blowing hot an'cold

[2nd row]

 WHY IS IT THAT WHEN YOUR HANDS ARE COLD YOU BLOW ON THEM TO WARM THEM UP,  AND YET, IF EATING   A  HOT BAG OF CHIPS, YOU BLOW ON THEM TO COOL THEM DOWN ?  ANSWERS PLEASE.

[ChocClare]

Good question! Maybe because your breath is always at your correct body temperature, and that's the temperature you're subconsciously trying to bring whatever you're blowing on up or down to? I know I find myself blowing on very icy drinks occasionally (though not in public, of course!).

[el duerino]

Because your breath is roughly at body temp 36 degrees (or is it 37 doh). the laws of thermodynamics state that a transfer of energy will occur if two interactive bodies are at different temps - hence cold hands will absorb warm breath, whilst hot foods will be cooled. It will always establish an average temp.

[AlFas]

Previous answers are true, but it's also because when you breathe on something to warm it up you use air from your lungs, and when you want to cool something down you use air from your mouth. The air from your lungs is at body temperature, the air in your mouth is cooler.

[smudge]

Good question & answers, now we know.

[bernardo]

Blowing on a chag of bips would also cause cooling because of the lower pressure (and therefore lower temperature) of the fast-moving air coming from the flow of breath.

[New Forester]

Blowing on hot food does cool it down because your breath is cooler than the food, and you are ventilating it -- you are moving hot air away and replacing it with cooler air.

 

However, a large part of the effect is that it increases evaporation of water in the food, by removing damp air and replacing it with drier.  You can easily demonstrate this.  Lick the back of one hand, then blow on the back of each hand -- which feels cooler?  In that case your breath is probably actually warmer than your hand.

 

Evaporation cools efficiently because the water is changing from a liquid to a gas, and the latent heat of evaporation sucks extra heat from the food.  It's why we sweat, and also how fridges and air-conditioners work.  It's also why scalds from steam are worse than those from the same mass of liquid water at the same temperature -- in that case the latent heat of evaporation is dumped into your skin.  In fact the same thing happens a bit when you blow on cold hands -- you'll notice that your hands get damper.

 

If hot food is dry, or if the water is not exposed to the air, blowing will be much less effective, as evaporation is not involved.  This is why it's so easy to scald yourself when drinking those soups which have a little oil on the top -- they can't lose heat by steaming.  Likewise, a cup of tea will stay hot much longer with a lid on it.

[New Forester]

AlFas - I'm not sure how you'd blow from your mouth without using air from your lungs.

 

Bernardo -- I don't think the Venturi effect lowers the actual pressure, just the effective pressure.  I think the air will remain at around body temp until it mixes with cooler air or hits the food.

[ezapf]

Based on the answer from AIFas, try this: Keep your hands close to your mouth to apply warm air from your body directly to your hands. If you move your hands away from your mouth and blow on them, you are circulating the surrounding air which is cooler.

[what is this]

hi