Ground inversion season - avoid a nasty surprise.

Hairy Dave's picture

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 familar 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 a 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.

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Hairy Dave's picture

More on landing in a wind gradient

A near-incident on top of Swanborough yesterday (11/8/12) reminded me that an unexpected dive into a landing is not limited to bottom landings in a ground inversion.

Even on a windy day, if the top of your hill has a fairly sharp, cliff like, edge the air will tend to overshoot and behave like a wing near to stall. For a proper cliff, the air forms turbulence or rotor, just like a stalled wing. For a sharp-ish edged hill like Swanborough the airflow isn't really "stalled" and turbulent, but the top landing zone is behaving like rear part of a wing near to stall. So, near the ground we find a much slower boundry layer. Yesterday this conditon was about at it's worst because the crop on the top is long, providing a lot of surface friction. The windspeed in the crop, the first few feet above the ground, is zero and it must increase as we go up to the free stream velocity well above the hill.

So the result is that a pilot, even with a bit of excess speed, can experience a nasty surprise coming in for a top landing and end up diving in for a hard one as the glider must accelerate to regain airspeed.

Hang gliders are much more suceptible to the problem of landing in wind gradients than paragliders because the wing is so much closer to the ground.

So any time there's even the faintest chance of wind gradient (almost always) a hang glider should come in with a lot (the more the better) of excess speed. With speed we have several advantages:

flying fast we have much better steering.

we are less efficient (especially with drag from legs down position) so the landing approach is steeper and so it's easier to hit the spot.

a few mph change of wind speed is less significent compared to our speed.

If we are doing, say 40 mph when entering a boundry layer where the wind changes from 20 mph to 10 mph, then all we have to do is push the bar out to the 30 mph airspeed setting and we still have plenty of airspeed. If we were to enter that boundry layer at 20 mph and try to push out to 10 mph we'd stall first. The other, more popular, choice is to enter at 20 mph and dive to regain airspeed only to discover the ground is in the way.

For me as a novice HG pilot, still the most shocking and amusing part of a flight is coming into land as fast as the machine will go, then rounding out just before impact. So far John B's advice was quite right and every landing has been a nice surprise!

Great write-up. Thanks Dave.

Great write-up. Thanks Dave.

JTurczak's picture

Just so I understand

Just a couple of questions to make sure I have understood the points above.
There are two risk periods with this cold inversion.
1. Start of the day.
2. End of the day

There are two major flying risks with both of these periods:
a) The sudden shift of wind at the boundary layer both in strength and possibly direction
b) Generally more turbulent air as the cold and warm air mix (more of a problem earlier in the day?)

Thanks, all very help ful in knowing what to look out for on the hill
And I guess it also shows that no matter how often you fly a site there is no substitute for assessing the site newly for that day at that time.

Hairy Dave's picture

Generally it becomes evident

Generally it becomes evident at the end of the day as the ground cools off. Evening flights will often end with a landing diving into an inverted layer near the ground, or a slope landing in nil wind when there's still a breeze across the top. Unless a change of air (front) or increased wind comes through, the cold layer will get thicker as the night goes on.

Some time the following morning the thermals + wind will stirr it all up and the inversion disappears in a puff of turbulance.

In the middle of winter, with very clear skies, weak thermals and light winds the inversion may never break and may get added to on successive nights.

The risks are, yes:

Climbing to meet the nasty shear layer. where it might be very turbulent and / or much more windy above.

Being anywhere in the air when the sudden mixing occurs. (If you are, get high and stay high - it's not a time to be near the ground, land when the mixing has finished). The later in the day that the mixing occurs, the nastier it's going to be due to more solar energy. Similarly the greater the difference in wind from low to high the worse it will be.

Stuffing up a bottom landing as you dive into the still boundry layer.

Stalling into a bottom landing as you don't dive into the still boundry layer. That's one of the ways I've seen a glider stall in big ears (see previous rant / technical advice posted about the safe use of big ears.

You're dead right about site assessments - they change minute by minute, and that's what makes flying our meterological research aircraft so much fun!

Steve Hampton's picture

An example

First arrivals at Bo Peep found the wind smack on the hill, about 10mph and feeling smooth, two pilots took off, soon got a couple of tucks in choppy air, and took the first opportunity to land.
The clues were all there though, the wind at the bottom was NW and smoke at least half a mile out indicated NW too. Firle Wendy (when it was much higher on the mast)was registering 15mph East, that's a whole heap of wind-shear.
The Met Office Ballooning forecast is a good one to check as it gives the expected height and duration of the inversion.
Steve Hampton

Carlo Borsattino's picture

Very good, Dave - and bloody useful for pilots too!

Very good, Dave - and bloody useful (potentially life and limb saving) for all pilots too!

Anyone who finds the above useful should buy Dave a pint when they next see him.

So that's 500 pints coming your way, Dave...


PS. I can't believe you dared to mention the 'A' word though. Everyone will run into hibernation now... ;o)