The Nature of Storms

The Nature of StormsChapter 13

13.1THUNDERSTORMSAt any given moment, nearly 2000 thunderstorms are occurring around the world.

Conditions of Formation of Thunderstorms

1. Abundant source of moisture in the lower levels of the atmosphere

2. A mechanism to lift the air to allow moisture to condense and release latent heat

3. The portion of the atmosphere where clouds grow must be unstable, to allow rising and cooling of air necessary to stop the cloud growth.

13.2 SEVERE WEATHER Occasionally weather events come together in such a way that there is a continuous supply of surface moisture, causing more severe storms.

Lightning

Lightning is electricity caused by the rapid rush of air in a cumulonimbus cloud.

A lightning bolt forms when friction between updrafts and downdrafts separates electrons from some of their atoms creating positive ions and negative ions.

This creates regions of air with opposite charges.

A channel of negatively charged air moves toward the ground and a channel of positively charged ions rushes upward from the ground to meet it, creating an illumination called lightning.

Thunder

• Lightning heats the air to around 30000°C, which is five times HOTTER than the surface of the Sun!

• As the super-heated air rapidly expands and contracts, it creates a sound called thunder.

• Because sound travels more slowly than light, you typically see lightning before you hear thunder even though they are actually generated at the same time!

Other Dangers of Thunderstorms

• Hail: precipitation in the form of balls or lumps of ice, occurring most frequently in the central US. Most common during Spring

• Flood: occur when a thunderstorm moves slowly over one location, dumping all its rain in one place

TornadoesA tornado is a violent, whirling column of air in contact with the ground

Characteristics• Before reaching the ground, it’s

called a funnel cloud

• Often associated with a super cell, which is the most severe thunderstorm

• Air is made visible by dust and debris drawn into the swirling column or by the condensation of water vapor into a visible cloud

• Over the area it covers, few storms on Earth can match a tornado’s violence

Formation• A tornado is caused by wind shear,

which is when wind speed and direction change suddenly with height – causing a horizontal rotation near the Earth’s surface

• An updraft can shift this rotating air into the vertical position

• As updrafts speed up the rotation, air pressure in the center decreases, creating a pressure gradient between the inside and outside of the tornado – creating violent winds

• Although devastating, tornadoes typically last only a few minutes.

Classification and Distribution

The Fujita Tornado Intensity Scale ranks tornadoes according to the path of destruction, wind speed, and duration• Bottom range is F0 - winds up to 118km/hr• Top range is F5 – winds of more than 500km/hr• Most tornadoes do not exceed the F1 category

Tornadoes can occur at any time and at any place, although some times and locations are more conducive to their formation• Most occur in the spring, during late afternoon and evening, when temperature

differences are greatest• Most common in the central United States due to colliding cP and mT air masses• More than 700 tornadoes touch down each year in the United States

13.3 TROPICAL STORMSThe most violent storm on Earth is within the calm, sunny tropics.

Tropical Cyclones

• Large, rotating, low-pressure storm• The strongest tropical cyclones are known in the United

States, Mexico, and the Caribbean as hurricanes• In the western Pacific Ocean, these storms are referred to

as typhoons• People living near the Indian Ocean refer to these storms

as cyclones

CharacteristicsTropical Cyclones / Hurricanes require two basic conditions to form:1. Abundant supply of warm

water2. Some sort of disturbance to lift

the warm air and keeprising it

These conditions exist in all tropical oceans except the South Atlantic Ocean and the Pacific Ocean west of the South American coast because waters in these areas are cooler.

Formation1. As water evaporates from the

ocean, heat is stored in the form of latent heat

2. The heat is released as the air rises and water vapor condenses into clouds and rain

3. The rising air creates a low pressure system at the ocean surface and more air moves in to replace it

4. The Coriolis Effect causes the air to turn counterclockwise, resulting in the rotation of the cyclone

5. Air moving towards center rises and rotates faster, further decreasing pressure in the center

Stages of a Hurricane

Tropical Disturbance - beginning stages, causes the air to rise

Tropical Depression - when a disturbance acquires a cyclonic rotation around a center of low pressure

Tropical Storm - when winds of the depression reach 65km/hr

Hurricane - when winds of the storm reach 120km/hr

Eye - calm center of the storm

Eyewall - a band immediately surrounding the eye where the winds are the strongest

Classifying Hurricanes

• Saffir-Simpson Hurricane Scale - classifies hurricanes according to wind speed, air pressure in center, and potential for property damage

• Ranges from Category 1 (74 mph winds) to Category 5 (+155 mph winds)

• At Category 3 it is classified as a major hurricane

Hurricane Hazards

• Hurricanes cause a lot of damage, especially along coastal areas

• Much of the damage results from violent winds

• Strong winds move onshore and are responsible for another major threat, storm surges, which is when winds move a mound of water over land

• Storm surges can be as high as 6m above normal sea level

• Heat released through condensation of vast amounts of water vapor fuels hurricanes. This condensation produces great amounts of rain

13.4 RECURRING WEATHERPersistent or repetitive weather can negatively affect agriculture, transportation, and recreation.

Floods• Natural occurrence in which water spills over the sides of a stream’s

banks onto land• Can be caused by long thunderstorms, hurricanes, and mild storms that

stay in the same area for extended times• The main cause of thunderstorm-related deaths in the United States

each year 

Droughts• A drought is an extended period of below average rainfall• Caused by large high-pressure systems in an area for an extended

period of time• One of the most extreme droughts occurred during the 1930’s in the

central United States (Dust Bowl)

Heat Wave• A Heat Wave is an extended period of

above normal temperatures

• Can result from long term high-pressure systems

• Few clouds to block the blazing Sun

• System barely moves because the air currents guiding the high-pressure system are weak

• Increases rate of humidity

• Can cause heatstroke, sunstroke, and/or death

Cold Wave• A Cold Wave is an extended period

of below normal temperatures, which can be caused by high pressure systems of continental polar (cP) or Arctic air

• Wind-Chill: because wind transports heat away from the body, the temperature may feel cooler than it really is

• The wind-chill factor tells how cold it actually feels to the human body

Heat Index

Due to humidity, air may feel warmer than the real temperature

The heat index tells how warm it feels to the human body