Science Questions

Is it really always coldest just before dawn?

Mon, 3rd Oct 2016

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Question

Elizabeth Louw asked:

Hi Chris, We're on a roadtrip to the coast and the sun is just coming up. We remembered the saying that it's always coldest just before the dawn (darkest as well but that makes sense). Is it true that it "suddenly" gets colder just before dawn? Love the show,

Elizabeth Louw

 

 

Answer

We put Elizabeth's question to Chris Smith...

Chris - Well, what do we think. Does everyone here agree that

Sunsetthe temperature appears to dip before, or around the time of dawn? Max...

Max - Yes, seems to be. Iím not sure it dips particularly but it feels like it slowly gets colder over the course of the night and then as a result itís coldest just before dawn.

Chris - So Max is disagreeing. Heís saying itís colder as the Sun comes.

Do you agree Caroline?

Caroline - I think Iíd agree with Max in that it seems that once the Sun sets the temperature seems to decrease consistently and then the Sun rises, would be my guess.

Chris - JudithÖ

Judith - I donít think thereís a particular dip just before the dawn.

Chris - There is a saying ďthe dawn dipĒ and itís based on reasonable principles in physics. Because the argument goes that, as you speculate Max, where does the energy on the Earth come from and the temperature it comes from the Sunís radiation warming the Earth.  When has the earth been not having any heat coming in for the longest? By definition that must be just before dawn because thatís when itís been dark for longest, therefore itís going to be coolest then. But there is the observation, certainly under certain circumstances, it can get colder just ahead of the Sun coming up and as the Sun rises.

Now why should this be? Well the argument put forward is that although you are still in darkness, thereís a patch of the Earthís surface quite close to you thatís now being illuminated and irradiated. So temperatures there are going to start changing reasonably fast and because the Sunís heat is warmin

g the Earthís surface this is, in turn, warming the air above the Earthís surface this is, in turn, creating a pressure difference around the Earthís surface, that pressure difference is going to drive winds. And this is going to draw

colder air in to replace the warm rising air which is going to include pulling

air in from where you are, so somewhere colder could pull itís air in over you and make you even colder than you were to start with.

So the argument goes that it does get a little bit colder, paradoxically, just as the Sun rises because you get these air movements which include movements of cooler air, which will chill you out a little bit.

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