Your connection with the sky

Soaring – The Nature of Thermals

To pilots schooled in the meticulous care and feeding of aircraft engines, the concept of extended flight which depends on something free and invisible called "Lift" verges on the magical.

Consider the ubiquitous "thermal." Imagine a cool, clear, summer morning. The sun is warming the earth as it has for billions of years. The air is still and cool but pockets of air, perhaps over plowed field or a sun facing slope, are warming faster than the rest. These pockets of warmed air expand into bubbles which are lighter than the surrounding cooler air. Like a hot air balloons, they begin to rise.

As the sun rises, solar heating becomes more intense and the bubbles form more rapidly, ascending in a continuous stream from favored thermal generating sites. As solar heating becomes still more intense, the bubbles merge into each other forming large columns. These columns of rising air are the "thermals" glider pilots speak of reverently. On a summer day, thermals generally start around 11 AM and last until around 7 PM.

A sailplane flown at 45 knots in a 45 degree bank will fly a circle about 300 feet in diameter. That circle will easily fit inside a thermal column. A sailplane sinks at around 100 feet per minute but the thermal column can be rising at more than 1000 FPM. The glider's variometer, a glider's special, fast response, rate of climb instrument, will show a 900 FPM climb. Sailplanes are solar powered. If the atmosphere is slightly moist, the thermal will generate a fair weather cumulus cloud as it reaches the condensation level thousands of feet above the earth. These clouds are welcome signposts glider pilots use to find thermals.

There are other signs. Glider pilots know bare earth, particularly sand and rock heat up faster than vegetation as do paved parking lots. Airports are almost always good thermal generators. In the deserts, dust devils may rise thousands of feet. Strong thermals may contain bits of trash or plant matter that give them away. Soaring birds are a good thermal markers as are circling gliders. Glider pilots often find themselves in the company of a hawks and eagles who have used THEM as a thermal marker.

As a rule of thumb, thermals are five times as far apart as they are tall. In other words, if thermals are 5000 feet tall, they are likely to be five miles apart. A glider with a glide ratio of 50:1 will pass ten thermals in a glide from 5000 feet. By watching carefully for signs, an observant pilot will find many thermals, one after another.

When a glide path intercepts a thermal, the variometer will indicate a climb beeping to alert the pilot. If the decision is to accept nature's offer of free energy, the pilot will fly a circle. The first circle probably won't be centered on the strongest lift so the pilot will shift the circle until it is. The pilot will continue circling and making small corrections until the thermal weakens or the glider nears cloud base before gliding off in search of the next thermal. Where I fly, thermals often reach the base of Class A airspace at 18,000 feet. Of course, we use oxygen at those altitudes.

Spending a hot afternoon high in a cool sky cruising quietly under cumulus clouds is a indeed a magical experience.

Next time, I'll write about lift which depends on wind and the fascinating ways it behaves as it flows across the earth. Imagine the view from 40,000 feet while surfing a monster called Mountain Wave - soarings equivalent of Mt Everest.

Meanwhile, see some great videos at:

3 Responses to “Soaring – The Nature of Thermals”

  1. Chris O'Callaghan Says:
    February 26th, 2009 at 9:14 pm

    Well said Bill...

    I learned to fly thirty-one years ago, soloing after only four days of lessons - on my 25th glider flight. Total cost to solo: $360. Since then I've accumulated more than 6,000 hours as pilot in command of more than 30 models of sailplanes. Today, I find flying very easy, but soaring remains no less challenging. Nature seldom repeats herself, so every flight is unique and as exciting (and satisfying) as my first.

    Perhaps the best part of soaring is the always changing, always beautiful skyscape. Though I live in an unremarkable region of costal plains and piedmont, the sky offers daily wonders.

    Keen observers of the air, glider pilots often see things others miss. The annual migration of raptors south. The gossamers of migrating spiders glinting on our wings. A monarch butterfly 4,000 feet above the ground, fluttering its way to Central America. A tumultous roll cloud, marking the turbulence beneath the glassy smooth but powerful lift of mountain wave, and the lens shaped clouds stacked above it. The grey tendrils that sometimes form below a cumulus cloud, marking especially strong eddies of lift. The concave bottoms of cumulus clouds that mark the very strongest of thermals. The extraordinay sensation of flying into a column of smoke rising from a field fire - opaque from the outside, but transparent once inside.

    These are my weekly delights... shared with friends at the gliderport.


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