CSULA - Geology: Severe Weather

Air Masses

Four basic types of air masses
- Continental polar (cP)
- Continental tropical (cT)
- Maritime polar (mP)
- Maritime tropical (mT)
- Air masses are classified on the basis of their source region
Air masses and weather
- North America (east of the Rocky Mountains)
  - Continental polar (cP)
  - From northern Canada and interior of Alaska
  - Winter – brings cold, dry air
  - Summer – brings cool relief
  - Responsible for lake-effect snows
  - cP air mass crosses the Great Lakes
  - Air picks up moisture from the lakes
  - Snow occurs on the leeward shores of the lakes
- North America (east of the Rocky Mountains)
  - Maritime tropical (mT)
  - From the Gulf of Mexico and the Atlantic Ocean
  - Warm, moist, unstable air
  - Brings precipitation to the eastern United States
- Continental tropical (cT)
  - Southwest and Mexico
  - Hot, dry
  - Seldom important outside the source region
- Maritime polar (mP)
  - Brings precipitation to the western mountains
  - Occasional influence in the northeastern United States causes the "Northeaster" in New England with its cold temperatures and snow

Fronts

Types of fronts
- Warm front
  - Warm air replaces cooler air
  - Shown on a map by a line with semicircles
  - Small slope (1:200)
  - Clouds become lower as the front nears
  - Slow rate of advance
  - Light-to-moderate precipitation Fronts
- Cold front
  - Cold air replaces warm air
  - Shown on a map by a line with triangles
  - Twice as steep (1:100) as warm fronts
  - Advances faster than a warm front
  - Associated weather is more violent than a warm front
  - Weather behind the front is dominated by
    - Cold air mass
    - Subsiding air
    - Clearing conditions
- Stationary front
  - Flow of air on both sides of the front is almost parallel to the line of the front
  - Surface position of the front does not move
- Occluded front
  - Active cold front overtakes a warm front
  - Cold air wedges the warm air upward
  - Weather is often complex
  - Precipitation is associated with warm air being forced aloft

Middle-latitude cyclone

Primary weather producer in the middle-latitudes
Idealized weather
- Middle-latitude cyclones move eastward across the United States
- First signs of their approach are in the western sky
- Require two to four days to pass over a region
- Largest weather contrasts occur in the spring
- Cloud patterns typically associated with a mature middle-latitude cyclone

Severe weather types: Thunderstorms

Stages
- Developing stage:
  - Warm, moist air moves up through condensation level
  - Developing cloud fed by latent heat of vaporization
  - Warm, buoyant air rises, condenses into towering clouds
  - More water vapor condenses, more latent heat, more cloud development = convection
- Mature Stage
  - Amount of ice crystals and rain exceeds updraft support = upper level precipitation
  - Falling rain creates downward drag and downdraft
  - Warm moist air going up, cold wet air going down = violent storm
- Dissipating Stage:
  - Downward air drag cools column enough to stop updrafts
  - Rainfall decreases in intensity
  - Cloud begins to evaporate
- Stages in the development of a thunderstorm
Tornadoes
- A rapidly spinning column of air descending from a large thunderstorm
- Have the highest wind speeds of any other weather phenomenon.
- In the U.S., tornadoes move northeast at speeds up to 100 km/hr (62 mph)
- Rotating wind speeds sometimes in excess of 500 km/hr (310 mph)
How a Tornado Forms
- Moist air from Gulf of Mexico
- Fast moving cold, dry air mass from Canada
- Jet stream moving east at 150 mph
- Sets up shearing conditions
- Warm moist Gulf air releases latent heat, creates strong updraft
- Updraft sheared by polar air, then twisted in a different direction by jet stream
Why do some thunderstorms spawn tornadoes while others do not?
Super Cell Thunderstorms
Scales
- The Fujita-Pearson Scale measures wind speed
  - The size of a tornado is not necessarily an indication of its intensity
  - Large tornadoes can be weak, and small tornadoes can be violent

Severe weather types: Hurricanes

Only natural disaster that is given a human name
- Actually large tropical cyclones
- Convert heat in the ocean into winds
- Exports excess heat from the tropics to the midlatitudes
How a Hurricane Works
- Tropical disturbance
  - Low pressure zone develops and draws in clusters of thunderstorms and winds
- Tropical depression
  - Surface winds strengthen, move about the center of the storm
  - Central core funnels warm moist air up towards stratosphere
  - Air cools, vapor condenses, latent heat released
  - Fuels more updrafts, cycle repeats, storm grows
- Tropical Storm
  - Storm has sustained surface wind speeds of +39 mph
- Hurricane
  - Surface winds consistently over 74 mph
- The Eye
  - As wind speed increases, winds are spiraled upwards prior to reaching the center
  - A distinctive clear “eye” is formed
  - Strongest winds are located on the walls of the eye Hurricane Origins
Form in the tropics ~ 5° and 20 ° latitude
- Cannot form at the equator (Coriolis effect = 0)
Hurricane Damages
- Storm Surges
  - Large mound of water builds up beneath the eye
  - Reaches land as a surge of water
  - Wind speed varies depending upon which side of the hurricane you’re on
  - Amount of damage on the coastline will vary accordingly
- Heavy Rains
- Mudflows and Debris Avalanches
- Flooding

Air Masses

Fronts

Middle-latitude cyclone

Severe weather types: Thunderstorms

Severe weather types: Hurricanes

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