-      Thunderstorms are the most common type of storm.

-      These bring heavy rain, thunder, lightning, hail, and the odd tornado.

-      Two conditions must exist for a thunderstorm to form:

  • Moisture is needed to form clouds and rain.
  • Warm air must be pushed up in the atmosphere to form large cumulus clouds

-      As warm, moist air rises, a large cumulus cloud is created.  This cloud warms the air around it therefore allowing it to rise even higher.

-      During a thunderstorm, lightning and thunder are produced.

-      Lightning is produced when electricity travels between negatively and positively changed parts of a cloud.

-      This electricity discharges from the cloud and lightning flashes (which can create temperatures up to 27,760oC).

-      When lightning occurs, air expands and contracts rapidly creating a loud noise called thunder.

-      If lightning and thunder occur at the same time, why do we not hear and see the two events at the same time?  This is because light travels a lot faster than sound.  Therefore, when there is a large gap of time between lightning and thunder, the thunderstorm is further away.


  • Never take shelter under a tree during a thunderstorm.


Lightning will ALWAYS strike the tallest object around so that it can take the shortest path possible to the ground.  If you are outside during a thunderstorm, stay low and stay away from any tall object.

  • Lightning never strikes the same place twice.


Lightning will always strike the tallest object around.  The CN Tower is struck multiple times during a thunderstorm.


-      Tornadoes form when fast-rising air (which occurs in a thunderstorm) begins to spin, forming a funnel of warm air and speeds.

-      As the air rises and cools, the spinning speeds up.

-      The funnel eventually forms an extremely low pressure area where it touches the ground.

-      The low pressure area acts like a vacuum therefore sucking up all the material around it.

-      Wind speeds in the tornado can be as high as 500 km/h.


-      Hurricanes are common in the late summer or early fall over warm waters just north of the equator.

-      Air is warmed from the ocean and an updraft is created.  The air pressure in the center of the updraft is low therefore producing a swirling, circling, counterclockwise movement.

-      More warm air is continuously pulled into the centre of the storm therefore adding more energy to the storm.

-      Once the winds of the storm rise above ­119km/h, the tropical storm becomes classified as a hurricane.

-      The center of a hurricane is called the eye.  The eye of a hurricane is calm and clear.

-      Hurricane winds are usually not as fast as tornado winds, however, hurricanes can last for a couple of weeks and therefore create more damage.


-      Blizzards are severe snowstorms with strong winds and low temperatures.

-      Blizzards occur when winds are at a speed of 55km/h, temperatures are very low, and visibility is less than 200m.

-      Blizzards develop the same way as thunderstorms except the low temperatures bring snow instead of lightning.

-      Due to the lake-effect (remember section 4.5 – Regional Weather?) the area around the Great Lakes experiences a lot of blizzards.


-      Floods can occur anywhere in the world except Antarctica.

-      Floods are an excess of water that cannot be absorbed quickly enough by the ground.

-      There are two types of floods: flash floods and broadside floods.

-      Flash Floods

  • These occur with little or no warning usually in cities where water cannot be drained away quickly enough by storm sewers.

-      Broadside Floods

  • These cover large areas of land and can last for months.
  • These floods can usually be predicted in advance.
  • These floods may occur if a winter with heavy snowfall is followed by heavy rainfall in the spring.  This can cause rivers to overflow and flood surrounding areas.
  • These floods can also be caused by tropical storms.

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