A thermal is a column of rising air, caused by the effect of the uneven heating of the ground by the sun. The air close to the ground heats up, and eventually separates as a bubble or column that is lighter than the surrounding atmosphere. As it rises it cools, and if the air is sufficiently moist, eventually the water vapour it contains will condense as a cumulus cloud, helpfully marking the area of lift for the glider pilot. On a cross country flight the pilot will circle in thermal lift under these cumulus clouds, and then glide towards the next thermal to stop for more lift along the way. Most long thermal cross-country flights, particularly in Ireland, are done throughout the warmer summer months. You will recognise these ‘thermal’ days by the fluffy white cumulus clouds dotted across the sky.
When the wind blows towards a hill such as the cliff face at Binevenagh near the Ulster Gliding Centre the air is forced up and over the hill. A glider can fly along in this updraft and remain aloft as long as the wind keeps going –all day if you like! Ridge lift is the most used form of lift at the Ulster Gliding Centre. The ridge there ‘works’ when wind has a westerly component and is more than 8 to 10 knots (10 to 12 mph) in strength.
In the right conditions when the wind passes over a mountain, it may bounce up again and cause a standing ‘wave’ of rising air. Very significant wave patterns can form in more mountainous areas such as the Sperrins in Northern Ireland or indeed the mountains of Wales or Scotland. These waves can be much higher than the mountains that caused them and can stretch for hundreds of miles. Occasionally we enjoy such wave conditions at Bellarena which enable glider pilots to fly at very high altitudes. Waves are often marked by ‘lenticular’ clouds that appear to be stationary, lined up perpendicular to the wind direction. Wave conditions at Ulster Gliding Centre are mainly found with Easterly winds.
Generally speaking, while you might not always get a ‘soaring’ flight, the only things that will stop you flying are persistent rain, low cloud and wind over 15kts particularly if it is gusty.