Is it really always coldest just before dawn?

04 October 2016

Question

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

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

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

Chris - So Max is disagreeing. He's saying it's colder as the Sun comes.

Do you agree Caroline?

Caroline - I think I'd 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 - Judith.

Judith - I don't think there's a particular dip just before the dawn.

Chris - There is a saying "the dawn dip" and it's 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 Sun's 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 that's when it's been dark for longest, therefore it's 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, there's a patch of the Earth's surface quite close to you that's now being illuminated and irradiated. So temperatures there are going to start changing reasonably fast and because the Sun's heat is warmin

g the Earth's surface this is, in turn, warming the air above the Earth's surface this is, in turn, creating a pressure difference around the Earth's 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 it's 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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