Thermals

It's not often that we can actually see atmospheric events, we can see the effect of them of course, like fallen trees in a storm etc. but we can't see the air moving at the time. Here is one example of where you can see air moving indirectly. I was out on my daily walk yesterday and noticed a flock of birds (mostly gulls) circling a lone Red Kite (harassing it) however the Kite was sitting in what's known as a "thermal" which is a column of rising air. You can tell the bird was in a thermal because it was rising up into the sky without flapping it's wings, it was effectively sitting in a rising air elevator along with it's tormentors. You can see the Kite in the middle of the cluster of birds and all the Gulls surrounding it.

Now, the air in a thermal is not only rising, it's also drifting according to the prevailing wind, i.e. it has horizontal movement as well as vertical movement, for the thermal in the picture I've shown this with the white arrows as well as suggested the extent or size of the thermal with the vertical (parallel) lines. Thermals will always have a source, usually a heat source that causes a big blob of air to have a temperature slightly higher than the surrounding air, hence it becomes less dense and rises. There are a number of possibilities for sources. In this particular case I reckon the dark coloured, smooth steel roof seen on the building in the bottom left of the picture is the source of this thermal, being dark it absorbs whatever infrared radiation is hitting it much better than the surrounding lighter surfaces, thus heating it up slightly above ambient temperature. Then, through convection the air above it is also heated thus causing the thermal to rise at a suitable density tipping point.

Back in the day when I used to do paragliding we watched out for birds doing this same thing as a fairly bullet-proof way of finding thermals that we could then ride for free (sometimes for many miles) - a little knowledge of physics can often lead to some really cool activities!