Transcript chapter8

Chapter 8
Atmospheric Pressure
What causes air pressure to change in
the horizontal?
 Why does the air pressure change at the
surface?

Atmospheric Pressure

Horizontal Pressure Variations
 It takes a shorter column of dense, cold air
to exert the same pressure as a taller
column of less dense, warm air
 Warm air aloft is normally associated with
high atmospheric pressure and cold air aloft
with low atmospheric pressure
 At surface, horizontal difference in
temperature = horizontal pressure in
pressure = wind
Atmospheric Pressure

Special Topic: Gas Law
P is proportional to T x ρ
P = pressure
T = temperature
ρ = density
Atmospheric Pressure

Daily Pressure Variations
 Thermal tides in the tropics
 Mid-latitude pressure variation driven by
transitory pressure cells

Pressure Measurements
 Barometer, barometric pressure
○ Standard atmospheric pressure 1013.25mb
 Aneroid barometers
○ Altimeter, barograph
Atmospheric Pressure

Pressure Readings
 Instrument error: temperature, surface
tension
 Altitude corrections: high altitude add
pressure, 10mb/100m above sea level
Surface and Upper Level
Charts
Sea-level pressure chart: constant
height
 Upper level or isobaric chart: constant
pressure surface (i.e. 500mb)

 High heights correspond to higher than
normal pressures at a given latitude and
vice versa
Surface and Upper Level
Charts

Observation: Constant Pressure Surface
 Pressure altimeter in an airplane causes
path along constant pressure not elevation
 May cause sudden drop in elevation
 Radio altimeter offers constant elevation
Newton’s Law of Motion
AN object at rest will remain at rest and
an object in motion will remain in motion
as long as no force is executed on the
object.
 The force exerted on an object equals
its mass times the acceleration
produced.

 Acceleration: speeding up, slowing down,
change of direction of an object.
Forces that Influence Winds

Pressure Gradient Force: difference in
pressure over distance
 Directed perpendicular to isobars from high
to low.
 Large change in pressure over s short
distance is a strong pressure gradient and
vice versa.
 The force that causes the wind to blow.
Forces that Influence Winds

Coriolis Force
 Apparent deflection due to rotation of the Earth (the
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rotation rate of Venus is so slow that the Coriolis
force is extremely small on Venus)
Right in northern hemisphere and left in southern
hemisphere
Stronger wind = greater deflection
No Coriolis effect at the equator greatest at poles.
Only influence direction, not speed
Only has significant impact over long distances
Forces that Influence Winds

Geostrophic Winds
 Earth turning winds
 Travel parallel to isobars
 Spacing of isobars indicates speed; close =
fast, spread out = slow

Topic: Math & Geostrophic Winds
Vg = 1 x Δp
fρ
d
Forces that Influence Winds

Gradient Winds Aloft
 Cyclonic: counterclockwise
 Anticyclonic: clockwise
 Gradient wind parallel to curved isobars
 Cyclostrophic near Equator

Observation: Estimates Aloft
 Clouds indicate direction of winds, place
pressure in location consistent with cloud
location.
Stepped Art
Fig. 8-29, p. 214
Forces that Influence Winds

Winds on Upper-level Charts
 Winds parallel to contour lines and flow west
to east
 Heights decrease from north to south

Surface Winds
 Friction reduces the wind speed which in
turn decrease the Coriolis effect.
 Winds cross the isobars at about 30° into
low pressure and out of high pressure
 Buys-Ballots Law
Winds and Vertical Motion
Replacement of lateral spreading of air
results in the rise of air over a low
pressure and subsidence over high
pressure
 Hydrostatic equilibrium and equation
 Topic: Hydrostatic equation

Δp = -ρg
Δz