Changes

As it requires rising heated air, thermalling is most effective in mid-latitudes from spring through late summer. During winter the sun's heat can only create weak thermals, but ridge and wave lift can still be used during this period.

== Ridge (Hill) lift ==

A ridge soaring pilot uses upward air movements caused when the wind blows on to the sides of hills. It can also be augmented by thermals when the slopes also face the sun. In places where a steady wind blows, a ridge may allow virtually unlimited time aloft, although records for duration are no longer recognized because of the danger of exhaustion.

== Wave lift ==

This lift is often marked by long, stationary lenticular (lens-shaped) clouds lying perpendicular to the wind. Mountain wave was used to set the current altitude record of 15,453 metres (50,699 ft) on 29 August 2006 over El Calafate, Argentina. The pilots, Steve Fossett and Einar Enevoldson, wore pressure suits. The current world distance record of 3,008 kilometres (1,869 mi) by Klaus Ohlmann (set on 21 January 2003) was also flown using mountain waves in South America.

The boundaries where two air masses meet are known as convergence zones. These can occur in sea breezes or in desert regions. In a sea-breeze front, cold air from the sea meets the warmer air from the land and creates a boundary between two masses of air like a shallow cold front. Glider pilots can gain altitude by flying along the intersection as if it were a ridge of land. Convergence may occur over considerable distances and so may permit virtually straight flight while climbing.

Glider pilots have occasionally been able to use a technique called "dynamic soaring"allowing a glider to gain kinetic energy by repeatedly crossing the boundary between air masses of different horizontal velocity. However, such zones of high "wind gradient" are usually too close to the ground to be used safely by gliders.

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