North Wales Gliding Club

Atmospheric lift

Gliders have the biggest engine of any aircraft in the sky. The glider's "engine" is the energy in the air and wind. Using this wind energy gliders have flown for thousands of kilometres - the current world record is 3,009km. Not bad for an aircraft with no fuel. So - how is it done?

Thermalling

How a glider thermals
How a glider thermals
To keep flying, gliders need lift from the surrounding air. One form of lift is a thermal. Thermals are created when the sun warms the air near the ground until. When this pocket of air becomes significantly warmer than the surrounding air, it starts moving upward through the atmosphere. This bubble of air can be several hundred feet across and involve as much as 50,000 tonnes of air moving upward at (typically) 500 feet per minute. When the bubble rises high enough any water vapour in it cools and will form a small fluffy cloud with a rounded top and a flat bottom.

Cross section of a thermal

The thermal consists of several bubbles of air moving upwards in a loose column. Within the thermal there is a movement of air such that the centre of the thermal can provide very strong lift whilst the outer edges provide very little lift. The glider will attempt to centre in the thermal's core and will fly in a circle to stay in the strongest lift.

Triggers

To get a series of thermal bubbles providing plentiful lift, it helps if the warm air on the ground is regularly disturbed to "unstick" the thermal bubble from the ground. Cars and agricultural machines are good sources of disturbance and are said to "trigger" the thermal.

Finding a thermal

There are three basic techniques for finding a thermal. One is to look for the cumulus cloud that can form at the top of a thermal whilst another technique is to fly downwind from a ground-based feature that is likely to create warm air, for example a dry, freshly ploughed, brown field or a large car park. The final trick is to look for somebody who has already found a thermal - either another glider or a bird.

Ridge Lift

To keep flying, gliders need lift from the surrounding air. One form of lift is ridge lift. This is caused when the wind blows across a feature in the landscape such as a long ridge or a hill. As the wind meets the obstruction it is forced up and over the ridge or hill and any glider flying in front of the ridge will also be lifted up.

Curl Over

As the wind moves over the top of the ridge it may become very turbulent instead of flowing smoothly. This is known as "curl over" or, more colourfully, as "the clutching hand". The glider is dragged down by the descending air almost as if someone had reached up and grabbed it. Fortunately curl over is easily avoided by simply staying in fron of the ridge.

Wave Lift

To keep flying, gliders need lift from the surrounding air. One form of lift is wave lift. This is caused when the wind blows across a feature in the landscape such as a hill or mountain and, if the wind is of sufficient strength, then the airflow begins to oscillate up and down like a large wave. This sort of lift is glider heaven. Wave systems can provide lift up to 20,000 feet and is very smooth to fly in.

Finding wave

Wave systems are easy to find. Their position is usually marked by very odd clouds which are very, very smooth and often very long and thin. The only problem is getting high enough to contact the wave system. At North Wales, we get strong wave from Snowdonia when the wind is in the west and blowing at just the right strength.