Air pressure and atmospheric
motion
Define the forces that generate winds
Explain why there is well-developed westerly flow in the upper troposphere
Differentiate between high and low pressure systems
Compare and contrast surface and geostrophic winds
Describe/read maps of constant pressure surfaces
Identify and discuss wind types and the forces that generate them
Recognize surface and upper-level atmospheric maps and identify general
patterns of windflow
Explain monsoonal flow and land-sea breezes
Air pressure and atmospheric
motion
Q: What makes the wind
blow?
A: Air pressure differences.
Air pressure
• Force exerted by
molecules in atm due to
gravity and temperature
Air pressure
Surface air pressure variability
• Average sea
level pressure
1013 mb
Mercury barometer
1013 mb = 29.91 inches
Aneroid barometer
Pressure systems
• Two types:
high and low
• Low:
associated with
clouds and
instability.
• High:
associated with
clear conditions
and stability
Low pressure systems
• Cyclone
– Converging rising air at
surface
– Diverging air aloft
– Winds rotate counterclockwise
in NH
– Areas of “light” atmosphere; air
is forced into these locations
– Unstable surface conditions
High pressure system
• Anticyclone
– Converging air aloft
– Diverging sinking air at
surface
– Winds rotate clockwise in NH
– Areas of “heavy” atmosphere;
air is forced out of these
locations
– Stable surface conditions
Is the location for these pressure systems the
northern or the southern hemisphere?
High and low pressure
systems
• Occur on a variety of spatial and temporal
scales
– Some pressure systems may be stationary for a
long period of time, others may migrate rapidly
around the planet
– Some pressure systems are closed, others are
more belt-like and open
Low pressure systems
• Types of low pressure systems: tornados,
thunderstorms, hurricanes….
Low pressure systems
….., midlatitude cyclones, the ITCZ, thermal lows
Low pressure systems: dust
devil versus ITCZ
Another secret of weather
forecasting
• The atmosphere is a collection of pressure
systems in three dimensions.
• Weather forecasting involves looking at
surface conditions as well as upper level
conditions (aloft, in the upper troposphere)
• http://weather.unisys.com/
“Deep” tropospheric phenomena
• A midlatitude cyclone is a low
pressure at the surface coupled to a
low pressure aloft in the upper
troposphere
• A hurricane is a high pressure aloft
and a low pressure at the surface
Midlatitude cyclones
• Strong, “deep”
interaction between
surface and upper
levels
• May travel large
distances around the
globe
Midlatitude cyclone
“Shallow” tropospheric phenomena
• Thermal low (warm)
• Thermal high (cold)
– Weak interaction
between surface and
upper levels
– May occur on a daily
basis or persist over
many months
L
H
Pressure differences initiate…..
…….winds
• Named according to the
direction they blow from
• Winds can blow at different
directions at different
altitudes in the atm
• Forces that act on winds:
– PGF
– CF
– Surface friction
1. PGF: Pressure gradient force
– winds blow from high to low
Where are winds the fastest?
2. Coriolis Force (CF)
• Apparent deflection of the winds due to
rotation of the Earth
– NH winds deflected right
– SH winds deflected left
Coriolis Force (CF)
• CF is not a true force; it is an apparent force
arising from the effect of the Earth’s rotation
• Deflection is strongest at poles and zero at
the Equator
• CF acts perpendicular to the direction of
motion
• CF deflects to the right in the Northern
Hemisphere
Misconceptions about the
Coriolis Force
• The CF does not determine the rotation
in a drain.
3. Surface friction (SF)
• Topography
(mnts, elevated
plateaus) deflect
winds
Two major types of winds
1. Geostrophic (upper
troposphere winds)
– Influenced by PGF and CF only
– Wind flow is parallel to
isobars/geopotential heights
– Geostrophic flow is westerly
(west to east) in NH
– Shown on geopotential height
maps
Geostrophic winds on
geopotential height map
Two major types of winds
2. Surface winds
– Influenced by
PGF, CF, and
SF
– Winds cross
isobars
Types of surface winds
• Monsoonal flow
– Creates extreme wet and dry seasons
Location of
thermal highs
and lows associated
with monsoonal
flow
Winter
Weak monsoonal flow in southwestern US
Summer
Types of surface winds
• Land-sea breeze circulation
• Many other different types of
surface winds based on local
physiography and
arrangement of pressure
cells. Many have unique
names.
Sea breeze blowing
from ocean to land