The following is an excellent article by Dave Lewis (Hairy Dave) Chief Coach for the Southern Hang Gliding Club:

I hate to mention the word, but it's beginning to feel autumnal early morning and late evening some days. What we're feeling is a ground inversion, which has provided more than a few nasty moments for pilots. It works like this:

We're all familiar with the ground heating in the day, heating the air touching it and making thermals. As the sun goes down the reverse happens.
The once-hot ground radiates it's heat energy. If there's no blanket of cloud to bounce it back, the energy is lost into space and the ground gets cold.
The air touching the cold ground gets cold and dense.

As evening draws into night, the layer of cold air gets thicker and colder, anything between a few feet and an up to several hundred feet, perhaps even a thousand.
We now have a lake of heavy, cold air over the land with the real air doing it's own thing over the top. The autumnal feel happens when we're standing in the cold layer - it's cold, humid (even forming dew or mist) and less windy than in the day.

Risks to the unwary pilot are:

• It's been windy all day and close to sunset the wind drops to flyable. If you fly and climb a bit, you might get up to the windy layer and find a turbulent surprise in the shear layer between the two. Above the shear layer it will still be as windy as it was all day.
• The wind early morning is light and flyable (possibly with plenty of wind from the north as the cold layer flows out to sea, just like a river). With some sun on the ground, thermals start and climbing begins. If you get a decent early climb, you might get to the shear layer and receive a battering from the turbulence. If you get through that in one piece there might be a lot more wind above.
• At about the point in the day when the thermals are strong enough to get our intrepid shear-layer researcher aloft, all those thermals belting through the cold layer stir up the whole system and mix the cold air in with the normal air above. That usually takes 10 minutes to half an hour and is not a nice time to fly. The thermals race up through the cold air, lumps of windy air from above are brought down and sensible pilots will be on the ground having coffee.
• Once the mixing is complete and the ground inversion is gone, all the cold air having mixed up with the air above, thermal activity will slow right down. That's because the air over the fields is now warmer and the fields and their thermals need to get hotter to make the required temperature difference. After another coffee it should get going again and this time the climbs will go all the way up.

Signs that this might be the situation:

• It's been a warm, breezy day and in the evening the wind slows as it cools off.
• The skies are fairly clear allowing the energy to escape.
• It's not windy on the ground, but the clouds are moving well.
• The forecast is for wind, but it's not windy on take-off.
• The isobars and other higher level wind forecasts show wind but the simple forecasts for ground level wind show much less.
• It's clearly shown on the forecast soundings.
• You're soaring around take off where there's plenty of wind but not much lift. You get a bit low and slope land only to discover there's no wind at all. You're in the cold lake and only the top bit of the hill was sticking out in the breeze.
• You're bottom landing into wind and at 10 feet the glider dives as it enters the cold, still layer and you land long (and fast). Hopefully you followed procedure and came in with excess airspeed and legs down so you didn't stall.
• You've watched someone take off half-way up, climb 100 feet above the hill, take a series of collapses, then start going backwards.