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

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

Midlatitude Cyclones & Cyclogenisis:

• 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

• recall that the 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

Thunderstorms

• defn: a storm containing lightning & thunder; it may produce gusty winds with heavy rain & hail; severe T'storms may spawn tornadoes

• the storm itself may be a single cumulonimbus cloud, a cluster of them, or even a line of clouds stretching for more than 100 km (62 miles)

• T' storms develop: within an airmass, at/ahead of fronts (particularly cold fronts), along the ITCZ, and at mountainous locations

• the birth of a T'storm occurs when warm humid air rises in an unstable environment

• in North America most t'storms occur in areas dominated by mT air masses

Life Cycle of an Air-Mass T'Storm

Lightning: flashes of light resulting from electrical discharges within the cloud (the majority of strikes), between the cloud and the ground (~20% of strikes), from one cloud to another, or from a cloud to the surrounding air

• lightning is created when there is a buildup of electrical energy between areas within the cloud, or between the cloud and the ground

• for lightning to occur, separate regions containing opposite electrical charges must exist in a cumulonimbus cloud

"Hail" - forms within cumulonimbus clouds when raindrops are repeatedly carried above and below the freezing level - layers of ice are added until it becomes too heavy and can no longer be supported by the updrafts
 

Tornadoes:

• are rapidly rotating winds that blow around a small area of intense low pressure

• are spawned from severe T'storms

• "funnel cloud" - the visible swirling circulation extending from the parent cumulonimbus cloud

• "tornado" - a funnel cloud that extends all the way to the ground

• tornado diameter: a few meters to a few hundred meters

• typical forward speed: ~20 to 40 knots

• most last only a few minutes & have an average path length of ~ 7 km (4 mi)

• horizontal wind speeds in tornadoes: can exceed 300 mph - most have lesser wind speeds

• 65% of tornadoes are weak (wind speed < 112 mph)
• 33% of tornadoes are strong (wind speed:113-206 mph)
• 2% of tornadoes are violent (wind speed >207mph)

• all 50 states have experienced tornadoes (Fig. 17.17)

• North America is by far the biggest recipient of Tornadoes (avg. 800 annually)

• in particular, the greatest number occur in the "tornado belt" (alley) of the Central Plains - from Texas to Nebraska - because of its unique geographical location with regard to contrasting air masses

• 3/4 of tornadoes occur from March- July; May has greatest #; most frequent in late afternoon (4-6 pm)

• in order for a t'storm to spawn a tornado, the updraft (& the t'storm itself) must rotate

• this rising, spinning column of air (5-10 km across) is called a "mesocyclone"

• inside the mesocyclone a spinning vortex (tornado) may appear near the mid-level of the cloud and gradually extend downward to the cloud base; and then perhaps down to the ground

• Defn: an intense storm of tropical origin, with sustained winds exceeding 64 kts (74 mph)

• tropical cyclones have different names in different regions of the world

• form within the homogeneous air of the tropics (23.5 S-23.5N)

• Form over the warm (790F) tropical waters where winds are light - from summer-fall in the tropical N. Atlantic & N. Pacific oceans

• (Hurricane Season: June-November)

• in order for a hurricane to develop from an unorganized mass of T'storms, the surface winds must converge

• surface convergence takes place on the eastern side of an "easterly wave" (a weak westerly moving area of low pressure within the trades)

• hurricanes will not form on the equator - no coriolis force for convergence

• it is the release of latent heat that drives & fuels the hurricane

• the heat released aloft warms the upper troposphere, creating higher pressre aloft and diverging air - surface pressures then fall

• as long as outflow aloft exceeds surface inflow, the hurricane will intensify

• when hurricanes move over land (or colder water) they lose their source of heat and fuel & die out