Different Flavors of Thermals

Understanding thermal types

By Honza Rejmanek
Originally published in USHPA Pilot, January/February 2019

It is noontime on a hot and sunny day. There is not a cloud in the sky. However, birds are spiraling skyward with encouraging climb rates. It might not be a record-breaking day but it looks soarable. The wind is light and the cycles on launch indicate that it should be possible to stay up. The only other pilot on launch looks over at you and says, “Looks good—you go first.”

Being proud of your thermaling skills you decide to take off. Losing the first thermal right off launch you find nothing but sink. Soon you are a third of the way down to the landing zone wishing you had insisted on the other pilot going before you. Suddenly you hit a strong punchy thermal. Knowing that this might be your only shot at staying up, you hyper-focus on finding the core and staying in it. Despite your best efforts, you cannot keep a full circle in lift. Passing in and out of this punchy small-cored thermal you eventually get back above launch but it feels like a lot more work than it should be. Regardless, you are glad to be airborne and you decide to make the best of the challenging conditions.

That evening you have a chance to talk with the other pilots who flew that day. They explain that it was a high-pressure day with “bullet thermals.” As you go to sleep that night you can’t help but wonder, “Why should thermals on high-pressure days be punchier, and presumably smaller? How does this contrast with days when thermals may be stronger and more usable for better climbs but not so rough-edged?”

If thermal quality were only related to surface-level pressure, then a simple barometer would tell us if we should head to launch. If great thermal conditions were only dependent on our location with respect to synoptic highs and lows, then all we would need is a recent surface-pressure chart to decide if it was worth going flying. Unfortunately, it is not quite that simple.

In order to judge the importance of one meteorological phenomenon with respect to another, it is important to keep in mind a sense of scale both for time and space. Sinking air in a high-pressure system descends on the order of approximately a centimeter per second and can persist for days. Thermals rise at meters per second and last minutes. Thus the generally sinking air in the center of a high might slow the thermal ascent rate down by 1% at most. This alone should not account for any detectable change in the behavior of thermals.

Rather than initially focusing on synoptic pressure to categorize the behavior of thermals, it is preferable to first focus on surface heating rates and depths of the convective boundary layer, the layer in which thermals are found. Only then should we focus on how high-and low-pressure systems might exert their influence.

With strong heating rates, such as would be typical of dry surfaces experiencing a high sun angle, it is possible to create strong contrasts between the temperature of the surface and the overlying air. The overlying air heats rapidly, becomes less dense, and quickly becomes buoyant. As enough of this air coalesces, it begins to rise as a thermal. How fast and how wide a thermal will eventually be is dependent on the depth of the convective boundary layer. Given the same surface-heating rate, a deeper boundary layer yields wider and stronger thermals, while a shallower boundary layer yields narrower and slightly weaker thermals. The thermals in the deeper boundary layer will be stronger because they will have had a longer vertical distance over which to accelerate compared to their shallow-boundary-layer cousins. A shallow boundary layer will have narrower thermals; nonetheless they can have significant punch to them because the greatest rate of acceleration occurs in the lowest layers where the thermal is most positively buoyant.

Convective boundary layers grow deeper by entrainment, or mixing at the top. Overshooting thermals chew away at the overlying capping inversion. They mix in some of this air from above, thereby increasing the depth of the boundary layer. The rate at which the convective boundary layer grows in depth is called the entrainment velocity and is dependent on surface heating rates and the strength of the capping inversion. Greater heating rates lead to an increase in entrainment velocity, whereas increasing the strength of the capping inversion decreases entrainment velocity. Typical entrainment velocities are on the order of centimeters per second. Additionally, the subsiding or sinking, stable free-atmosphere air above the boundary layer is warming as it sinks. This warming above the boundary layer top is having a stabilizing effect similar to that of warm-air advection above the boundary layer top. Remember that warming from above slows down the rate of boundary-layer convection despite strong heating at the surface.

The center of a surface low pressure is experiencing low-level convergence and this leads to general ascent of the whole airmass. Capping inversions become less stable in a rising airmass, and the air above the boundary layer is cooling due to the ascent. Cooling at the top of the boundary layer promotes convection within the layer. Thus several factors conspire to deepen the boundary layer. As a result, wider thermals will reach greater heights. Of course, with enough moisture present we often develop widespread clouds that shut off the surface heating rate, thus weakening or all together shutting down the production of thermals.

In the center of a high pressure, air is descending. The capping inversion is becoming stronger and whether it descends or rises is dependent on the entrainment velocity. At night the capping inversion will certainly descend due to the sinking airmass. During the day, the capping inversion might continue to lower over humid regions that do not experience strong surface heating. This is because much of the net radiation goes into the evaporation of water from plants. After several days of shallower boundary layers and narrow thermals, the capping inversion can lower enough to where it might eventually be eroded much like a surface inversion. Good thermals might return once again despite the fact that the high pressure remains.

In high mountain deserts the summertime daytime entrainment velocities can more than keep up with the sinking air in the center of a high. Thus convective boundary layers may remain deep with great thermals despite the big H on the surface chart.

Lastly it should be noted that in populated areas the lack of strong winds in the center of a high-pressure system can allow for the accumulation of aerosols, and the air in the boundary layer can get quite polluted. This has a stabilizing effect, especially in the early morning and late afternoon as the sun’s rays are intercepted by more of this dirty air before reaching the surface.

The take-home message is that narrow “bullet” thermals can result in high-pressure scenarios in certain locations. However, high pressure over your flying site is not enough to write off a flying day. A sounding offers far more insight than a surface pressure chart for predicting thermal characteristics.

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