A. Air Masses

air mass (Defn)- a huge volume of air (hundreds of square miles in horizontal size) - uniform in its temperature and humidity characteristics

air mass advection (Defn) - movement of an air mass (and thus its properties of temp & moisture) in the horizontal from one location to another

Air Masses Affecting North America

• continental arctic (cA) - very cold & dry
• continental polar (cP) - cold & dry
• continental tropical (cT) - hot & dry
• maritime polar (mP) - cool & moist
• maritime tropical (mT) - warm & moist

B. Atmospheric Lifting Mechanisms

1) Orographic Lifting - air is lifted upslope as it is pushed against a mountain; the airs cools adiabatically as it is lifted

• the "windward" slope receives more precipitation than the "leeward" (downwind) slope of the mountain

2) Convection (convectional lifting) - a cooler (& moist) air mass moves over a warmer land area -heating from the land causes the air to rise - if the atmosphere is unstable it continues to rise (e.g., occur at: tropical islands, Florida, ITCZ)

            
Average Annual # of T'Storm Days

3) Fronts - also provide a means of atmospheric lifting

C. Fronts

Front (defn) - a transition zone between two air masses having different densities (temperatures)

• fronts typically separate air masses with different temperatures and humidities

• since air masses have both a vertical and horizontal extent, a front also extends upward in the vertical - the "frontal surface" (frontal zone)

Fronts, Air Masses, Pressure Systems
 

(1) Cold Front

• cold, dry, stable polar air is advancing into warm, moist, unstable tropical air

• a cold front is represented by a solid blue line with triangles pointing in the direction that the front is moving

(a) sharp temperature changes in the horizontal

(b) sharp moisture (Td) changes in the horizontal

(c) wind direction shifts

(d) clouds and precipitation
 
 

• advancing warm, moist, tropical air replaces colder air
• is depicted by a solid red line with half-circles pointing in the direction of movement (movement is toward the colder air)

• warm fronts move more slowly than cold fronts (~1/2 the speed of cold fronts)

• the rising of warmer, less-dense air over the colder more-dense air is referred to as "overrunning"

• overrunning produces clouds and precipitation well in advance of the front's surface boundary

• warm fronts have more inclined slopes than do cold fronts

(3). Stationary Fronts

• there is essentially no movement of the front

• is depicted by alternating red semicircles and blue triangles (the blue triangles point into the warmer air; while the red semicircles point into the colder air

• occur when a cold front catches up with and overtakes a warm front, and all the warm air is forced aloft

D. Midlatitude Cyclones & Cyclogenisis:

                    
Satellite Image of a Mid-Latitude Cyclone

• the frontal systems that we've been examining are part of a larger storm system referred to as a "middle latitude (wave) cyclone"

• midlatitude (wave) cyclones - are migrating centers of low pressure - the air flows inward and CCW in the N. Hemisphere (inward & CW in the S. Hemisphere) - there is rising motion in the center of the low/cyclone > clouds & precip.

• Norwegian meteorologists (just after W.W. I) developed a model explaining the life cycle of the mid-latitiude cyclone called the "polar front theory"

• according to this theory/model the cyclone begins its development along the polar front

• cyclogenisis (defn) - the development or strengthening of a mid-latitude cyclone

• in addition to the polar front, there are other preferred areas of cyclogenisis: eastern slopes of the Rockies ("lee-side-lows"), the Gulf of Mexico, and the Atlantic Ocean east of the Carolinas.

• Lifecycle of a midlatitude cyclone:

• "birth", "maturity", & "death"

Cyclogenisis

• why do some waves grow into big storms, while others never really develop?

• we need to look to the upper level winds for the answer !!

 Upper-Level Mid-Latitude Westerlies

• the air-flow aloft is comprised of a series of waves, both long and short

• longwaves (Rosby Waves): wavelengths of thousands of miles; 4-to-6 around the globe; move very slowly

• shortwaves: move more quickly through the longwaves; deepen/weaken when they approach a trough/ridge

• these deep troughs are important for the development of surface lows because they are associated with divergence and convergence

• convergence - the piling-up of air above a region - aloft it leads to the formation & maintenance of surface anticyclones (highs)

• divergence - the spreading-out of air above a region - aloft it leads to the formation & maintenance of surface cyclones

• for a surface low (cyclone) to be maintained/deepened, divergence aloft must exceed surface convergence

• for a surface high (anticyclone) to be maintained/intensified, convergence aloft must exceed surface divergence

• convergence aloft occurs on the west side of the trough; divergence aloft occurs on the east side of the trough