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The comment on my previous entry raises a few  issues with the way we feel heat.  (NB for those who normally read this blog on http://www.sciblogs.co.nz , you’ll need to go onto physicsstop to see the comment – http://sci.waikato.ac.nz/physicsstop ) 

How hot we feel has more to do than just what the temperature is.  Anyone who has stood outside in a gale will know that it feels much colder than what the thermometer reads. That’s the windchill.  The temperature is the same, but the rate at which heat leaves your body is much higher when the airflow past you is greater. That’s because in still air, your body heats up the air around your skin, so unsurprisingly it feels warmer to you (because the air next to your skin really is warmer). But in a strong wind, that warm air is just blown straight away.

Humidity plays a key role too. Water requires energy to evaporate, and it takes that energy from what it is in contact with. So when sweat evaporates, it cools the skin. It evaporates more readily in low humidity conditions, so here it takes energy from you at a greater rate than in high humidity. Thus a dry heat might feel more tolerable than a wet heat.

Evaporation is what makes you feel cold the second you step out of a swimming pool, especially in nice sunny weather. All that water on you starts evaporating, and it sucks heat out of your body. So there’s a vicious irony here – when you jump in the swimming pool to start your swim, it feels cold (water is better at carrying heat away from you than air), and when you get out of the pool at the end of your swim, it also feels cold.  Why can’t it feel warmer both ways?