Atmospheric Motion – Pressure and Winds Outline
Vicki Drake
Santa Monica College
I.
Atmospheric Pressure
A. Air pressure is a measure of the weight of the air above the point of
observation
B. Barometer: a means of measuring air pressure
i. Height of mercury column
C. Mean sea level is considered at 0 meters of elevation
D. Pressure changes more quickly with vertical distance change than
horizontal distance change
E. Isobars are lines connecting points of equal barometric pressure
i. Spaced 4 mb apart
II.
Charts
A. Constant Height Chart
i. Represents atmospheric pressure at constant elevation (sea
level, 1000 meters, etc.)
B. Constant Pressure Chart (Isobaric Chart)
i. Shows variations along an equal pressure (isobaric) surface
ii. Change in Air temperature =change in air density and pressure
iii. Contour lines – altitude above sea level for 500 mb pressure
readings
1. bends into ridges (elongated Highs)
2. bends into troughs (elongated Lows)
C. Upper Air Charts (@500 mb pressure)
i. Show wind-flow patterns of importance in weather forecasting
ii. Track movement of weather systems
iii. Predict behavior of surface pressure systems
III. Pressure Systems
A. Surface air pressure systems develop due to either Thermal and/or
dynamic conditions
B. Thermal Conditions – differential heating and cooling of landmasses and
water
C. Warm surface (landmass or water) –
a. Columns of warm rising air
b. As air leaves surface, less air available to maintain stable pressure
c. Low Pressure cell develops with a Warm core
d. Surface air is pulled into the center of the Low pressure cell,
heated and rises
e. Circulation of air about the Low Pressure is Cyclonic.
i. Northern Hemi – counterclockwise and into the center
ii. Southern Hemi – clockwise and into the center
D. Cold Surface (landmass or water)
a. Columns of cold sinking air
b. As air “piles” up at surface, more air available for pressure
c. High Pressure Cell develops with a Cold core
d. Surface air is pushed out from the center of the High Pressure cell
as the air is cooled and sinking
e. Circulation of air about the High Pressure is Anticyclonic
© Vicki Drake
Santa Monica College
Fall 2000 Geography Lectures
1
III.
i. Northern Hemi – clockwise and out from the center
ii. Southern Hemi – counterclockwise and out from the center
Air Movement
A. Newton’s Law of Motions
i. An object in motion or at rest will tend to stay in motion or at
rest until a forced is exerted upon it - INERTIA
ii. The force on the object is equal to the mass of the object times
the acceleration produced by the force – F=ma
B. Forces involved in Air Movement
i. Pressure Gradient Force
1. The net force produced when there are difference in
horizontal air pressure
2. The force is directed from High Low, and move at right
angle to the isobars
3. Pressure Gradient – the amount of pressure change over
a given distance
4. Strong PGF = strong winds: Weak PGF = weak winds
5. Controls both direction and velocity (speed) of winds
ii. Coriolis Force
1. Apparent deflection of any free-moving object from a
straight-line path
2. Deflection is to the right of the original path of the
object in the Northern Hemi, and to the left of the
original path in the South Hemi
3. Magnitude of deflection depends on (1) initial speed of
object and (2) latitude of moving object and (3) altitude
of moving object
4. Objects moving with high speed = more Coriolis effect
5. Objects closer to Poles = more Coriolis effect
6. Objects closer to Earth’s surface = more Coriolis effect
7. Controls only direction of wind
iii. Friction
1. A boundary layer effect close to Earth’s surface
2. Objects come into contact with “obstacles” in
atmosphere – air pressure changes, air density,
water/land boundaries, etc.
3. Controls only speed (velocity) of wind
C. Geostrophic Winds
i. Winds blowing parallel to isobars
ii. Found at high altitudes above Earth’s boundary layer ~1000
meters above surface
iii. Develop as PGF = CF
iv. Jet Streams are examples of Geostrophic Winds
D. Local Winds
i. Onshore-Offshore Winds (Sea Breezes-Land Breezes)
1. Daytime – thermal conditions create Low over land, High
over ocean – air moves Onshore (Sea Breeze)
2. Nighttime – thermal conditions create Low over Ocean,
High over land – air moves Offshore (Land Breeze)
ii. Santa Ana Winds – Hot, dry, high velocity winds
1. High Pressure over area of Utah
2. Change in altitude from Utah to Southern CA produces
adiabatic warming of air
© Vicki Drake
Santa Monica College
Fall 2000 Geography Lectures
2
IV.
3. Passage of air over southern California’ deserts produces
dry air
4. Funneling of air through canyons and over San Gabriel
Mountains produces high velocity
5. Southern California’s Fire Season
iii. Chinook Winds (Mistral Winds)
1. Air parcel lifted up and over a mountain
2. Releases precipitation on windward side, dried out on lee
side
3. Compressional heating and drying produces another hot,
dry wind (“Snow-eater” in Colorado)
iv. Katabatic Winds
1. Shallow high pressure dome at top of ridge
2. Shallow low pressure trough at base of ridge
3. Downslope winds created – can be extremely cold and
high velocity (usually occurs when ridge is snowcovered)
v. Haboob
1. Sand Storms created in downdraft of advancing
Thunderstorms in desert regions
2. Dust curtain seen for many kilometers, can cover 100 of
km, rising vertically to base of Thunderstorm
3. May give rise to Dust Devils, and even Tornadoes
vi. Monsoons
1. “Gigantic” Thermal Sea and Land Breezes
2. A Change of wind direction due to a change in seasons
3. Winter conditions for India
a. Cold continental land mass = high pressure cells
b. Clockwise rotation pushes air off continent
c. “warmer” Indian Ocean Low not strong enough to
push moisture onto land
d. Clear, dry, conditions prevail
4. Summer conditions for India
a. Hot continental land mass – Low pressure cells
b. Counterclockwise rotation pulls air in off Indian
Ocean, Bay of Bengal, South China Sea, etc
c. “Cooler” ocean High pressure feed moistureladen air onto land
d. Rising air columns cool, condense and produce
enormous amounts of rain over a short period of
time
e. Record precipitation: 1042 inches (between April
and October) – Record one month: 366 inches
Dynamic Pressure Systems
A. Creation: processes related to rotation of Earth and mixing of air
masses
B. Not ground controlled (i.e. thermal), but assert control on temperature
of surfaces
C. Dynamic High: characterized by presence of High Pressure at Earth’s
surface and High pressure aloft.
i. Air descends within the system, diverging in a clockwise flow at
surface (Northern Hemi)
© Vicki Drake
Santa Monica College
Fall 2000 Geography Lectures
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ii. Subsidence of air produces heating of air (compressional
heating) within the core and surface below is also heated
iii. Pressure of system persists throughout core and anticyclonic
flow persists and intensifies aloft – Core is Warm
iv. Associated with clear skies, warm temperatures
D. Dynamic Low: characterized by converging and ascending air at the
surface and aloft
i. Pressure decreases rapidly in the core with increasing height
above the surface
ii. Rising air is cooled adiabatically and the Core is Cold with
cyclonic flow
iii. Associated with cloudy, rainy, and stormy conditions
© Vicki Drake
Santa Monica College
Fall 2000 Geography Lectures
4