Flying Cross Country

How does a glider fly cross-country?

Very simply, by finding air which is going up faster than it is going down! In short, a glider regains the height (and therefore, its potential energy) by flying in upcurrents that are rising faster than it is falling. This energy comes from the sun and manifests itself in three main ways: as thermals, as slope lift and as a phenomena called wave. The latter two are caused by the wind blowing onto a hill or a mountain. Although glider racing generally uses thermals, slope and wave lift can be also be used. We will concentrate on thermal driven cross-country.

Thermal cross-country

Thermals are basically rising currents warm air (and if they could be visualised, they would like the blobs in a lava lamp). When air near the ground is heated to a higher temperature than its surroundings, it will tend to rise when disturbed. It will continue to rise if the air around it is cooler, as it is less dense. For instance a cornfield, or a town, will tend to generate and trap a lot of warm air in the sunshine. When the warmer air is gently blown towards another feature, such as line of trees, or a lake (which has cooler air over it), it will then separate and rise. Thus to generate the best thermals, you need energy (the sun), a varying landscape to give contrast and colder air above the ground. The best days are often those that have fair weather cumulus; a cumulus cloud is formed by the water in the rising air condensing out as the air cools. However, even on cloudless days (called 'blue days'), there are usually thermals. There are no clouds because the rising air doesn't cool enough to form a cloud (or it contains too little water). The gusts of wind we experience on the ground on warm sunny days are caused by the ascent of thermals - you can often feel the wind suddenly get warmer then colder.

These thermals supply the energy a pilot of a glider needs to go cross-country. On a good day, thermals can be found every 5-15 miles, depending on the terrain and how high they are going. Generally speaking, the higher the better. In the UK, this can be anything from a 1000ft to 10,000ft, depending on the time of year, how much moisture is in the air and how cold the air is. Good soaring days are often preceded by cold nights. Unfortunately, what goes up, must come down. Between the thermals, the air is usually coming down (and is known as 'sink'), so pilots try and minimise the time they spend in the sink and maximise the time they spend in the 'lift'. Thus, to go cross-country, a pilot will circle in the rising air to gain height, then cruise at a higher speed to the next thermal (and will go especially fast in the sink, thus reducing the time in bad air). The speed at which the pilot goes cross-country depends on many factors.

Next: Racing