Readers of my blog will probably be aware that gliding is one of my biggest loves which means that you develop an interest in clouds. Fluffy cotton wool cumulus clouds for example, typically mark the top of thermals or bubbles of rising air. Birds of prey like falcons and eagles use these to stay aloft for hours and hardly move their wings.
Other ways of staying up without an engine are hill or ridge lift where a wind is forced up over an obstacle. Again birds use these and save themselves a lot of energy.
Directly related to ridge lift is wave lift where after blowing over a ridge, the wind will sometimes bounce and set up standing waves downwind from the ridge, You can see this effect easily in a fast flowing stream where water is flowing over a rock creating a series of ripples downstream.
Wave lift forms best in stable air while thermals form best in unstable air and the two conditions don’t mix well. Flying in Nevada few years ago, the local guys told me that in summer, one had to take off at 4.30 am to catch the wave and maybe get up to 30,000 feet. take off much later and the Sun’s energy would warm the ground creating thermals which broke up the smooth air. Standing waves move around slightly as wind changes direction and speed, but basically sit over and downstream of the mountains that create them.
Now nature shows us something new. In Northen Australia is a dynamic wave which seems to be in class of its own. Called Morning Glory, it is a line of cloud up to 600 miles wide which travels across the ground at speeds of up to 60 kph pushing the air up over it. Imagine a snake travelling sideways. The film below was taken from a powered plane, but nothing like surfing something like this in a glider – without an engine.
Reminds me of The Severn Bore which can travel up to 8 miles inland.
More stuff like this at The Cloud Appreciation Society