An unstable atmosphere can provide great conditions for thermal soaring. If the atmosphere is too moist and unstable, however, cumulonimbus (Cb) or thunderclouds can form. Cb clouds are the recognized standard marker of thunderstorms. When Cb builds sufficiently, it changes from rainstorm to thunderstorm status. Well developed Cb clouds are thunderstorms. Not all precipitating, large cumulo-form clouds are accompanied by lightning and thunder, although their presence is usually an indication that conditions are ripe for full-blown thunderstorms. Forecasters sometimes use the term “deep convection” to refer to convection that rises to high levels, which usually means thunderstorms. The tremendous amount of energy associated with Cb stems from the release of latent heat as condensation occurs with the growing cloud.
Thunderstorms can occur any time of year, though they are more common during the spring and summer seasons. They can occur anywhere in the continental United States but are not common along the immediate West Coast, where an average of only about one per year occurs. During the summer months, the desert southwest, extending northeastward into the Rocky Mountains and adjacent Great Plains, experiences an average of 30 to 40 thunderstorms annually. Additionally, in the southeastern United States, especially Florida, between 30 and 50 thunderstorms occur in an average year. Thunderstorms in the cool seasons usually occur in conjunction with some forcing mechanism, such as a fast moving cold front or a strong upper-level trough. [Figure 9-15]
The lifecycle of an air-mass or ordinary thunderstorm consists of three main stages: cumulus, mature, and dissipating. The term “ordinary” describes the type of thunderstorm consisting of a single Cb, since other types of thunderstorms (described below) can occur in a uniform large scale air mass. The entire lifecycle takes on the order of an hour, though remnant cloud from the dissipated Cb can last substantially longer.
The cumulus stage is characterized by a cumulus growing to a towering cumulus (Tcu), or cumulus congestus. During this stage, most of the air within the cloud is going up. The size of the updraft increases, while the cloud base broadens to a few miles in diameter. Since the cloud has increased in size, the strong updraft in the middle of the cloud is not susceptible to entrainment of dryer air from the outside. Often, other smaller cumulus in the vicinity of the Tcu are suppressed by general downward motion around the cloud. Towards the end of the cumulus stage, downdrafts and precipitation begin to form within the cloud. On some days, small cumulus can be around for hours, before Tcu form, while on other days, the air is so unstable that almost as soon as any cumulus form, they become Tcu. [Figure 9-16]
As the evolution of the thunderstorm continues, it reaches its mature stage. By this time, downdrafts reach the ground and spread out in what are known as downbursts or microbursts. These often lead to strong and sometimes damaging surface winds. Gliders should not be flown or exposed to microbursts and the associated windshears. While increased headwinds can momentarily improve performance, the distance from the landing area may be increased beyond the capabilities of the glider. Attempting to launch with possible windshears can result in fatal consequences.
Note: Launching of a glider either behind a towplane or ground launch is risky. Given the size of windshears, the towplane could be in a tailwind situation while the glider is in a headwind situation. With the likely outcome of that combination of factors, and the hazards of ground handling, operations in such conditions must be severely discouraged.
Pilots need to watch apparently dissipating thunderstorms closely for new dark, firm bases that indicate a new cell forming. In addition, outflow from one Cb may flow several miles before encountering an area where the air is primed for lifting given an extra boost. The relatively cool air in the outflow can provide that boost, leading to new a Cb, which is nearby but not connected to the original Cb.
The LI is determined by subtracting the temperature of a parcel that has been lifted to 500 mb from the temperature of the ambient air. This index does not give the likelihood of thunderstorm occurrence; rather, it gives an indication of thunderstorm severity if one does occur. In Figure 9-17, an LI of –5 indicates moderately severe thunderstorms if they develop.
The KI is used to determine the probability of thunderstorm occurrence and uses information about temperature and moisture at three levels. It is given by the equation KI = (T850 – T500) + Td850 – (T700 – Td700). Here, T stands for temperature, Td is the dewpoint, and 500, 700, or 850 indicates the level in mb. All values are obtained from a morning sounding. Using the following values from a morning sounding KI = (16 – [–9]) + 12 – (6 – 0) = 31. This indicates about a 60 percent probability that thunderstorms will occur. [Figure 9-18] As discussed below, charts showing both the LI and KI for all the sounding sites in the continental United States are produced daily.
Thunderstorms have several hazards, including turbulence, strong updrafts and downdrafts, strong shifting surface winds, hail, icing, poor visibility and/or low ceilings, lightning, and even tornadoes. Once a cloud has grown to be a Cb, hazards are possible, whether or not there are obvious signs. Since thermal soaring weather can rapidly deteriorate into thunderstorm weather, recognition of each hazard is important. Knowledge of the many hazards may inspire the pilot to land and secure the glider when early signs of thunderstorm activity appear—the safest solution.
Moderate turbulence is common within several miles of a thunderstorm, and it should be expected. Severe or even extreme turbulence (leading to possible structural failure) can occur anywhere within the thunderstorm itself. The inside of a thunderstorm is no place for glider pilots of any experience level. Outside of the storm, severe turbulence is common. One region of expected turbulence is near the surface gust front as cool outflow spreads from the storm. Violent updrafts can be followed a second or two later by violent downdrafts, with occasional side gusts adding to the excitement—not a pleasant proposition while in the landing pattern. At somewhat higher altitudes, but below the base of the Cb, moderate to severe turbulence can also be found along the boundary between the cool outflow and warm air feeding the Cb. Unpredictable smaller scale turbulent gusts can occur anywhere near a thunderstorm, so recognizing and avoiding the gust front does not mean safety from severe turbulence.
Large and strong updrafts and downdrafts accompany thunderstorms in the mature stage. Updrafts under the Cb base feeding into the cloud can easily exceed 1,000 fpm. Near the cloud base, the distance to the edge of the cloud can be deceptive; trying to avoid being inhaled into the cloud by strong updrafts can be difficult. In the later cumulus and early mature stage, updrafts feeding the cloud can cover many square miles. As the storm enters its mature stage, strong downdrafts, called downbursts or microbursts, can be encountered, even without very heavy precipitation present. Downbursts can also cover many square miles with descending air of 2,000 fpm or more. A pilot unlucky enough to fly under a forming downburst, which may not be visible, could encounter sink of 2,000 or 3,000 fpm, possibly greater in extreme cases. If such a downburst is encountered at pattern altitude, it can cut the normal time available to the pilot for planning the approach. For instance, a normal 3-minute pattern from 800 feet AGL to the ground happens in a mere 19 seconds in 2,500 fpm sink!
When a downburst or microburst hits the ground, the downdraft spreads out, leading to the strong surface winds, known as thunderstorm outflow. Typically, the winds strike quickly and give little warning of their approach. While soaring, pilots should keep a sharp lookout between the storm and the intended landing spot for signs of a wind shift. Blowing dust, smoke, or wind streaks on a lake indicating wind from the storm are clues that a gust front is rapidly approaching. Thunderstorm outflow winds are usually at speeds of 20 to 40 knots for a period of 5 to 10 minutes before diminishing. However, winds can easily exceed 60 knots, and in some cases, with a slow-moving thunderstorm, strong winds can last substantially longer. Although damaging outflow winds usually do not extend more than 5 or 10 miles from the Cb, winds of 20 or 30 knots can extend 50 miles or more from large thunderstorms.
Hail is possible with any thunderstorm and can exist as part of the main rain shaft. Hail can also occur many miles from the main rain shaft, especially under the thunderstorm anvil. Pea-sized hail usually does not damage a glider, but the large hail associated with a severe storm can dent metal gliders or damage the gelcoat on composite gliders, whether on the ground or in the air.
Icing is usually a problem only within a cloud, especially at levels where the outside temperature is approximately –10 °C. Under these conditions, supercooled water droplets (water existing in a liquid state 0 °C and below) can rapidly freeze upon contact with wings and other surfaces. At the beginning of the mature stage, early precipitation below cloud base may be difficult to see. At times, precipitation can even be falling through an updraft feeding the cloud. Snow, graupel, or ice pellets falling from the forming storm above can stick to the leading edge of the wing, causing degradation in performance. Rain on the wings can be a problem since some airfoils can be adversely affected by water.
Poor visibility due to precipitation and possible low ceilings as the air below the thunderstorm is cooled is yet another concern. Even light or moderate precipitation can reduce visibility dramatically. Often, under a precipitating Cb, there is no distinction between precipitation and actual cloud.
Lightning in a thunderstorm occurs in cloud, cloud to cloud (in the case of other nearby storms, such as a multicell storm), or cloud to ground. Lightning strikes are completely unpredictable, and cloud-to-ground strikes are not limited to areas below the cloud. Some strikes emanate from the side of the Cb and travel horizontally for miles before turning abruptly towards the ground. Inflight damage to gliders has included burned control cables and blown-off canopies. In some cases, strikes have caused little more than mild shock and cosmetic damage. On the other extreme, a composite training glider in Great Britain suffered a strike that caused complete destruction of one wing; fortunately, both pilots parachuted to safety. In that case, the glider was two or three miles from the thunderstorm. Finally, ground launching, especially with a metal cable, anywhere near a thunderstorm should be avoided.
Severe thunderstorms can sometimes spawn tornadoes, which are rapidly spinning vortices, generally a few hundred to a few thousand feet across. Winds can exceed 200 mph. Tornadoes that do not reach the ground are called funnel clouds. By definition, tornadoes form from severe thunderstorms and should obviously be avoided on the ground or in the air.