Thermals are warm bubbles of air generated by the sun’s heating of the earth’s surface. Early in the morning, the sun warms the earth which in turn then warms the air adjacent to the surface. Throughout the day, a mixed layer grows through turbulent thermals originating at the surface. At times, this motion may result in the development of cumulus clouds, creating a self limiting process as the clouds shield the earth’s surface from the sun. Of course, if the clouds are scattered in nature, thermals will continue bubble. This process will continue until either the solar source is cut off from the surface or until the suns angle becomes low enough where the sun can no longer heat the surface of the earth effectively. Once this occurs, the stronger winds from aloft are no longer mixed down to the surface and the overall wind speed and gusty conditions at the surface will likely decrease. This usually occurs within a few hours of sunset. Therefore, flying within a couple hours within sunset not only allows the winds at the surface to decrease but also decreases your potential of encountering a thermal.
The strength of each thermal varies, but because of simple physics, the same amount of displacement upward needs to be equalized by the equal amount of downward motion. Usually, the upward motion is much more concentrated than the areas of downward motion, so being aware of where thermals form at the surface can help you avoid these areas while in flight. Areas that absorb energy and warm relatively quickly are favored areas for thermal development. Some areas that are favored include: areas covered by rock (since rock heats relatively quickly), urban development with substantial areas of concrete, and dark areas of ground cover (recently plowed fields). Time of day can also influence thermal development in a particular area. If one area is exposed to more direct sunlight than another, it has a better chance of producing thermal activity. For example, if a dark colored hillside faces east it is more likely to develop thermals in the morning hours under the influence of direct sunlight. Additionally, you can use signs mother nature to identify the presence of a thermal. If you see two flags pointing towards each other, there is likely a thermal between them since the air is flowing toward a common point and can’t go into the ground. If you see something like this, do what you can to avoid the area.
As alluded to earlier, thermals can at times cause cumulus clouds to develop. These types of clouds are usually scattered throughout the sky, but the taller they are, the more unstable the atmosphere is, the stronger the thermals are, or a combination of the two. Cumulus clouds form at the top of thermals, but dissipate relatively slowly. If a cloud is fairly crisp looking, the cloud is fairly fresh and thermals are likely present in the area. Likewise, if the cloud is fuzzy on the edges there were likely thermals earlier in the day, but they have since weakened or dissipated. Don’t solely rely on the formation of clouds for identifying a thermal though, since a lack of clouds does not necessarily imply a lack of thermals.
Time of year also plays a significant role in the development of thermals. In the spring and summer, the sun’s angle results in more direct and rapid warming of the surface of the earth. However, in the late fall and winter much less energy reaches the surface of the earth, and some of the incoming energy may be reflected away by snow cover. Therefore, thermals are often times weaker or nonexistent.
If you are planning for an upcoming flight, you may be wondering how quickly thermals develop/subside? Unfortunately, there is no concrete answer, as the atmosphere is constantly changing; however, there are clues that will let you know how the atmosphere is expected to behave. For example, you may be able to use the dew or frost to determine how quickly thermals develop. The more overnight cooling, the greater the potential for dew. Ideal conditions for dew are clear skies and light winds, similar to conditions for the development of an inversion. If you remember from last month’s article, inversions are very stable layers of air in the atmosphere and actually suppress the development of thermals. Therefore, the heavier the dew, the greater the likelihood that a fairly strong inversion exists. As the sun rises and attempts to warm the earth, some of the solar energy is sacrificed to the process of evaporation. Therefore, as a general rule, thermals do not develop until the dew has begun to burn off. In the evening, you can use wind gusts as a clue to when the development of thermals decrease. In the Upper Midwest, thermals often times transport stronger winds from aloft down to the surface. As the thermals decrease in intensity, the wind gusts also decrease.
Thermals, which are ultimately powered by the sun, vary in both time as space as the atmosphere changes. The strongest thermal activity typically occurs in the late morning and afternoon hours, times at which balloons typically are not flying. However, thermals can be present at other times during the day as well. Knowing where thermals are likely to form at the surface, and keeping track of Mother Nature’s warning signs will help make your flight safe!
Over the past several months, I have received several questions about weather web sites. Several new sites have popped up over the last year or two, and in the next article we will discuss some of these sites. In the meantime, hope you have smooth, thermal-free sailing.
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