Hello and welcome to more or less on the BBC World Service.

We are your friendly guide to the numbers that explain our lives, and I'm Tim Haafard.

It's been chilly this week, hasn't it?

Dangerously cold temperatures are expected to impact millions of people this weekend across

the Northern United States.

It's going to be bitterly cold in the Northeast, so negative 22 in Caravu, that is rivaling

pretty much anything that I've seen this month.

And that raises an age old question.

Should I wear a hat?

How much heat do you lose from your head anyway?

When I was growing up, I was told it was more than 40 percent,

and the cold weather survival chapter of the US Army field guide says,

Always keep your head covered.

You can lose 40 to 45 percent of body heat from an unprotected head.

But is that view out of date?

Because these days a spot of light googling produces a very different result.

The head accounts for about 7 percent of the body surface area,

and the heat loss is fairly proportional to the amount of skin that's showing.

That's a big difference.

Which views right?

I spoke to a South African academic, Dr. Theopritorius,

who's studied the history of the science of heat loss.

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