In the UK the official weather forecast is issued by the Met Office, The nearest forecast is given for Waresley 3km from the airfield. The Met Office does a 5 day forecast, although their 3 day forecast is more accurate.
Typically in the UK most days are flyable, but more days are flyable in the summer and fewer days are flyable in the winter.
Rain - If it is raining there will be no gliding, however if there are showers is may be possible to fly in between them.
Low Cloud - If the cloud is below 1000ft (300m) it will not be possible to fly
High Wind - Winds over 40 mph are unflyable, winds above 30 mph in certain directions are also likely to be unflyable. Winds above 15 mph are likely to restrict flying for inexperienced pilots, for example an early solo pilot may not be allowed to fly solo.
Gusts - (sudden bursts of fast wind) can be particually diffcult to manage for inexperinece pilots if gusts are large, flying may be stopped even if the background win levels are manageable.
Waterlogged Airfield - During the winter is is possible for the airfield to get waterlogged, in this case even if the weather is good on the day no flying can take place, look for long periods of rain before the day you intend to fly.
Misting - In the winter there can be a problem of the canopies of gliders misting up, this only happens when the air is very humid and the air cools before sunset.
Sunlight - Gliding only takes place in the day, so when the days are shorter during the winter so is the gliding time.
While it is reasonably easy to work out if it will be flyable, it is much harder to predict soaring weather. In general thermals are created when sun hits dry ground, heats it up more than the surrounding ground. Therefore, the conditions need to be sunny and dry, but not necessarily hot. In the UK RaSP provides the best soaring forecast.
RaSP - gliding forcast
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Met Notes- Lots of gliding weather information and news