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Chapter 8
Weather
Robert W. Christopherson
Charlie Thomsen
Crime scene evidences of violent weather!
Rising average damage: $2 (1975) to $10 (1995) billion
Hurricane Katrina: $125 billion damage
Weather and Climate
Weather:
Short-term day-to-day condition of the atmosphere.
Climate:
Long-term average (over decades) of weather conditions.
e.g. mean annual temperate, mean total precipitation, etc.
Important Weather Elements
Temperature
Air Pressure
Relative Humidity
Wind Speed and Direction
Seasonal Factors: insolation, day length, sun angle
Air Mass
A distinct body of air with relatively homogenous
characteristics in temperature, humidity and
stability, which is formed over a relatively large
and uniform Earth surface area.
Air masses are the actors/actresses in the weather
drama.
Air Masses Affecting North America
mP: Aleutian
Low resides.
cP: Major player for
middle- and highlatitude weather
mP: Icelandic
Low resides.
c: continential, m: maritime
A: arctic; P: polar; T: tropical; E=equatorial; AA: antarctic
Figure 8.2
Air Mass Modification
As air mass migrates away from its source region, its
temperature and humidity characteristics modify
and gradually takes that of the land over which
they pass.
e.g. cP can move as far south to Florida. The air
mass warms significantly when reach Florida.
When cP passes the great lakes, it picks up heat
and moisture and dump tremendous snow in the
Ontario, Michigan, Penn, and NY (lake effect
snow)
Lake-Effect
Snowbelts
Air mass modification
Figure 8.5
Atmospheric Lifting Mechanisms
Convergent Lifting
Convectional Lifting
Orographic Lifting
Frontal Lifting (Cold and Warm Fronts)
Atmospheric Lifting Mechanisms
(e.g. ITCZ)
(e.g. plowed dark soil, urban)
Figure 8.6
Local Heating and Convection
Figure 8.7
Convection over Florida
Low pressure convergence
Convection lifting by
localized heating
Figure 8.8
Orographic Precipitation
Mountains set precipitation record: e.g. Cherrapunji, India (elevation: 1313, 25oN of the hills of
Himalayas) has 9300 mm of precipitation.
Figure 8.9
Orographic
Patterns
rain shadow
State of Washington:
Olympic Mountain and
Cascade Mountains
orographically lift invading
mP air mass.
Figure 8.10
Frontal Lifting
Front: the leading edge of an advancing air
mass is its front.
Cold Fronts
Cold air forces warm air aloft
400 km wide (250 mi)
Warm Fronts
Warm air moves up and over cold air
1000 km wide (600 mi)
Cold Front
Aftermath of a cold front: Northerly winds in Northern Hemisphere and high air pressure.
Figure 8.11
Warm Front
Cold air is heavier than warm air. It is more difficult to push back, thus a prolonged “battle” front.
Gentle lifting of the warm air, producing drizzly rain showers, in contrast to more dramatic
precipitation associated with the cold front.
Figure 8.13
Stationary Front
Produce moderate precipitation because the air is not being rapidly uplifted.
Figure 8.15
Midlatitude Cyclones
Cyclogenesis
Open stage
Occluded stage
Dissolving stage
Midlatitude Cyclone
When two high
pressure air mass
meet, they create a
low pressure trough.
The cold air moves
southernly and the
warm air move
northernly, creating a
convergence on the
surface, and the air
rise up to form a low
pressure center.
Figure 8.14
Midlatitude Cyclone
The signature “comma” clouds for a mature mid-latitude cyclone.
Figure 8.15
Average and Actual Storm Tracks
Figure 8.16
Weather Forecasting
Step 2: 3-D computer models
Step 1: Collecting data
Figure 8.17
Violent Weather
Thunderstorms:
Tornadoes
Tropical Cyclones
Thunderstorm
A moist warm air uplifted (local insolation, heating, two winds
converge forcing uplifting, or orographic lifting causing warm air to
cool and releases tremendous latent heat which further warms the air
and causing violent updrafts, resulting cumulonimbus clouds, heaving
precipitation, lightning, thunder, hail, and may be tornados. Raindrop
form and fall causing violent downburst (can suck down an airplane).
Signature anvil shaped cumulonimbus clouds
Thunderstorms
Why there are such a strong latitudinal
gradient in T-storm occurrences?
Figure 8.19
Thunderstorm counts by Satellites
Why there are such a shift of T-storm between winter and summer?
Figure 8.20
Hailstones
Hailstones are by-product of T-storm. The violent up and down turbulence in the cumulonimbus
clouds keep raindrop and ice co-exist, letting the ice particle to grow until it is sufficiently large and
fall: hailstone
Figure 8.21
Mesocyclone and
Tornado
Figure 8.22
Tornado
Figure 8.22
Tornado Path
Figure 8.22
Tornadoes
Based on the mechanism how tornados form, it is
not difficult to understand the smooth gentle slope
at the western side of central plane is ideal for
tornado formation.
Figure 8.24
Tropical
Cyclone
Formation
High wind shear
Low wind shear
1. Entirely within tropical
air masses, not involving
any frontal system.
2. A powerful manifestation
of Earth-atmosphere
energy exchange,
converting heat energy
into mechanical energy in
wind.
3. Conditions to form:
(1) warm water
(2) low winder shear
(3) Thunderstorms: ITCZ
and eastward moving
atmospheric waves
earterly.
Figure 8.25
Tropical Cyclones
Figure 8.26
Hurricanes Gilbert and Catarina
Find the difference.
Figure 8.26
Profile of a Hurricane
Figure 8.27
Hurricane Isabel,
Cape Hatteras
and the
Outer Banks
Figure 8.28
2005: Record-Breaking Storm Season
Why most of the hurricane eventually turn it’s direction toward northeast?
Figure FS 8.1.1
End of Chapter 8
Geosystems 7e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen