Well, we have now moved into our new house. We moved last Friday, mostly dodging the heavy showers that have been marauding around the country for the last week. We are slowly unpacking – the place is looking a lot tidier now than it did at the weekend, but it will take a while to find a home for every object we own. Mizuna the cat has taken a little bit of convincing about the move – but he has now emerged from under a bed and seems more at home and is very happy that our ex-neighbour’s cat is no longer is around to bother him.
It’s a two-storey house, so slightly unusual for New Zealand, but one part of it is just one-storey with a very high ceiling. The layout makes for an excellent demonstration of the fact that hot air rises. (N.B. It’s hot air that rises, not heat, as is often mistakenly said.) With the log fire up and going on the ground floor, the area on the first floor above it gets really nice and toasty, whereas the ground floor itself is not so warm. The warm air has risen, has been trapped by the ceiling, and so the higher up you are the warmer you are going to be, pretty much.
So, here’s a conundrum. If hot air rises, why is it colder at the top of a mountain than the bottom? It’s certainly warmer at the top of our house than the bottom. What’s the difference?
It’s because there’s another very effect going on. As you go upwards, atmospheric pressure reduces, and so air expands. Expanding air does work (e.g. compressed air or steam expanding to drive a piston to do useful work) and so must lose heat energy. And that means it cools down. A quick bit of thermodynamics allows you to estimate how quickly temperature must drop with height. Ignoring the effect of moisture, it’s about 1 degree C for every 100 metres climed. This is known as the lapse rate. Consequently you can expect the top of Mt Te Aroha (about 950 m altitude from memory) to be around 9 or 10 degrees colder than Te Aroha township, to give a local example.
That’s if the air is dry. It gets more complicated if there’s moisture involved, because of latent heat. When water vapour condenses to form water droplets (e.g. a cloud) it gives off heat. That prevents the temperature from dropping as quickly with height. So, in damp conditions (i.e. this week) the lapse rate can be rather lower than this. Not that I have any intention of climbing Mt Te Aroha in this weather to demonstrate it.