When is a thermal not a thermal?

[Rayven]

We live on a penninsula on the tip of Puget Sound and rarely get "real" thermals. With our limited sun and constant cooling winds from the Straits of Juan de Fuca, any thermal that do form are quickly cooled and blown downwind--its been frustrating for us glider-guiders to say the least...But lately we've been watching the birds and noticing some incredible soaring activity at the margin between the land and the sea.

Today, with very light westerly winds on the straits--and light southerlies on the opposite side of the penninsula--we spotted eagles soaring at about 1 to 3 thousand feet. Seagulls were struggling to get into the lift and as we watched a couple did get into it. Most didn't. This phenomenon has been noticed before by us, but today it was really dramatic.

The thing is, I don't think it could have been thermals--the sun was pretty weak today. Anybody else observed anything like this?? Our working theory is that the warmer airmass of the land is getting pushed upwards by the cool air coming off the water. If, like the eagles, you can get high enough to ride about the boundry layer, you can get quite a ride!

[Peytr]

I think you saw thermals all right.

I fly at two locations, a flat land site aout 50 miles from the coast (allways some thermal activity in spring, summer and fall. Mostly good) and at the coastal dunes for slope soaring. Though eveybody tells me there hardly are any thermals to be expected in front of the dunes (actually over the beach /sea) I have seen thermals on a few occasions. One is when the sun is heating the dunes and sand, wind dropping (so no slope lift) but continuing and very docile lift over the beach. The other is more spectacular, when wind is blowing harder from the sea. The real challenge is to get to the lift which often is high up and a bit too far from the slope to reach. I have been flying very high and out over the sea at one or two occasions. I spotted the lift by looking at birds, which can be very high up. Once you're flying more than 4 to 5 times the height of the slope, or flying in no wind conditions, you can assume you're in some kind of thermal lift.

[Ollie]

http://www.charlesriverrc.org/articles/flying/markdrela_ds.htm
http://www.sciencedaily.com/encyclopedia/dynamic_soaring
http://www.wfu.edu/albatross/atwork/dynamic_soaring.htm

[flystoolow]

...when two air masses colide, a great quantity of air rises as it has no where else to go. It's referred to as "Convergence." When conditions are right, be it in a valley, across a ridge, or along the coast, your 200' slope flight may just turn into a 2000' soaring flight. A long, rolling eddy of stable air usually quickly forms along the entire coastline to a hight of aprox 200', so if you miss the boat, so to speak, you're often out of luck without an engine or highstart. It's a sneaky feeling to be soaring at 2000' when your buddies show up at the slope, only to find the wind at launch is actually blowing toward the sea at about 2mph. Once last summer, I was flying in these conditions when I saw about 6 guys pull up the road to join me flying. Well, my plane was at about 2000' so I trimmed it out for a slow continuous turn and had a seat on a park bench. When my buddies walked up with their gliders, I had the transmitter sitting on the bench beside me and started chatting. "No good", I said, "The wind turned offshore and I lost my plane in the ocean." I had them fooled for a couple of minutes, until one of the guys finally noticed my plane.

The seaguls seem to just love this lift, and often climb in groups of 200 along our coast. It's quite a sight.

[Rayven]

...when two air masses colide, a great quantity of air rises as it has no where else to go. It's referred to as "Convergence." When conditions are right, be it in a valley, across a ridge, or along the coast, your 200' slope flight may just turn into a 2000' soaring flight. A long, rolling eddy of stable air usually quickly forms along the entire coastline to a hight of aprox 200', so if you miss the boat, so to speak, you're often out of luck without an engine or highstart. It's a sneaky feeling to be soaring at 2000' when your buddies show up at the slope, only to find the wind at launch is actually blowing toward the sea at about 2mph. Once last summer, I was flying in these conditions when I saw about 6 guys pull up the road to join me flying. Well, my plane was at about 2000' so I trimmed it out for a slow continuous turn and had a seat on a park bench. When my buddies walked up with their gliders, I had the transmitter sitting on the bench beside me and started chatting. "No good", I said, "The wind turned offshore and I lost my plane in the ocean." I had them fooled for a couple of minutes, until one of the guys finally noticed my plane.

The seaguls seem to just love this lift, and often climb in groups of 200 along our coast. It's quite a sight.


I think this might be exactly what happens here on the penninsula. Depending on wind direction, it can happen on either side--and it doesn't seem to be related much, if at all, to sun-heating of the ground. It also seems only to happen when the winds are very light--as in under 5mph. So I don't think its related to a ds type conditions either. In any case, the birds around here seem to understand it rather well!!
Thanks for everybodies response!

[Ollie]

http://www.sciencedaily.com/encyclopedia/dynamic_soaring

"In his 1978 book Streckensegeglflug (Cross-Country Soaring), Helmut Reichmann describes a flight made by Ingo Renner in a Libelle sailplane over Tocumwal in Australia on 24 October 1974. On that day there was no wind at the surface, but above an inversion at 300 metres there was a strong wind of about 70 km/h (40 knots). Renner took a tow up to about 350 m from where he dived steeply downwind until he entered the still air; he then pulled a sharp 180-degree turn (with very high g) and climbed steeply back up again. On passing though the inversion he re-encountered the 70 km/h wind, this time as a head-wind. The additional air-speed that this provided enabled him to recover his original height. By repeating this manoeuvre he successfully maintained his height for around 20 minutes without the existence of ascending air, although he was drifting rapidly downwind. In later flights in a Pik 20 sailplane, he refined the technique so that he was able to eliminate the downwind drift and even make headway into the wind."