming it slightly, but it's where there are small breaks in the cloud that the sun heats the land quickest. As soon as a break appears then the atmosphere starts to mix quickly due to rising thermals. This then mixes the atmosphere in surrounding areas and it's this mixing which evaporates the low cloud and the process begins to accelerate as more breaks form.
So, next time you see low cloud forming overnight and then breaking up in the morning and then "Burn off back to the coasts." It's more than those six words, it's nature and physics in action all around you.
METEOROLOGIST : MARSH
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