The picture was taken a while ago; it’s Butser Hill. It’s a lovely view, but for today it’s the clouds. I was sat in the chair at the bottom of the garden, and my head was back, just looking at the sky.
What had started off as a calm day was turning into a breezy one. The anemometer on my weather station was quite happily whizzing around, and the weather vane couldn’t make up its mind which direction the wind was coming from, except that it was mostly westerly.
The thoughts I had about the butterflies in the garden, I’ll leave to the next Letter to Steve; it was the clouds that caught my attention. I have a chart on the wall of my office; it shows me the different cloud types. I’m fairly sure what I was looking at were altocumulous, probably 6 – 7km high. They were barely moving at all.
I knew that was deceptive; they were definitely moving, but you really had to sit and watch them to detect it. I realised afterwards that I should have filmed them. My assistant can estimate cloud speed from a short video. Based only on my description, she reckoned they were probably travelling at something like 20–30 km/h. It’s remarkable that something moving at the speed of a car can appear almost stationary simply because it’s several kilometres above your head.
Which, of course, led us into a completely different conversation about how high airliners fly, how far away their horizon is, and how differently we perceive movement at those distances. One observation in the garden had opened the door to a whole chain of questions. That’s one of the things I enjoy most about simply sitting and watching.
Bills Posts 