Severe Weather Thunderstorms, Tornadoes, and Hurricanes Thunderstorms One of the most thrilling and dangerous weather phenomena Over 40,000 occur throughout the world each day Two main types ordinary and severe Ordinary Thunderstorms Common summer storm Usually last about an hour Precipitation includes rain and small hail Cumulonimbus clouds can reach 12,000 m Severe Thunderstorms Very dangerous Can produce baseball-sized hail, strong winds, intense rain, flash floods, and tornadoes Can last several hours Cumulonimbus clouds can reach 18,000 m Severe Thunderstorms This severe thunderstorm shows a shelf cloud that passed over Little Chute, WI on June 13, 2004. Stages of a Thunderstorm 1. Cumulus Stage Sun heats Earths surface during day Air is heated and rises (called updrafts) Warm moist air condenses into cumulus clouds Continues to grow as long as warm air rises Stages of a Thunderstorm 2. Mature Stage Water becomes large and heavy Raindrops start to fall through the clouds when the rising air can no longer hold them up Cool dry air enters the cloud and starts to descend (called downdrafts) Downdrafts pulls heavy water downward forming rain Is now a cumulonimbus cloud with lightning, thunder, hard rain, wind Stages of a Thunderstorm 3. Dissipative Stage Downdrafts dominate over updrafts Warm moist air can no longer rise Cloud droplets can no longer form Storm dies out with light rain Cloud disappears from top to bottom Stages of a Thunderstorm Hail Large frozen raindrops that are tossed around the cloud by the updrafts and downdrafts Forms in a cumulonimbus cloud where temperatures are cold enough to freeze water Grows larger when water droplets freeze on contact with the hailstone When its too heavy it will fall to the ground Hail This is one of the largest hailstones ever observed in the United States. The stone fell on June 22, 2003 in Aurora, Nebraska. Lightning Electrical energy - a BIG spark between the cloud and the ground/tree/another cloud Spark tries to balance the difference in charges Ice crystals are positively charged Hailstones are negatively charged Updrafts and downdrafts separate charges Thunder The noise made as the air around lightning is heated. The air expands faster than the speed of sound - you hear that expansion Tornadoes Winds that rotate very quickly Can have winds up to 310 mph Tornadoes Occur along a front between cool, dry air from the north and warm, moist air from the south Formation of Tornadoes From severe thunderstorms Wind coming into storm starts to swirl and form a funnel Spins faster and faster Creates a low pressure area and sucks more air (and other things) into it Funnel Cloud - part of a cloud that reaches down from the sky Tornado Hurricanes Hurricane Alex, a category 3 storm at its strongest, traveled north along the east coast of North America in August 2004 causing flooding, strong waves, and rip tides along the coast. Hurricanes Most powerful of all weather systems Are huge average 340 miles across Also called cyclones and typhoons Form from tropical depressions Winds are greater than 74 mph Coriolis effect causes the rotation of the storm 5. Eye of the Hurricane Weather is calm Sky is clear Winds are light Air from the storm sinks down in the center of the eye Eyewall (dense clouds surrounding the eye) has the strongest winds Eye of the Hurricane Eye of the storm - very low pressure center Naming Tropical Storms and Hurricanes Started in 1953 using only female names - in 1979, names alternate between male and female There is a list of pre - approved names each year They have 6 different lists that rotate Start at the beginning of the alphabet each year - if they get through Z, they start naming with Greek Letters (alpha, beta, gamma) Dont use Q or U for naming Names can be excluded if the hurricane was extremely devastating