A thermal is a rising mass of buoyant air. Thermals are the most common updraft used to sustain soaring flight. In the next sections, several topics related to thermal soaring weather are explored, including thermal structure, atmospheric stability, the use of atmospheric soundings, and air masses conducive to thermal soaring.
Convection refers to an energy transfer involving mass motions. Thermals are convective currents and are one means by which the atmosphere transfers heat energy vertically. Advection is the term meteorologists use to describe horizontal transfer; for instance, cold air advection after the passage of a cold front. As a note of caution, meteorologists use the word “convection” to mean deep convection, that is, thunderstorms. Unfortunately, there is often a fine meteorological line between a warm, sunny day with plenty of thermals, and a warm, sunny day that is stable and produces no thermals. To the earthbound general public, it matters little—either is a nice day. Glider pilots, however, need a better understanding of these conditions and must often rely on their own forecasting skills.
Thermal Shape and Structure
Two primary conceptual models exist for the structure of thermals: bubble model and column or plume model. Which model best represents thermals encountered by glider pilots is a topic of ongoing debate among atmospheric scientists. In reality, thermals fitting both conceptual models likely exist. A blend of the models, such as individual strong bubbles rising within one plume, may be what occurs in many situations. These models attempt to simplify a complex and often turbulent phenomenon, so many exceptions and variations are to be expected while actually flying in thermals. Many books, articles, and Internet resources are available for further reading on this subject.
The bubble model describes an individual thermal resembling a vortex ring, with rising air in the middle and descending air on the sides. The air in the middle of the vortex ring rises faster than the entire thermal bubble. The model fits occasional reports from glider pilots. At times, one glider may find no lift, when only 200 feet below another glider climbs away. At other times, one glider may be at the top of the bubble climbing only slowly, while a lower glider climbs rapidly in the stronger part of the bubble below. [Figure 9-7] More often, a glider flying below another glider circling in a thermal is able to contact the same thermal and climb, even if the gliders are displaced vertically by 1,000 feet or more. This suggests the column or plume model of thermals is more common. [Figure 9-8]
Which of the two models best describes thermals depends on the source or reservoir of warm air near the surface. If the heated area is rather small, one single bubble may rise and take with it all the warmed surface air. On the other hand, if a large area is heated and one spot acts as the initial trigger, surrounding warm air flows into the relative void left by the initial thermal. The in-rushing warm air follows the same path, creating a thermal column or plume. Since all the warmed air near the surface is not likely to have the exact same temperature, it is easy to envision a column with a few or several imbedded bubbles. Individual bubbles within a thermal plume may merge, while at other times, two adjacent and distinct bubbles seem to exist side by side.
No two thermals are exactly alike since the thermal sources are not the same.
Whether considered a bubble or column, the air in the middle of the thermal rises faster than the air near the sides of the thermal. A horizontal slice through an idealized thermal provides a bull’s-eye pattern. Real thermals usually are not perfectly concentric; techniques for best using thermals are discussed in the next chapter. [Figure 9-9]
The diameter of a typical thermal cross-section is on the order of 500–1,000 feet, though the size varies considerably. Typically, due to mixing with the surrounding air, thermals expand as they rise. Thus, the thermal column may actually resemble a cone, with the narrowest part near the ground. Thermal plumes also tilt in a steady wind and can become quite distorted in the presence of vertical shear. If vertical shear is strong enough, thermals can become very turbulent or become completely broken apart. A schematic of a thermal lifecycle in wind shear is shown in Figure 9-10.
Stability in the atmosphere tends to hinder vertical motion, while instability tends to promote vertical motion. A certain amount of instability is desirable for glider pilots; without it, thermals would not develop. If the air is moist enough and the atmospheric instability is deep enough, thunderstorms and associated hazards can form. Thus, an understanding of atmospheric stability and its determination from available weather data is important for soaring flight and safety. As a note, the following discussion is concerned with vertical stability of the atmosphere. Other horizontal atmospheric instabilities, such as the evolution of large-scale cyclones, are not covered here.
Generally, a stable dynamic system is one in which a displaced element returns to its original position. An unstable dynamic system is one in which a displaced element accelerates away from its original position. In a neutrally stable system, the displaced element neither returns to nor accelerates from its original position. In the atmosphere, it is easiest to use a parcel of air as the displaced element. The behavior of a stable or unstable system is analogous to aircraft stability discussed in Chapter 3, Aerodynamics of Flight.
For simplicity, assume first that the air is completely dry. Effects of moisture in atmospheric stability are considered later. A parcel of dry air that is forced to rise expands due to decreasing pressure and cools in the process. By contrast, a parcel of dry air that is forced to descend is compressed due to increasing pressure and warms. If there is no transfer of heat between the surrounding, ambient air and the displaced parcel, the process is called adiabatic. Assuming adiabatic motion, a rising parcel cools at a lapse rate of 3 °C (5.4 °F) per 1,000 feet, known as the dry adiabatic lapse rate (DALR).
The DALR is the rate at which the temperature of unsaturated air changes as a parcel ascends or descends through the atmosphere which is approximately 9.8 °C per 1 kilometer. On a thermodynamic chart, parcels cooling at the DALR are said to follow a dry adiabatic. A parcel warms at the DALR as it descends. In reality, heat transfer often occurs. For instance, as a thermal rises, the circulation in the thermal itself (recall the bubble model) mixes in surrounding air. Nonetheless, the DALR is a good approximation.
The DALR represents the lapse rate of the atmosphere when it is neutrally stable. If the ambient lapse rate in some layer of air is less than the DALR (for instance, 1 °C per 1,000 feet), then that layer is stable. If the lapse rate is greater than the DALR, it is unstable. An unstable lapse rate usually occurs within a few hundred feet of the heated ground. When an unstable layer develops aloft, the air quickly mixes and reduces the lapse rate back to DALR. It is important to note that the DALR is not the same as the standard atmospheric lapse rate of 2 °C per 1,000 feet. The standard atmosphere is a stable one.
Another way to understand stability is to imagine two scenarios, each with a different temperature at 3,000 feet above ground level (AGL), but the same temperature at the surface, nominally 20 °C. In both scenarios, a parcel of air that started at 20 °C at the surface has cooled to 11 °C by the time it has risen to 3,000 feet at the DALR. In the first scenario, the parcel is still warmer than the surrounding air, so it is unstable and the parcel keeps rising—a good thermal day. In the second scenario, the parcel is cooler than the surrounding air, so it is stable and sinks. The parcel in the second scenario would need to be forced to 3,000 feet AGL by a mechanism other than convection, such as being lifted up a mountainside or a front. [Figure 9-11]
Figure 9-11 also illustrates factors leading to instability. A stable atmosphere can turn unstable in one of two ways. First, if the surface parcel warms by more than 2 °C (to greater than 22 °C), the layer to 3,000 feet then becomes unstable in the second scenario. Thus, if the temperature of the air aloft remains the same, warming the lower layers causes instability and better thermal soaring. Second, if the air at 3,000 feet is cooler, as in the first scenario, the layer becomes unstable. Thus, if the temperature on the ground remains the same, cooling aloft causes instability and better thermal soaring. If the temperatures aloft and at the surface warm or cool by the same amount, then the stability of the layer remains unchanged. Finally, if the air aloft remains the same, but the surface air cools (for instance, due to a very shallow front), then the layer becomes even more stable.
An inversion is a condition in which a layer warms as altitude increases. Inversions can occur at any altitude and vary in strength. In strong inversions, the temperature can rise as much as 10 °C over just a few hundred feet of altitude gain. The most notable effect of an inversion is to cap any unstable layer below. Along with trapping haze or pollution below, it also effectively provides a cap to any thermal activity.
So far, only completely dry air parcels have been considered. However, moisture in the form of water vapor is always present in the atmosphere. As a moist parcel of air rises, it cools at the DALR until it reaches its dewpoint, at which time the air in the parcel begins to condense. During the process of condensation, heat (referred to as latent heat) is released to the surrounding air. Once saturated, the parcel continues to cool, but since heat is now added, it cools at a rate lower than the DALR. The rate at which saturated air cools with height is known as the saturated adiabatic lapse rate (SALR). Unlike the DALR, the SALR varies substantially with altitude. At lower altitudes, it is on the order of 1.2 °C per 1,000 feet, whereas at middle altitudes it increases to 2.2 °C per 1,000 feet. Very high up, above approximately 30,000 feet, little water vapor exists to condense, and the SALR approaches the DALR.