GEOG 140
Intro Physical Geography
Lecture Notes
Weather
Air masses
-very large parcel of air in the boundary layer of the troposphere that possesses
relatively uniform qualities of
-temperature
-density
-humidity
-North American air masses
-continental polar (cP)
-forms only in the Northern Hemispher
-important in middle and high latitude weather
-cold dense air forces warm air aloft (fronts)
-area under-cold, stable air, clear skies, high pressure,
anticyclonic flow
-maritime polar (mP)
-cool moist unstable conditions throughout year
-subpolar low-pressure cells reside within mP air masses
-more pronounced in winter
-martime tropical (mT)
-Gulf/Atlantic
- very unstable late spring to early fall
-high moisture content
-Pacific
-weaker, less moisture
-semi-arid western US
Precipitation
-four lifting mechanisms that produce precipitation
-convergent-lifting precipitation
-warm moist air flows converge near surface
-northern and southern hemisphere trade winds come together in
the ITCZ
-convectional precipitation
-convection usually localized to a few square miles/kilometers
of the surface
-column of rising air (convection cell)
-cools at dry adiabatic rate
-continues as long as relative humidity is under 100%
-dew point reached –condensation and release of latent heat
-heat increases lifting/updraft
-three required conditions
-sufficient water vapor to sustain cloud formation
-unstable atmosphere –upward airflow continues
-weak winds aloft-do not disrupt vertical development
-life cycle of a thunderstorm-see fig 8.6b in book
-orographic precipitation
-mountain barrier forces moist maritime air aloft
-example and figure 8.6c in book
-frontal (cyclonic precipitation )
-associated with collision of air masses of significantly different
temperature/moisture/density
Fronts
-cold front
-cold air replaces warm air
-dense air replaces lighter air
-example-cP replaces mT
-summer contrast due more to contrast in water vapor pressure than temperature
-dense air at the surface creates frictional drag
-results in a steep edge to the front
-uplift restricted to a narrow zone along the leading edge
-frontal weather depends on stability of warmer air
-relatively stable
-nimbostratus and altostratus clouds
-showery precipitation
-relatively unstable
-vigorous uplift
-cumulonimbus clouds
-thunderstorms-possibility of strong winds and violent weather
-rapid frontal movement – squall line of vigorous thunderstorms
develop ahead of the front
-warm front
-warm air replaces cold air
-lighter air overrides denser air
-gently sloping frontal surface – hundreds of miles ahead of surface warm front
-cloud sequence (500-600 miles in advance)
-cirrus
-cirrostratus
-altostratus
-nimbostratus
-large amounts of rain for 24 hrs or more
-intensity light to moderate
-soil infiltration of precipitation
-unstable warm air – more vigorous storms
-must be sufficient contrast between air masses for stormy weather
-stationary front
-neither cold or warm air is advancing
-occluded front
-contrasting air masses (cold, warm, and cool) interact
-form boundaries above the ground and at the surface
-cold occlusion
-cold air overtakes cool air
-cold-warm boundary aloft
-behaves similar to a cold front
-warm occlusion
-cool air overtakes cold air
-warm-cold boundary aloft
-surface weather similar to a warm front
Mid Latitude Cyclones
-principal weather producer in the mid latitudes
-counterclockwise/inward surface flow around a center of low pressure
-brings together contrasting air masses
-birth of a cyclone
-surface westerlies meet and override polar easterlies along the polar front
-under an area of upper level divergence
-diverenge aloft exceeds convergence at the surface
-surface air pressure drops and surface horizontal pressure gradient
develops
-cyclonic circulation develops and a storm is born
-initial stage –develops along polar front
-polar front stationary-trough of low pressure-high pressure on either side
-surface winds blow parallel to the front in opposite directions
-cold air north of the front-easterly
-warm air south of the front-westerly
-surface winds converge while winds aloft diverge
-pressure dropping aloft causes surface air to rise and surface pressure to drop
-front begins to move
-west of the low –moves southeast as a cold front
-east of the low –moves northward as a warm front
-forms a wave (wave cyclone)
-as it matures
-central pressure continues to drop and the horizontal pressure gradient
steepens
-counterclockwise winds increase in velocity
-both cold and warm fronts are active
-travel with counterclockwise flow
-cold front moves faster than warm front
-forms a right angle with the warm front
-mature
-warm side and cold side
-coldest air to the west of the low
-winds northwesterly
-warm air to the southeast
-winds southerly
-region of air between cold and warm fronts
-warm sector
-weather different on either side of system
-north track (warm side of the storm)
-rain and drizzle
-partial clearing
-scattered showers and T-storms as cold front passes
-clearing skies
-south track (cold side of the storm)
-rain and snow
-precipitation tapers
-temperatures drop
-skies clear
-completion of life cycle
-faster cold front catches up with warm front
-forces warm air aloft
-cuts of supply of warm moist air to storm center
-skies remain cloudy
-steady precipitation gives way to drizzle
-low begins to fill
-central pressure rises
Hurricanes
-differences between hurricanes and mid-latitude cyclones
-develop in a uniform mass of very warm, humid air
-no fronts/frontal weather
-air pressure symmetric around system center
-central pressure lower
-horizontal pressure gradient steeper
-isobars form concentric, closely spaced circles
-smaller system - 1/3 size of mid-latitude cyclone
-life cycle
-two conditions must be met
-warm surface ocean water (min. 81°F)
-speed determined by temperature difference between sea surface
and top of storm
-sustains hurricane circulation
-latent heat of evaporation released during condensation
within the hurricane
-equivalent to 400 20 megaton bombs or half the U.S.
energy needs for a year
-warm water more readily vaporized
-pass over cool water/land - loose energy source
-significant Coriolis effect
-minimum lat. with strong enough effect is 4°
-conditions exist in four main areas
-western tropical north Pacific
-west of 180° - typhoons
-east of 180° - hurricanes
-Indian Ocean and north of Australia
-cyclones
-tropical north atlantic off west Africa
-Pacific Ocean southwest of central America
-absence off South America - cold ocean current
-seasonal
-requirement of high surface temperatures
-oceans thermally stable (high thermal inertia)
-peak temps lag peak solar radiation
-Atlantic - August to October
-convective activity/organized thunderstorm activity along ITCZ
-low pressure trough in westerlies intruding into tropics from mid-latitudes
-wave in easterly trades
-central air pressure drops
-water vapor condenses
-releases latent heat
-heated air rises
-more condensation
-more release of latent heat aloft
-rising temperature-outflow of air aloft
-sharp drop in air pressure
-convergence of surface air
-more condensation and release of latent heat
-tropical depression (low intensifies)
-winds strengthen
-40mph - tropical storm
-74mph - hurricane
-drift with the trade winds (easterlies) to the west
- 6-12mph
-western Atlantic
-speed up
-curve north/northeastward
-mid-latitude westerlies
-30°N latitude - acquire mid latitude (extratropical) characteristics
-cold air drawn into system
-fronts develop
-follows life cycle similar to mid-latitude cyclone
-ends by occluding over the north Atlantic