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How to spot thermal sources and triggers in the flatlands from XC Magazine

Few flying areas are really flat – there are always gentle contours. There may be gentle hills and ridges but we have to assume that none will be dynamically soarable in the flats. I will always try to avoid reaching any ridge at a height that would put me low on its windward face. It’s most likely that the thermal will be triggering off its peak or lee side so you must be able to over-fly it on your last ditch attempt to get up. It’s very common for inexperienced XC pilots to try and ridge soar a slope, whilst simply flying past it would have worked far better.

In a completely flat landscape with no lee side protection from the wind then even a crop of tall, dry wheat can give good enough protection from the wind to allow higher temperatures to be reached amongst the stalks. Early in the day this can be more effective than a dark, ploughed field. However, later in the day naked earth or rock will maintain a higher surface temperature allowing quick heating of the air above it and possible provide a near constant column thermal. The field of wheat, having once released its thermal, will take longer to re-heat and re-release.

Apart from the usual physical triggers like irregularities in the landscape, small hills, tree lines, conurbations, buildings and bodies of water, you’ll also find certain airflows trigger thermals too. A small hill can split the wind and when it converges again behind it it may draw air from your source. Cold air will undercut and release warm air at the source too – for example air blowing off water or coming from a shaded area – so it’s a good idea to fly the shadow line of a cloud as you fly down wind.

Sometimes you can get a source and trigger combination to fly along. In very dry flatland environments like Manilla and central Spain river courses work well. They are slightly lower than the surrounding land, allowing some protection from the wind for better heating and the surrounding air will have a slightly greater relative humidity. Vegetation along the banks gives contrast and protection from the cooling wind and trees lining the banks act as useful triggers.

See you in the air!

Cross Country Magazine Team

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