Sunday, December 20, 2009

A Thermal Made Visible

For as long as I can remember I have occasionally seen flocks of seagulls gathering over our town - up to about 200 at a time, wheeling in circles in a group a few hundred feet tall. Later on I remember seeing the ones at the top heading out in a line, usually toward the ocean. I thought it looked like a thread being pulled from a spool - the flock eventually turned into a long line of gulls and disappeared. I never really noticed that they entered at the bottom and left from the top - I just figured it was some sort of holding pattern as they arrived from feeding all over town.

That was before I knew about thermals.

After I learned to soar, I realized that the gulls in those flock never flap - they're soaring in some thermal over a parking lot or dark roof or something. They're obviously "tanking up" before heading out on a long flight. The dimensions of the flock - horizontal and vertical - probably define the size and shape of the thermal.

So far, so obvious.

This is one of the few times that a thermal is actually visible. The other time that thermals are visible is when they're filled with dust and we call them a "dust devil". Usually those are tall and thin and often moving laterally, but are pretty much vertical columns of dust.

Yesterday I saw another seagull-filled thermal as I was driving on the freeway. The day before we had had "Santa Ana winds" - a local name for a foehn wind. This day the wind had settled down to a breeze - but was still probably between 5 and 10 knots. As I passed by the flock, I could see that the column of gulls was skewed at a 45-degree angle to the vertical. They were still thermaling, but were being carried downwind at the same rate they were rising.

We glider pilots all know that thermals are skewed by the wind - but we very rarely get to see it. The hundreds of gulls made the distortion perfectly clear. What a great teaching tool that would be. Now if only we can arrange for flock of seagulls during ground school...

1 comment:

Dewlish said...

Hi Roger, don't know if you have looked at my blog
I enjoyed my gliding like you, but it all took place a long time ago.
At my club there was allways lots of discussion about finding thermals, and there were a number of local hot spots that could be relied upon, including the RAf Upavon cookhouse!
Enjoy your flying and all the best for 2010