STORMS AND SEVERE WEATHER

STORM: violent or severe disturbance of the atmosphere that usually creates dangerous, destructive, or unpleasant conditions on Earth’s surface.

Storms and Severe WeatherMost storms are associated with: High winds Heavy precipitation Fronts Low air pressure of cyclonic conditionsPeople need to be aware that storms can: Cause loss of life Injuries Loss of property

Mid-latitude or Cyclonic Storms

Caused by movement of air masses along polar fronts of the middle latitudesResults in: formation of low pressure storm systems (mid-latitude cyclones)Often associated with many thunderstorms and associated hail and tornadoes.

HURRICANES

An Intense low pressure system that starts as a tropical low.– Winds sustained at 74 mi/hr (120 km/hr)– Storm surge causes most of the damage.– Energy from latent heat released during condensation

fuels the storm– Spin due to the Coriolis Effect– Characterized by rotating bands of precipitating clouds.– Lowest pressure is in the center of the storm (cloudless

eye)

Hurricanes continued. High winds can get to over 150 mi/hr When a hurricane passes over land or cold

ocean water, it loses its energy source of evaporating water and slowly dissipates.

Greatest threat is when a hurricane hits land.

In the U.S. the southern and eastern coasts are most vulnerable.

Hurricane Formation

Saffir-Simpson ScaleCategorizes hurricanes

THUNDERSTORMSThey are heavy rainstorms accompanied by thunder and lightning from Cumulonimbus cloudsA line of thunderstorms is called a squall

Thunderstorm lifecycleThe typical lifecycle of a thunderstorm consists of three stages: towering, mature cumulus, and dissipating stage

• Towering cumulus cloud indicates lifting air.

• Little if any rain

• Lasts about 10 minutes.

• Occasional lightning

Mature Stage

• The air continues to rise up to the tropopause, forcing the air to be spread out.

• Hail, heavy rain, lightning, strong winds likely.

• Lasts an average of 10 to 20 minutes .

Dissipating Stage• Thunderstorm

dissipates and is dominated by a downdraft.

• Rainfall decreases in intensity.

• Three important ingredients:

1. Moisture2. Instability3. Lifting mechanism

Thunderstorm hazards Thunderstorms can create many hazards including flooding, hail, high

winds, and lightning.In many years, more people get killed from lightning than any other

weather-related cause.

Hail Formation

TORNADOES

Most violent of all storms. By definition, a tornado is a rapidly

rotating extremely low pressure funnel that hangs down from a column of air extending from a thunderstorm to the ground.

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Tornado characteristics

• Wind speeds 40 mph to 110 mph

• Often travel a few miles before dissipating.

• Rotate counterclockwise in NH. Opposite in SH

Tornado formation

Before thunderstorms develop, vertical wind shear creates an invisible, horizontal spinning effect in the lower atmosphere

Updraft within the thunderstorm tilts the rotating tube from horizontal to vertical.

An area of rotation of few miles wide is formed within the storm. Most tornadoes form within this area of strong rotation

Tornado scales

Types of Tornadoes