{ 4.2 Low-pressure systems can become storms

 A tropical storm is a low-pressure system that starts near the equator and has winds that blow at 65km/h or more.  A hurricane is a tropical low-pressure system with winds blowing at speeds of 120km/h or more – strong enough to uproot trees.  DID YOU KNOW? Hurricanes are called typhoons or cyclones when they form over the Indian Ocean or the western Pacific Ocean. Hurricanes

Formation of hurricanes Begins as a low-pressure area over warm water, or a tropical disturbance As winds spiral inward, bands of high winds and heavy rains form. Warm, humid air rises and begins to spiral. As air rises,,ore warm, moist air is drawn into the system and the hurricane gains energy.

Structure of a hurricane In a hurricane, air moves rapidly around a low- pressure area called the eye.

 Can pound a coast with huge waves and sweep the land with strong winds and heavy rains.  It’s winds can lift cars, uproot trees and tear the roofs off buildings. Effects of hurricanes

 They also produce tornadoes that cause even more damage.  Heavy rains from hurricanes make rivers overflow their banks and flood nearby areas. < National Geographic Video > < National Geographic Video >

 When a hurricane moves into a coastal area, it often pushes a huge mass of water known as a storm surge.  In a storm surge, the sea level rises several meters and floods the shores.  They can be destructive and deadly with large waves adding to the destruction. Storm Surge

 Before a storm, prepare a plan to leave the area and gather emergency supplies.  Listen to weather reports for storm updates.  Secure loose objects outside, and cover windows.  If ordered to leave, evacuate immediately.  During a storm, stay indoors and away from windows.  After a storm, be aware of power lines, hanging branches and flooded areas. Safety Tips

 Winter storms are part of low-pressure systems that form when two air masses collide.  There are national weather services to alert people to dangerous weather.  A winter storm warning means that dangerous conditions are already present or will affect an area soon. Winter storms

 Blizzards are binding snowstorms with winds of at least 56km/h and low temperatures below -7C.  Strong winds and large amounts of snow can knock down trees, power lines and make it difficult to see or dangerous to travel.  Deep, heavy snow on top of buildings may cause roofs to cave in. Blizzards

 As cold air moves across the warmer water, it becomes more humid as water vapor evaporates from the lake’s surface.  When the air reaches land and cools, snow falls.  The lake effect can cover areas downwind of the Great Lakes with clouds and snow even when the rest of the area has clear weather. Lake-Effect Snowstorms

 Ice storms occur when rain falls onto freezing cold ground. The cold rain freezes as it touches the ground. The cold rain freezes as it touches the ground. This freezing rain covers everything with heavy, smooth ice.  The ice-covered roads become slippery and dangerous because: - cars become difficult to steer and control. - cars become difficult to steer and control. - branches or whole trees break due to the weight of the ice. - falling branches block roads, destroy power and telephone lines. Ice Storms