Severe Weather

QuickTime™ and a decompressor

are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.

QuickTime™ and a decompressor

are needed to see this picture.QuickTime™ and a

decompressorare needed to see this picture.

What is severe weather?

• Refers to any dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life.

• Types of severe weather vary depending on latitude, topography, and atmospheric conditions.

What causes severe weather?

• Severe weather occurs from the same conditions that generate ordinary thunderstorms: atmospheric moisture, lift, and instability.

QuickTime™ and a decompressor

are needed to see this picture.

Thunderstorms• Thunderstorms result from the rapid upward movement of warm, moist air.

• They can occur inside warm, moist air masses and at fronts.

• As the warm, moist air moves upward, its cools, condenses, and forms cumulonimbus clouds that can reach heights of over 20 km.

Thunderstorms• Thunderstorms can generally form and develop in any geographic location, perhaps most frequently within areas located at mid-latitudes when warm moist air collides with cooler air.

QuickTime™ and a decompressor

are needed to see this picture.

Lightning and Thunder

• Lightening– The movement of air can cause different parts of the cloud to become oppositely charged.

– When current flows between regions of opposite electrical charge, lightning flashes.

Lightning and Thunder• Thunder

– Results from the rapid heating of air around a bolt of lightening.

– Lightening can reach temperatures of 30,000°C, which is 5x hotter then the surface of the Sun.

– This extreme heat cools quickly and contracts.

– The rapid movement of molecules form sound waves heard as thunder.

What is a Tornado?• A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground.

• The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more.

• Damage paths can be in excess of one mile wide and 50 miles long.

QuickTime™ and a decompressor

are needed to see this picture.

What causes a Tornado?

• Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts

• Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east.

What Causes a Tornado?

• During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours.

How do Tornadoes Form?• Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.

• Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.

How do Tornadoes Form?

• An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

Classifying a Tornado• Forecasters and researchers currently use

a wind damage scale created in 1972 by T. Theodore Fujita to classify tornadoes and sometimes the damage done by other wind storms. The F (for Fujita) - scale uses numbers from 0 through 5. The ratings are based on the amount and type of wind damage.

Fujita Tornado Damage Scale

Tornadoes in the News• April 25-28

– Super Outbreak- 300 confirmed tornadoes with 317 fatalities through Midwest and parts of the Northeast.

– On April 27, a large tornado struck Tuscaloosa Alabama, killing at least 41 people. The Tuscaloosa mayor called the damage "catastrophic.

• May 21-26– very large and intense multiple vortex tornado resulted in catastrophic damage in Joplin, Missourii.

– Many houses and businesses were flattened and some even were blown away in Joplin.

Hurricanes• Hurricanes are also known as a Tropical Cyclone.

• They are characterized as large low pressure centers and numerous thunderstorms that produce strong winds and heavy rain.

How do Hurricanes Form?

• They are born in moist tropical air. • In developing tropical cyclones, strong thunderstorms occur. Air pressure drops at the surface of these storms.

• This low pressure attracts warm moist air from the ocean's surface.

• The Coriolis Effect causes the low-level winds to spin in a counter-clockwise direction around the center of the low pressure system.

Tropical Cyclone Stages• Tropical Disturbance

– A tropical disturbance is a discreet system of clouds, showers, and thunderstorms that originates in the tropics and maintains its identity for 24 hours or more.

Tropical Depression– When a tropical disturbance develops a closed circulation. Has sustained wind speed of 38 mph.

Tropical Cyclone Stages

• Tropical Storm– A tropical cyclone is given a name once it reaches tropical storm status. Must has sustained winds of 39-73mph.

• Hurricane– Must have sustained winds of at least 74mph. Most hurricane winds are stronger.

Categorizing a Hurricane

• Hurricanes are categorized on a scale of 1 to 5 based on – Wind speed– Barometric Pressure– Destructive Potential

•Based on the Saffir-Simpson Scale

Blizzard

• A severe snow storm that is categorized by strong winds.

• To be a blizzard, a snow storm must have winds in excess of 56 km/h (35 mph) with blowing or drifting snow which reduces visibility to 400 meters or ¼ mile or less and must last for a prolonged period of time — typically three hours or more.[

Nor’easter• A blizzard type storm that occurs along the East coast of the US and Canada.

• So named because the storm travels to the northeast from the south and the winds come from the Northeast.

• It is a low pressure system with the center just off the east coast, in which the counter-clockwise rotation brings precipitation on to the land.

Nor’easter

• Can cause coastal flooding, hurricane force winds and coastal erosion.

• They can happen at anytime of the year but are most known for their presence in winter.

• Nor’easters form due to converging air masses cold air from the poles and warm air masses from the south.