Lec 4 - Ch 7,8
Ch 7 - Transient Disturbances (aka storms)
• dramatic
•influential to mankind
•damaging but also positive
•promote vegetation with life-giving water
I. Air masses
•important to have basic understanding because
these are responsible for storms
A. characteristics
•large
•uniform properties (temperature, humidity same;
mass stable)
•retains original characteristics even while it’s moving
B. origin
•characteristics developed over land or sea as it picks
up the temp and humidity from the source area
•usually high pressure, because mass must be stable
enough to stay close to ground
•Source regions - a few to remember Fig 7.3
•warm and cold land (continental tropical, polar)
•warm and cold water (maritime tropical, polar)
•this text includes arctic and equatorial source regions
as well
C. Classification
•all based upon the source region - Table 7.1, Fig 7.2
•cT, cP
•mT, mP
•A, E
D. Movement / modification
•air mass moves and modifies weather as it travels,
taking the source region characteristics to other areas
Fig 7.4, 7.5
• mP hits the Pacific NW
•CP affects central US Plains
•mT brings moist air up all through eastern US
III. Fronts
•important Fig 7.6
•boundaries between air masses
•think in 3D
•different precip patterns
•different slopes (cold steep, warm more horizontal)
•few basics •fronts usually move
•warm air always on the top Fig 7.6
•cold front acts like a wedge to drive warm air up
•warm front climbs up the trailing edge of cold air
Warm air
Warm front
Cold front
Cold air
Symbols
warm front with circles
cold front with triangles.
•warm front brings warm air (Fig 7-9)
•clouds and rain may be well ahead of warm frt
position on ground
•cold front brings cold air Fig 7-12
•clouds and rain turbulent, violent, very near front
position on ground (steep)
other fronts include stationary, occluded Fig 7-10
IV Atmospheric Disturbances (severe weather)
A. midlatitude “cyclones”
•large low pressure systems (Fig 7-13)
•bring most of our weather
•counterclockwise circ brings warm air from south
up to meet cool air from north
•system of 2 fronts: one warm, one cold Fig 7-13,
each with precip
•life cycle Fig 7-15 an interesting story - mixing at
first, utimately horiz layered
•Conditions aloft important to keeping things moving
•Fig 7-17 - important - note high and low pressure on
ground tied to conditions above
B. Tropical disturbances (hurricanes)
•low pressure too, but very intense, strong gradient
(therefore wind speed FAST)
•Fig 7-24 - parts include eye, eye wall
•origin in warm moist tropical areas, where energy
found in latent heat of condensation to keep driving
the engine
•on either side of equator (fig 7-25)
•tracks shown in fig 7-27
•very destructive - wind, high seas
•low pressure acts like a vacuum cleaner, “sucks
up” surface of ocean to a bulge, creates shortterm sea-level rise that floods low-lying coasts
•most deaths are drownings
•example - Galveston TX 1900 - 20 foot storm
surge killed 6000 people
•almost unbelievable in India:
•1737 - 300,000 people
•1876 - 300,000
•1970 - 500,000
•1991 - 175,000
C. minor disturbances (thunderstorms, tornadoes, etc)
•thunderstorm (fig 7-31) has a cycle of updraft
followed by downdraft
•tornadoes most destructive of all disturbances (Fig 733)
•most extreme pressure gradients known, hence
fastest wind speeds known (200 to 500 mph
•“exact mechanism of formation still not wellunderstood”
•usu spawned with cold fronts, usually come
directly out of the base of a cumulonimbus
thunderhead
•seasonal (fig 7-34)
•concentrated in US (fig 7-35)
•note “Tornado Alley” in central plains of US (Fig
7-36)
Lec 4 (cont)
Chap 8 - Climatic Zones
I. classification
•temp and precip are the 2 biggest keys for
classification
•why do it?
•organization
•simplification
•learning tool
A. many types of schemes
•ancient Greeks (fig 8.1, 8.2) had 5 zones of
temperate, torrid, frigid
•we use 5 basic ones today (fig 8.3)
•equatorial warm/wet
•tropical hot/dry
(precip the big diff w/ these 2)
•subtropical warm/temperate
•midlatitude cool/temperate
(temp the big diff w/these 2)
•high latitude cold
•note this is similar to Greeks - hot (2), temperate
(2), cold
B. Koppen system in text
•keys - avg precip, temp determine the zones
•vegetation limit determines the boundaries
•Fig 8-4 shows system parameters
•A,C,D,E based on temp progression from equator to
pole
•B based on wet vs dry, crosses temp boundaries
•Fig 8-5 shows the world based upon Table 8-2, 8-3
C. Climograph
•a device for showing monthly info Fig 8-6
•useful in classification determination
II. World Distribution of Climate
focus on :
•where are the types?
•What are the characteristics?
•Why are the characteristics what they are?
A. tropical humid Fig 8-7
•all land 15-20 degress either side of equator
•truly “winterless”
•quite wet (moisture and uplift mechanisms present)
•several variations - Af (wet), Aw (savanna), Am
(monsoon)
B. dry Fig 8-18
•dry due to lack of uplift, not necessarily a lack of
moisture
•lack of uplift due to subsiding high-pressure air
•tend to be subtropical and midlatitude in location
•primarily “desert” (almost rainless) and “steppe”
C. mild midlatitude Fig 8-31
•great variability; neither constant heat nor constant
cold
•air mass contrasts
•variations are mediterranean, humid subtropical,
marine west coast
Idealized pattern of mild climates - Fig 8-42
•note the broad zones, and transitions, generally
associated with high, low pressure global circulation
cells
Humid subtrop - low pressure
Desert - high pressure
0 degrees
Tropical humid - low pressure
D. severe midlatitude
•N hemisphere only - no land masses in S hemi at
these latitudes
•NE US, Canada, Alaska
•little maritime influence
•4 clearly defined seasons
E. Polar
•cold
•dry