Question #a56a1
1 Answer
...because convection causes updrafts that can be ridden to higher altitudes.
Explanation:
The sun shining on the earth's surface causes different parts of it to heat at different rates.
Hot patches on the earth's surface in turn heat the air immediately over them. At intervals, bubbles of relatively hotter air detach from the surface and rise.
This allows soaring birds to ascend without the exertion of flapping their wings. Anytime you see a vulture or eagle just circling in the sky, it probably means they've detected one of these thermal updrafts and are riding it upwards. Keep your eye on them for a minute or so and see if they don't gain altitude.
It also allows gliders and sailplanes to ascend without an engine. (Which I know because I build model gliders.)
