1. Severe Weather Weather Dynamics Science 10

2. Today we will learn about: Thunderstorms Updrafts Downdrafts Tornados Hurricanes Typhoons Tropical Cyclones Tropical Depression Tropical Storms Monsoons

3. 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

4. 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

5. 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.

6. 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.

7. 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.

8. 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.

9. 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!

10. 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.

11. 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.

12. 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.

13. 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.

14. 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

15. 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.