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Severe Weather
Weather Dynamics
Science 10
Today we will learn about:
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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?
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It comes from thermal energy of warm, tropical, ocean water.
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Remember our trade winds who make warm water pile up on
the eastern coasts (El Niño)?
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These areas with deeper warmer water cause updrafts which
can form into massive storms.
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This is why hurricane season is typically certain months –
August, September, and October – after the water has been
warmed all summer.
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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.