Storms

Understanding Fronts

Fronts occur at the boundaries of converging air masses

Cold Fronts

• A cold front is defined as the transition zone where a cold air mass is displacing a warmer air mass

• Air associated with a cold front is usually unstable and conducive to cumulonimbus cloud formation

• Usually, rainfall associated with cold fronts is in the form

of heavy deluge.

Cold Fronts

Warm Fronts

• A warm front is defined as the transition zone where a warm air mass is replacing colder air mass.

• A warm front exists when warm air is rising over cold air

• As the air lifts into regions of lower pressure, it expands, cools and condenses water vapor as flat sheet cloud (altostratus),

Warm Fronts

Stationary Fronts

• A cold front is the boundary between cool and warm air when the cool air is replacing the warm air. A warm front is the boundary when the warm air is winning the battle. When the pushing is a standoff, the boundary is known as a stationary front. Stationary fronts often bring several days of cloudy, wet weather that can last a week or more.

Stationary Fronts

Supercell

• A thunderstorm with a persistent rotating updraft.

• Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds.

Supercell

Tornado Formation

• Warm moist air shoots upward meeting colder, dryer air.• Warm moist air is lighter than the cold dry air making for a

strong updraft within the thunderstorm. • If these varying winds are staggered in just the right

manner with sufficient speed, they will act on the upward rising air, spinning it like a top

Tornado FormationBefore 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

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.

How are Tornadoes measuredThe Fujita Scale

Tornado Rating Wind Speed Level of Damage

F0 40mph Light

F1 (weak) 73-112mph Moderate

F2 (strong) 113-157mph Considerable

F3 (strong) 158-206mph Severe

F4 (violent) 207-260mph Devastating

F5 (violent) 261-318mph Incredible

Hurricanes

• They are tropical, meaning that they are generated in tropical areas of the ocean near the Equator.

• They are cyclonic, meaning that their winds swirl around a central eye.

• The eye of a hurricane is always a low-pressure area. The lowest barometric pressures ever recorded have occurred inside hurricanes.

• The winds swirling around the center of the storm have a sustained speed of at least 74 mph

How is a hurricane formed?• Hurricanes usually originate as a cluster of showers

and thunderstorms in tropical waters

• Hurricanes form in tropical regions where there is warm water (at least 80 degrees Fahrenheit / 27 degrees Celsius), moist air and converging equatorial winds.

• Hurricane winds in the northern hemisphere circulate in a counterclockwise motion around the hurricane's center or "eye," while hurricane winds in the southern hemisphere circulate clockwise.

Some important definitions:

• Tropical Disturbance: An organized area of thunderstorms that holds itself together for at least 24 hours.

• Tropical Depression: A tropical cyclone (an area of low pressure that develops over tropical or subtropical waters with a definite organized surface circulation) in which the maximum sustained wind speed is 38mph or less.

• Tropical Storm: A tropical cyclone with maximum sustained winds of 39mph to 73mph.

The Saffir-Simpson Scale

Category Wind Speed Storm Surge Damage

Category 1 74-95mph 4-5 feet Minor

Category 2 96-110mph 6-8 feet Considerable

Category 3 111-130mph 9-12 feet Extensive

Category 4 131-155mph 13-18 feet More Extensive

Category 5 Greater than 155 18 and higher Catastrophic

Diagram of a Hurricane

Storm Surge

http://www.usatoday.com/graphics/weather/gra/gsurge/flash.htm

Nor'easter

• Gulf Stream low-pressure system - (counter-clockwise winds) These systems generate off the coast of Florida. The air above the Gulf Stream warms and spawns a low-pressure system. This low circulates off the southeastern U.S. coast, gathering warm air and moisture from the Atlantic. Strong northeasterly winds at the leading edge of the storm pull it up the east coast.

• Arctic high-pressure system - (clockwise winds) As the strong northeasterly winds pull the storm up the east coast, it meets with cold, Arctic air blowing down from Canada. When the two systems collide, the moisture and cold air produce a mix of

precipitation.