Re: is it true that hair on your head keeps you warm

A layer of hair on any part of the skin will keep you warm.  It does this 
in the same way as clothing or a duvet, by trapping a layer of air.  The 
heat escaping from your body warms this trapped layer of air which then 
stays put.  If it drifted away, your body heat would have to keep on 
warming more and more fresh layers of air, so cooling your body down.

Because it is important for mammals to be able to regulate their body 
temperature, there is a mechanism to make the layer of hair more or less 
efficient at trapping air.  It is called piloerection - which means that 
the hairs are able to stand up or lie down to some extent.  There are tiny 
muscles attached to each hair.  When it is cold, the muscles contract, 
making the hair stand up.  This increases the depth of the layer of hair, 
and forms a more efficient insulating layer.  Even though our own body hair 
has very little warming effect, piloerection still happens if we are 
chilled, causing 'goose bumps'.

Piloerection is also connected to mood or emotion.  You may have seen how a 
cat or dog can make some of their hair stand on end when they are 
frightened.  Likewise, we speak of creepy things making 'the hairs on the 
back of my neck standing on end'.

The hair on your head does keep your head warm.  It cannot cool you down, 
but can vary the amount of warming through piloerection.