Amy Roberts pulled the rope release handle on the instrument panel of the Schweizer 2-33 training glider she piloted. She was 3,000 feet above the ground. Roberts turned right, separating from the tow plane, an old crop duster, which turned left and dived.
“We found a thermal nearly right away and it was way stronger than anything I'd ever been in," explains Roberts. "So, we started turning. And I'm turning and turning and turning, and we were going higher and higher and higher, like, really fast.”
Spring is one of the best times to encounter thermals- rising columns or bubbles of air -and to fly gliders, also known as sailplanes. In Virginia, springtime air is often unstable and moist. On a sunny, partly cloudy day, the surface of the Earth is heated unevenly. Things on the ground like parking lots, roads, crop fields, quarries, chicken barns and big box stores, absorb the sun’s heat. This stored energy becomes dislodged by surrounding cooler air, sometimes by cold fronts, and then it rises, like the waxy bubbles in a lava lamp. This rising air—a thermal— is also called lift.
A lava lamp is a good way to visualize how thermals act.
Some people confuse gliders with hang gliders, those large triangular kites with a pilot suspended beneath them, but they’re not. Instead, think of a glider as an airplane with no engine. For example, in 1903 the Wright brothers stuck an engine on one of their glider designs and ushered in the dawn of powered flight. I like to think that all airplanes are gliders. Most, like Cessnas, Pipers and even Boeing 747s, just happen to have engines. But, since pure gliders have no engines, they have to be towed into the air, tethered to another plane by a remarkably thin rope. Once up at altitude, two to three thousand feet above the ground, the glider releases the rope and is flying on its own. Certain times of the year, like winter, the glider floats back to Earth much in the way a paper airplane is thrown and glides back to the ground. Other times of the year a glider can use thermals to stay aloft for hours, fly overland hundreds of miles, and even climb high in the sky.
In May 2020, Amy Roberts was still working toward her private pilot glider license when she encountered a monster thermal near Merlin Aerodrome, a glider port in Amelia County, Virginia. (Full disclosure: I was Amy's flight instructor, and was in the plane with her during this flight).
“It’s actually turbulence.” Roberts explains, “The first thing you feel when you enter a thermal is a bump in your seat.” That is, the thermal pushes the glider upward, and as a result, your seat cushion pushes upward against you. When a glider pilot thinks they have entered a thermal, they check their variometer (or vario) . This is a gauge on the glider’s instrument panel that senses changes in barometric pressure. It has a needle that points upward when the glider is going up, and points downward when the glider is going down.
Numbers above or below zero indicate a climb or descent in hundreds of feet per minute.
"My variometer,” Roberts says, “instead of showing 100 feet a minute up or 200 feet a minute up, [which indicates moderately strong thermals] was showing like 600 feet a minute up. It was the biggest, strongest thermal I've ever been in.” Some pilots experience thermals so strong that the variometer needle reads 1,000 feet per minute up, which is the highest limit on the variometer scale.
But remember, the vario needle also points down. Where there is lift, there is also a phenomenon called sink, like when the bubbles in the lava lamp float to the top, they cool down, often moving to sides of the lamp where they sink back to the bottom. There, they are reheated, and the process repeats. On a day when a glider pilot, like Amy, finds thermals going up at 600 feet per minute, they are just as likely to find sink surrounding the thermals descending at 600 feet per minute. Eric Lambert, an experienced glider pilot who also owns Merlin Aerodrome, once likened this to a huge imaginary doughnut. The doughnut hole is where the lift occurs, while the doughnut around the hole is where the sink happens.
Thermals are heated rising air surrounded by cooler sinking air.
So, glider pilots try to stay in the thermal, the doughnut hole, and stay out of sink. When a pilot finds lift, they fly circles within the thermal, hopefully avoiding the sink around it. Think of those bubbles in the lava lamp again. You need to stay in the bubble in order to go up with it.
Those waxy bubbles in the lava lamp are opaque and are lit by a colored light. But thermals are invisible to the eye. So, how do you find them? You look for indicators on the ground, like parking lots and fields, you watch for the variometer needle to go above zero, but in the spring especially, you can also just go outside and look up at the sky. One of the best indicators for thermals are cumulus clouds. They look like puffy cotton balls hanging in the air. They sometimes have flat, dark bottoms which indicate that a thermal is pushing that cloud upward, The bottom of the cloud is generally how high the thermal goes. Cumulus clouds are abundant in the spring. Sometimes they are small, sometimes they are huge, and sometimes they are so tall they can reach into the stratosphere, forming towering cumulus, which often become thunderstorms.
Roberts circled her glider under a billowy cloud, perhaps a mile in diameter. “We were pretty high, like 6000 feet. We'd already gained 3000 feet in altitude. “And I'm looking around and I'm noticing that we are getting blown away from the airport [by the wind]. And I'm like, should we turn back to the airport? And you were in the back seat saying, ‘Yeah, yeah, keep going. Keep going.’ So I kept going and going and going, and at one point we’re at 9000 feet. I’m looking outside and you can see the the curvature of the Earth. And it's cold.”
For every 1,000 feet of altitude, the temperature drops roughly 3.5 degrees Fahrenheit or 2 degrees Celsius. That day, it was 70ºF on the ground. At 9,000 feet, it was 38.5º. Gliders don’t have heat.
“We are so high and we got up here without an engine. We’re just two people flying around, and we are at 9000 feet, higher than jets flying to and from Richmond [International Airport], it's crazy.”
This was an exceptional day. Sometimes thermals will reach to only three, four or five thousand feet above the ground. At Merlin Aerodrome, glider pilots have also been able to take advantage of other sources of lift, like mountain wave: strong updrafts caused by high winds deflected upward by mountain ranges like the Blue Ridge mountains . Those pilots have reached altitudes of over 15,000 feet and flown their gliders hundreds of miles over central Virginia. But gliders have gone even higher. The world record ascent of a glider was in 2018, when a pressurized glider called the Perlan, flown by pilots wearing spacesuits, used Mountain wave over Patagonia to climb to 76,000ft.
Still, climbing to 9,000 feet without an engine over Amelia County is quite an experience. “I was exhilarated,” Roberts says about her flight. “I'm sure I couldn't sleep that night, or even for days. I still think about it.”
Amy Roberts earned her Private Pilot Glider license in December 2021. She was 57 years old. She is currently working on adding a “powered rating”—airplanes with engines and propellers— to her license. Paul Tait Roberts, Amy’s husband, was her glider instructor. Their marriage is healing nicely, thank you.