An immense body of air, 1000 miles across or more, which originate in a 'source region.
A Source Region is...
Where air masses originate. An ideal source region has:
an extensive and uniform area (flat land or body
stagnant atmospheric conditions (high presssure
*midwest is not a good source region because of a lack of stagnant atmosphere
Identifying Air Masses
Two letters. the first describes moisture content and the second describes temperature
Continental Polar Air Mass (cP)
Originates in Canada
Cold and Dry in winter, relatively cool and dry in summer
Continental Artic Air Mass (cA)
Originates over the artic ice caps - north of the cP region
Very cold and dry; colder than cP
Frigid weather in winter/temps below zero could last for more than a week
Continental Tropical Air Mass (cT)
Originates in Mexico/SW United States
Hot, dry air
Only found in summer and usually stays in sourcee region
Can advect north and give the S. Plains droughts
Maritime Polar Air Mass (mP)
Originates in cool ocean waters of Pacific or Atlantic
Cool, moist air that brings precip to coasts in winter but pleasant temps in summer
*midwest is rarely affected by mP air masses
Maritime Tropical Air Mass (mT)
Originates in the Gulf of Mexico and warm ocean waters off the coasts
Warm, moist air that is responsible for most of our precipitation throughout the year
Gives us heat waves in summer (oppressive hot, humid conditions)
Indicated by an area that was occupied by cool air that is moving away and is being replaced by warm air (a warm front will "overrun" the less dense cold front
Indicated by the boundry where a colder air mass is advancing on a warmer air mass; the slope of the cold front is steeper and forces the warm air to rise over it more vigorously
Indicated by cooler air mass is not moving toward or away from the warmer air mass not moving the boundry between (thus stationary)
Indicated (usually) by cold air behind a cold front moving faster than the cool air ahead of the warm front is retreating (ie; the cold front moving faster than the warm front)
What is a Dryline?
A dryline is indicated when air masses of like temperatures but different amounts of moisture collide
Hot, dry air is behind the dryline, with very warm, moist air ahead of it
Thus a cT behind the dryline and a mT ahead
What is the Polar-Front Theory
An idealized model of how a mid-latitude cyclone develops in conjunction iwth a polar front
Cyclones that, in the winter, form in the province of Alberta, Canada and move rather quickly bring cold air and some snow to the Upper Midwest
What is the most common area for Midwestern lows to form?
East Colorado. (These lows could be reforming from Pacific disturbances or they could be brand new formations due to the unstable Gulf of Mexico air advecting in)
Iowa can get large amounts of snow when...
The Colorado Lows track east to SE Kansas and then turn NE through Missouri
What is the "steering level" for mid-latitude cyclones?
500 mb the cyclone path tends to be directed by the 500 mb wind directions
the speed of the cyclone is generally about half as fast as the 500 mb wind speeds)
What are Blocking Highs
Highs that are strong and cover a large area and not move for weeks.
Areas under the High get dry conditions
Air pollution episodes often occur in large cities affected by blocking Highs because they inhibit vertical motion
Form from cumulonimbus coulds in warm, humid, and unstable enviroments (the warmer and more humid the more thunderstorms)
Need a lifting mechanism such as a front
To be classified as severe, a Thunderstorm must produce what?
1. Wind speeds of 50kts (58mph) or higher
2. Hail in diameter of 1 inch or larger3. A tornado
What is Vertical Wind Shear?
Changes in wind speed and/or direction with height
*necessary for severe thunderstorms*
What does a Roll/Shelf cloud mark?
The leading edge of the cold downdraft air that hit the ground and spread out pushing ahead of the storm (a "cold pool")
Define a Gust Front
It is the leading edge of a cold pool because it is where winds change direction and become stronger and gusty
Hurricanes form where?
Between 5 and 20 degrees latitude
Profile of Hurricane
Sustained winds of at least 74 mph
Diameters range from 100-1500km (avg 600km)
Pressure in/around a Hurricane
Drops from outer edge to center
spirals inward (from high to low pressure)
Winds increase closer to the center/stronger pressure gradient means stronger winds
Define an Eye Wall
intense convection surrounding the center of a hurricane
*the fastest winds and heaviest rainfalls*
Define an Eye
zone in the center of the hurricane where precipitation ceases and winds subside but still some clouds (a deceptive break in the storm)
A hurricane needs ____ to form
Large, continuous quantity of warm moist air
Ocean waters at at least 80 degrees
Enough spin (Coriolis force) to generte rotary motion
Saffir Simpson Scale
Measures Huricane destruction:
Cat 1: 79-95 mph (minimal)
Cat 2: 96-110 (moderate)
Cat 3: 111-130 (extensive)
Cat 4: 131-155 (extreme)
Cat 5: >155 (catastrophic)
Damage Caused by hurricanes is divided into what three classes?
Storm Surge: a dom of water 40-50 miles wide that sweeps across the coast near where the eye makes landfall (90% of hurricane deaths)
Wind Damage: covers large area though not as catastrophic as a storm surge (flying debris and tornados may form)
Inland Flooding: torrential rains with a system may affect places farther inland for several days after landfall (damage can sometimes exceed a storm surge)
An overshooting top indicates what?
That a storm is very strong and possibly severe.
Forms when both vertical speed and directional shear are present.
Favorable speed shear=winds increase with height
Favorable directional shear=winds turn clockwise with height
***The directional shear works with the updraft and creates the rotation***
What is a squall line?
A line of strong-to-severe thunderstorms (or mesoscale convective systems MCS)
What does it mean if the upper-air wind is "zonal"
The wind is flowing west to east and is nearly geostrophic (makes it difficult for mid-latitude cyclones to develop)
Stages of a Hurricane
Tropical Depression (TD) - once circulation develops
Tropical Storm (TS) - once sustained winds are between 39 and 73 mph (named here)
Hurricane - for winds above 74 mph
Hurricane Diminish when they:
Move over cooler ocean waters
Move onto land (rapid demise as water is cut off and the friction decreases wind speads)
Move into an area of unfavorable upper level flow
Front Develops - two air masses of different dens. are moving parallel in opposite directions
Wave Develops - instability aloft moves in and causes the movement of the air to begin and pressure to fall in the center (divergence aloft)
Cyclonic circulation - cold air moves behind the Low and warm air replaces cold ahead of the Low
Occlusion - cold air catches up to the front edge of the warm air. (cyclone is at the most intense stage)
Cyclone dissipates - occlusion lifts all the warm air off the ground eliminateing the temp gradient and creating stability
How does lightning form?
Particles in a cumulonimbous could become charged and separate into positive and negative; this eventually creates such a difference in charges that it must conduct electricity (takes electrons from the cloud and gives them to the ground)
occurs within a cloud or between clouds. represents 80% of observed lightning
Enhanced Fujita Intensity Scale (EF-scale)
Tornados are rated based on wind speed estimates obtained by evaluating the damage.