In the science and weather books I am reading, a thermal is described as a “pocket,” “bubble,” “blob,” “column,” or “parcel” of air. Two of my meteorology textbooks include illustrations of thermals and diagrams of how thermals produced clouds. I studied the illustrations carefully, read the relevant chapters on cloud formation, but when I began explaining how a warm patch of ground became a thermal, I couldn’t do it. I was not willing to write “warm air rises and develops into a thermal.”
If I can't explain how a thermal develops, I can't explain how convection works. And if I can't do that, I can't tell you how a cumulus cloud forms.
So I e-mailed my meteorologist for help. He referred me to a specialist.
The specialist is a friend of his who specializes in cloud dynamics. I wrote the specialist a long e-mail describing my "thermal problem" in great detail. I attached the photographs I had taken of the thermals in my meteorology books--the lovely, simple ones posted here.
Within an hour, he wrote back.
“Those are awful pictures of thermals,” he began.
I was strangely relieved.
“My concept of thermals is based on the results of numerical modeling studies and flying a sailplane in them.”
“Near the ground the air is very turbulent or chaotic. But several hundred feet above the ground these turbulent eddies coalesce to form a buoyant blob of air which we call a thermal…Some thermals are more or less bubbles of buoyant air that rise and expand. Some thermals have long tails and resemble more a jet of buoyant fluid.”
I had to pause and think about this. I imagined chaotic air whirling around my bare ankles and blowing my hair blowing into my face. I imagined eddies of water swirling around boulders in a river. I thought about the rising bubbles in lava lamp and about jelly fish with long tentacles. I pushed images of tidy air “parcels” from my mind.
“…thermals are more jet-like under low wind conditions,” he continued, “and you can sit on the ground when it is almost calm and recognize the passage of a thermal overhead by a short-lived gust of wind. Sometimes thermals become rotating columns of air which we can see as dust devils or blowing leaves, etc.”
This gave me goosebumps. I had never thought of such gusts as anything but isolated, inexplicable puffs and swirls of wind. I began looking forward to experiencing my next gust, knowing it might be connected to a thermal and a potential cloud.
And then, best of all, the specialist included this image of thermals.
This shows rising heat--not so much in columns or blobs--but in more organic shapes like mountain peaks or stalactites. In this modeled image the thermals are rising up--but have not yet reached the condensation level. The might never reach it, but sink back to earth instead.
NEXT BLOG: More on how thermals form and rise to become clouds.