K Index < 15 15 to 20 21 to 25 26 to 30 31 to 35 36 to 40 > 40 Thunderstorm Probability (%) near 0 20 20 to 40 40 to 60 60 to 80 80 to 90 near 100  Figure 9-18.  K-Index (KI) versus probability of thunderstorm  occurrence. 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  9-13