Again, poor Icarus! If you read this column yesterday you will recall that Icarus, en route from Crete to Naples with the aid of wax-and-feather wings, flew too close to the sun and ignominiously crashed. More intriguing in a way, however, is that his father, Daedalus, similarly equipped, is presumed to have reached his destination.
The question arises: how did Daedalus succeed? With no artificial power, and presumably lacking sufficient strength to flap their wings effectively, both aeronauts, once launched, should have had a tendency to drift slowly back to Earth. To remain aloft and gain height, they would need to find areas of the sky where the lift provided to their wings was sufficient to overcome the pull of gravity.
It is a challenge familiar to every glider pilot, and there are two common ways in which the required lift can be achieved. First, ridge soaring relies on the fact that air approaching a sizeable hill does not go round the obstacle, but climbs over it, forced upwards in a gentle sliding motion.
Thus, on the windward side of hills, and for some considerable altitude above them, there is a body of rising air that can be used systematically by gliders to gain height. The mountains of Crete would have served our heroes well in this respect.
The second major source of lift comprises thermals, invisible currents of ascending air that rise through the atmosphere from patches of ground that have been warmed by the sun. A volume of air resting on ground that has been heated in this way becomes warmer than its surroundings, achieves a certain buoyancy as a result and begins to bubble upwards rather like the bubbles often seen in boiling water.
Thermals are usually present underneath cumulus clouds, which are themselves a consequence of the ascending air, but they frequently occur well away from clouds of any sort. They tend to slope forward with the wind, increasing in diameter with height, and when a strongish breeze is blowing they may be carried bodily downwind by the movement of the air, rather than remaining static over one spot.
A good thermal not long after take-off would have provided Icarus and Daedalus with the opportunity of an exhilarating upward spiral, from the summit of which they could glide gently westwards in search of another rising current.
Icarus, as we know, was too enthusiastic in his use of these atmospheric fountains, but the wiser Daedalus, despite his grief, would have used them systematically as stepping stones to reach his distant Italian destination.