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Severe Weather Weather Dynamics Science 10 Today we will learn about: • • • • • • • • • • Thunderstorms Updrafts Downdrafts Tornados Hurricanes Typhoons Tropical Cyclones Tropical Depression Tropical Storms Monsoons Thunderstorms • Thunderstorms can be very destructive. • They bring lightning and strong gusting winds • They also bring torrential rains that can cause flash floods. • They can form “out of the blue” in a very short time What Causes Thunderstorms? • They form from cumulus clouds that continue to grow and develop into cumulonimbus clouds. • Only a small percentage of cumulus clouds ever develop into thunderclouds. • Thunderstorms are formed in 3 stages Stage 1 of Thunderstorm Development • Formation of a cumulus cloud (puff puff puff). • Cumulus clouds form when warm air rises up quickly. • This may happen because very warm ground is heated quickly by the Sun and starts the process of convection. Stage 2 of Thunderstorm Development • Even more warm air rises and condenses. • The condensation (changing gas to liquid) releases energy (remember heat of vaporization). • This energy is turned into heat which further heats the air and makes it rise even more. • This creates an updraft that pulls in more and more air from below, cycling over time. Stage 2 of Thunderstorm Development • The warm air rises so high that the top of the cloud freezes. • This very cold air at the top of the cloud starts to fall, creating a downdraft. • With updrafts and downdrafts moving throughout the cloud, much turbulence is created. If enough is created, hail will even form. Stage 2 of Thunderstorm Development • It is during the second, or mature, stage that the thundercloud produces lightning. • The extreme turbulence in the cloud causes ice crystals, snow particles, and water droplets to collide with great force. • This strips electrons from some of the water molecules, giving the cloud a positively charged top and negatively charged bottom. • Eventually the forces are so great between this charged parts of the cloud that a tremendous electrical discharge occurs. Stage 3 of Thunderstorm Development • Eventually the rain and the downdraft of cool air cut off the updraft of warm, moist air (the source of the storm). • The rain continues until the supply of moisture runs out. • Then the storm is over! Did you know? Throughout the world, there are nearly 40 000 thunderstorms every day?! Did you know? The largest thunderstorms deliver as much energy as a nuclear bomb! Tornadoes • The massive amount of energy delivered by a thunderstorm is sometimes released in the form of a tornado! • A tornado is a swirling funnel, or vortex, of air. • It destroys nearly everything in its path. • Until recently, tornadoes were difficult to study since they destroyed any instruments used to document them. • Today, we use radar to measure the speeds of tornadoes. • Tornadoes winds range from 60 km/hr to as high as 500 km/hr. Tornadoes • Nearly all thunderclouds have small cyclones of rotating air in them due to updrafts. • When any rotating object becomes narrower, it spins faster. • If the cloud and hence the cyclone happen to compress together, a “funnel cloud” or the start of a tornado is born. Hurricanes • A tornado may be more destructive to everything in its path, but a hurricane is much larger and lasts much longer! • Thus, hurricanes leave as much, if not more, damage than tornadoes. • Like tornadoes, hurricanes are related to thunderstorms • Hurricanes can be described as gigantic, 500km wide, whirling, moving thunderstorm. Where does all this hurricane energy come from? • It comes from thermal energy of warm, tropical, ocean water. • Remember our trade winds who make warm water pile up on the eastern coasts (El Niño)? • These areas with deeper warmer water cause updrafts which can form into massive storms. • This is why hurricane season is typically certain months – August, September, and October – after the water has been warmed all summer. • Hurricanes are called typhoons in the West Pacific and Tropical Cyclones in the Indian Ocean. Severe Weather • All severe weather storms involve fast winds. • When winds reach 37km/hr the storm is classified as a tropical depression • When winds reach 65km/hr the classification changes to tropical storm. • If the winds reach 120km/hr, the storm earns the classification “hurricane”. • All wind storms have a calm central zone known as the eye Monsoon • A monsoon is a system of winds that causes torrential rain and extensive flooding in the summer • It causes very dry conditions in the winter. • Southern Asia is the only region of the world that has unique positioning of land and oceans to create intense monsoons. • The flooding causes serious damage in the summer. • The absence of monsoons for even one season, however, would result in crop losses and famine due to lack of water.