Wednesday, July 12, 2017

Air Currents at work

Cirrus clouds are very high clouds made up of ice crystals. And when they form to look like horses’ tails, their frozen droplets are being driven by the jet stream (high, strong currents of air that have an effect on the weather beneath). So this is moving air that you are actually able to see, because of the ice crystals. Looking at the passage of clouds tells you much the same as they are moved by wind.
When leaving Granada, in Spain, by air, we had to skirt around the lower parts of forming cumulonimbus clouds. These particular clouds are made by violent up-currents of air, blowing moisture high up (generally from a wet surface below, like a lake) with such power that cloud moisture forms into raindrops. Heavy rainstorms may result.
As these raindrops rise in the up-currents they may freeze in the cold of altitude and form into hailstones. These fall under their own weight, either as small hailstones or get sent aloft again by the up-currents of air to gather more moisture and freeze once more, making larger hailstones – and so on.
In learning meteorology as a trainee pilot in the war, the instructor told us that these cumulonimbus clouds were most interesting clouds. They must be, he said, as airmen had flown into them and never come out again.
Ice forming on aeroplanes’ wings can be deadly, but is now dealt with efficiently by the heated leading edges of aeroplanes’ wings. But it was splashed water from the ground that once froze to ice under the wing of my aeroplane that caused it (and me) to crash – rather violently.
So the pilot from Granada was wise as he flew around those dangerous clouds when leaving southern Spain. And the view of them from the aeroplane’s windows was magnificent.
We are unable to see the upward-rising air currents in these clouds, but we can see the outside movement at work as they billow aloft. This was such an occasion.
Even on a very still day of no wind or breeze you may see a leaf on the ground or in a bush suddenly move, telling that there are nearly always air currents around, sometimes minute ones, even on the stillest of days.
One of the best ways to “see” air currents is when they are combined with smoke.
Aircraft land against the wind. This slows down the landing speed, making it safer.

When flying training years ago, you always wanted to know wind direction when in the air – just in case of engine failure and a forced landing necessary. To look at the direction of chimney smoke or bonfire smoke was always a good indication of it. But nowadays there is not much smoke to be seen and, thankfully, aviation in all its aspects has advanced enormously.