1. Weather Patterns and Severe Storms

2. Air Masses and Weather
An air mass is an immense body of air that is characterized by similar temperatures and amounts of moisture at any given time.
Carries temp and moisture conditions with it when it moves, but can change (modify) over time

3. Classifying Air Masses
Named according to their source region
Continental (land)
Maritime (water)
Polar (high latitudes)
Tropical (low latitudes)
Describe both moisture and temperature

4. Weather in North America
cP - cold & dry in winter and cool and dry in summer; “lake-effect” snow
mT - warm, moist and unstable
mP - North Pacific and Atlantic; rain and snow
cT - least influence for NA; very hot and dry; drought-like conditions

5. Front Formation
When two air masses meet, they form a front, which is a boundary that separates two air masses.
Often associated with some form of precipitation.
When collide, pressure differences influence how the air masses will behave (one moves faster).
Classified according to the temperature of the advancing front.

6. Warm Front
Forms when warm air moves into an area formerly covered by cooler air
Warm air rises (less dense) on top of colder air (more dense)
Gradual slope, so light-to-moderate precipitation
Gradual increase in temperature when passes

7. Cold Front
Forms when cold, dense air moves into a region occupied by warmer air.
Steep slope as cold air wedges and forces warm air aloft; moves faster than warm front
Can lead to heavy downpours and severe weather
Temperatures drop when a cold front passes
Stationary front forms when air masses run parallel to each other

8. Occluded Front When an active cold front overtakes a warm front
Complex weather patterns
Much warm front precipitation, yet advancing cold air produces its own precipitation pattern

9. Middle-Latitude Cyclone
Main weather producer
Large centers of low pressure that generally travel from west to east and cause stormy weather
Counter-clockwise air movement
Most have a cold front and often a warm front
Forceful lifting so much precipitation

10. Thunderstorms A storm that generates lightning and thunder.
Frequently produce gusty, winds, heavy rain, and hail.
May be produced by a single cumulonimbus cloud or clusters of along a cold front
About 45,000/day
Most frequent in tropics

11. Thunderstorm Development
Form when warm, humid air rises in an unstable environment
Cumulus stage: strong updrafts provide moisture
Mature stage: heavy precipitation (winds, lightening, hail)
Dissipating stage: falling precipitation and descending cold air from above causes storm to die down

12. Tornadoes
Violent windstorms that take the form of a rotating column of air called a vortex which extends from a cumulonimbus cloud.
770/year in U.S.
Greatest frequency April through June (mT meets cP)
“Tornado Alley”

13. Tornado Development Typically form with severe t-storms
Often begin as a mesocyclone (“rolling” air at surface pushed up from rising air)
Air pressure 10% lower than surrounding air, so air near ground rushes in and spirals upward
Fujita scale measures intensity

14. Hurricanes
Whirling tropical cyclones that produce winds of at least 199 kilometers per hour
Generate high waves at sea and strong winds and flooding inland
Most form near equator (warmer ocean waters)
Most powerful storms on Earth

15. Hurricane Development
Fueled by energy (latent heat) when large quantities of water condense
Develop most often in late summer when water temperatures very warm
Begin as tropical disturbance (t-storms)
As warm air rushes in to core, air turns upward and rises creating doughnut-shaped wall (eye wall) where greatest winds and rain are found
Weakens over land and colder water (no “fuel”)

16. The bottom line… Severe weather occurs when unstable air is forced up
Severe weather is seasonal
Frontal wedging between cP and mT (Spring)
Warm ocean waters (Summer and Fall)
Isolated T-Storms (localized convective lifting in summer)
Lake-effect snow (cP moves over water)