If you've just bungee launched a glider off a hill, then? Do you have to go and collect it from the bottom of the hill to have another go? Luckily, there are ways for gliders to stay airborne for hours, or more in a lot of cases. The atmosphere is always shifting around, with air moving both horizontally (as wind) and vertically. If we can position the glider in the upwards moving air, and the air is moving upwards faster than the glider is sinking downwards, we can then gain height. Think of it as a person trying to walk down a fast escalator: if the escalator is moving fast enough, the person will be taken upwards, despite the fact that they are walking down it.
There are 3 basic types of lift that gliders use to stay airborne for lengths of time:
In the case of the glider we have bungee launched off a hill, it can gain height using ridge lift. Bungee launching is only done when there is wind blowing against the hill, for 2 reasons: First, it makes launching the glider a lot easier - it already has some relative airspeed even when it is still on the ground (equal to the wind speed), so achieving flying speed is a lot easier. Secondly, when the glider is launched, it can use the ridge lift generated by the wind blowing against the hill to gain height, and land back on the airfield for another launch.
Ridge lift is the easiest to imagine - the wind blows against a hill, and the hill deflects the air upwards. The glider can fly in this upward moving air to gain height or stay at the same height. The closer to the hill you are, the stronger the lift will be.
In the summer (or even in spring and autumn when the sun is shining) different areas of the ground will heat up in the sun faster than others (for example, dark fields). These warmer areas heat up the air around them, and this air eventually breaks away from the ground as a bubble of warmer, rising air. These are called thermals, and gliders can use them to gain height on good days. Since they are roughly cylindrical in shape, gliders will turn in reasonably tight circles in order to stay in them. On a summer's day, you can often see gliders turning in circles inside thermals to gain height.
As the air rises, it cools down, and eventually it will reach a height where the water vapour inside the air will condense, and form a cumulus cloud. These are the clouds that glider pilots look for in order to find where the thermals are located. The white, fluffy clouds you see on a summer's day are all indicators that there is a thermal underneath them.
This is probably the hardest type of lift to understand, not even the experts When a strong wind blows across a series of obstacles (usually a range of hills or mountains), the air will flow up one of the obstacles and down the other side, up the next one, down the other side and continue like this. This oscillation can continue for hundreds of miles past the obstacles, and can also grow in height far beyond the height of the original mountains or hills.
Many height records have been set in wave, up to the heights commercial airliners fly at, and sometimes even higher. The world record for height in a glider stands at over 50,000ft. Pretty impressive for something that is only using the natural currents in the atmosphere to gain height!
We occasionally get wave at Cranwell on a strong wind day, but the best places to look for wave without leaving the UK are Scotland, Northern England and Wales. We organise an expedition to Portmoak in Scotland every year to try and sample some of these unique conditions!