weather expert/scientist needed, please

roseangel said:

My fiance, who is from Holland, has this bit of weather advice he likes to give me:
If you are want to know if it will freeze tonight, check the skies; if you have clear skies it'll get cold, if there are clouds it wont get cold.

I want one of my characters to use that bit of advice, but wanted to know if it has any scientific basis to it.
It doesn't sound right to me, but I am unsure.
Anyone have an answer for me?

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Not that I'm an expert or anything, but clouds act like insulation and keep the residual warmth from the day. If the skies are clear, the warm air dissipates.
It is absolutely true that clear nights in winter are more likely to bring frost, if the temperature is low enough.

Your fiance is absolutely right. As Firedrake said, the clouds provide insulation and hold in the heat from the earth.

Another one I've seen proved right almost every time is that it's always colder when there's a full moon. Where I am in the US, we can almost be guaranteed our killing frost will happen on one of the autumn full moon nights. Puma

The term for the earth dissipating heat is called "radiational cooling" I believe, if you want to look it up. As others have said, the clouds form an insulating layer that doesn't allow this re-radiated heat to escape back into space.

Aerial

It's true. The temperature drops faster without clouds. But it is not a certain way to tell whether it will freeze or not. Clouds are only one of many factors.

We notice it significantly here on the West Coast of Canada because clouds usually make the difference between freezing and above freezing temperatures. It rarely snows here because of the phenomenon (rarely as in once or twice a year).

I believe this is also the reason why deserts get so cold at night and hot during the day, there is no humidity to hold in the heat or the cold.

Drachen Jager said:

We notice it significantly here on the West Coast of Canada because clouds usually make the difference between freezing and above freezing temperatures. It rarely snows here because of the phenomenon (rarely as in once or twice a year).

I believe this is also the reason why deserts get so cold at night and hot during the day, there is no humidity to hold in the heat or the cold.

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I lived in Arizona for 8 years. In the winter, the temperature can reach mid the mid 60s F, during the day and drop down to below freezing at night. A difference of 30 to 40 degrees in one 24 hour period.

I agree with the above, and also:
The approach of most storms is signalled by clouds.
Most storms have a warm front and a cold front
Warm front arrives first, is signaled by gradually descending cloud layers.

During winter here in Northern California we look forward to storms as it usually warms up ten degrees or so.

Being a bit pedantic, clouds don't insulate the earth but they irradiate the earth's surface with absorbed heat. Because it is the surface that absorbs the radiated heat, then a ground frost is less likely if it is cloudy.

Another thing to consider is that clear skies are often associated with low wind speeds, which means that advective heat transfer is less effective (the air is less likely to warm a cold surface).

Deserts can be particularly cold because of the low heat capacity of the (dry) surface. Similarly, the surface can get very hot during the day.

Cloud cover is a big factor in surface temp - it can prevent it getting too cold, or too hot.

Also, the temp in built up areas and areas surrounded by hills tends to be slightly higher than the temp in an open, exposed place.

shaldna said:

...Also, the temp in built up areas and areas surrounded by hills tends to be slightly higher than the temp in an open, exposed place.

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The first is definitely true and is called the 'urban heat island' effect. Things are more complex for places surrounded by hills though. Frost pockets tend to occur when cold dense air accumulates at the bottom of dips or valleys. So in that sense, places surrounded by hills can be much colder. The wind flow in hilly areas is quite complicated and I think the answer to the second part is 'it all depends'.

Drachen Jager said:

We notice it significantly here on the West Coast of Canada because clouds usually make the difference between freezing and above freezing temperatures. It rarely snows here because of the phenomenon (rarely as in once or twice a year).

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Uhm, probably not. It mostly has to do with this really big, nearly constant temperature thingy called the Ocean that has the biggest effect on your weather patterns. Cloud cover really plays such a small roll in the west coast getting snow that it's probably meaningless (other than to get precipitation, you need clouds).

Plus, it snows a LOT with above freezing surface temperature. It'll even stick sometimes if it's coming down hard enough.