Pace - I had noticed that nobody had answered your question.
I'm not an expert; my experience of such matters is limited to a fortnight of gliding over the last three years in the Pyrenees mountains in Spain with a dozen or so flights of 3hrs duration up to an altitude of 13,000ft using mountain wave.
Surely at altitudes of even 10K to 20K the OAT could be minus 20 or less and sitting stationery for hours surely it would get pretty chilly?
Yes - it can be very very chilly at altitude - even at ground level - it may be cold enough to maintain snow.
Flying at altitude - most people find that feet & hands are the first parts of the body to get cold and numb.
How do glider pilots keep warm on high altitude flights ?
Some gliders have an extended canopy that makes sure that the sun reaches everywhere from the feet to the head. Also best to wear dark colours. You will be amazed at how hot it can be sitting in what is effectively a greenhouse. Although, with typically less than 1cm between flesh and atmosphere - the top may be warm enough - but the rear can be cold...
Quite a few of the female pilots I know carry a blanket which they can put over them when they start feeling the cold. Mary.M has also said as much in a previous post somewhere on this site. I don't know many male pilots that admit to carrying blankets. Although, I know of at least one that carries a blanket "just in case he has to sleep in the glider waiting for a retrieve" - he won't admit to using it in flight though.
There are custom designed suits for gliding/paragliding etc... that are designed for easy movement in confined spaces in sub-zero temperatures with the ability to unzip various parts.
Clothing, gloves and boots for glider pilots, paragliders, hang gliders
- Heated Motorcycle Clothing
For the very serious - kitting oneself out with heated glove inserts or heated socks or heated jackets from the motorcycle shops. Some of the motocycle gear now is extremely light, flexible, and can be powered by small portable batteries for very reasonable amounts of time.
7V heated products - Gerbing: Leading in Heated Clothing However, heated clothing does divide people at the club bar where there are those that think heated clothing is the next sliced bread and those that like pointing out the virtues of Li-ion batteries and their ability to combust when impacted and therefore, Li-ion batteries should be banned. I've always considered that if any of my Li-ion batteries are impacted enough to ignite - I'll be in a whole heap of pain and Li-ion batteries will be the least of my worries. However, the RC model flyers have flame-proof Li-ion battery bags - and having Li-ion batteries in locations easy to reach and easy to jettison I believe should be enough.
Are there heaters?
The glider doesn't come with a heater in the same way a SEP or Microlight may have. I've only heard about heated clothing.
Or is there different dress sense for the levels you will fly at ?
If your not into heated clothing - there is the standard trick of multiple layers. Typically - it is hottest while waiting on the ground with the canopy closed for the aerotow to start. The trick is to have an idea what the temperature should be at the altitude your expecting to fly at. Glider pilots will have that info from the morning briefing anyway from the daily sounding charts. You put on as many layers as you believe is necessary and have as many layers with front zips as possible with maybe a few layers with zip-on sleeves etc... You sit on appropriate impact foam and parachute to insulate the body from the glider fuselage. Then as you climb - you start doing up the zips one layer at a time etc... This is manageable as in wave at high altitude its as smooth as silk and you can loosen the straps enough to do this.
Hope this helps.