Hot air rises

I was reminded while driving in to work this morning that we’re getting into hot air ballooning season in Hamilton – with a balloon hanging nicely over the road to district drivers like myself.  Flight with hot air balloons isn’t exactly rocket science – quite simply hot air is less dense than cold air, so hot air rises. Encase it in a balloon, and up goes the balloon.

Those of you who live in a house with more than one storey might have noticed during the recent high temperatures that the upper storeys are rather warmer than the lower ones – same effect going on. But, if that’s the case, have you thought about why it is that mountains are colder at the top. If hot air rises, shouldn’t it be warm at the top of Mt Cook?

The explanation is that there is another process going on. As we go upwards, the air pressure decreases (since there is less air above us ‘squashing’ us). This means that, as a lump of hot air rises, it expands.  A consequence of the laws of thermodynamics is that something expanding must cool down. (Try it – if you use an aerosol can feel the temperature of the nozzle afterwards – it should be cold. The converse happens with a bicycle pump – feel the nozzle after use – it will be hot, because you’ve been compressing the air).  So our rising hot air doesn’t stay hot. By the time its reached the top of a mountain, it is very cold.

If you’ve seen photos of super-altitude helium balloons you’ll know that they look a bit funny when on the ground – they seem to have a huge balloon that is barely inflated. But all that balloon is needed – as they rise the gas inside will expand, and at high altitude they will look much more balloon-like.

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