ATMOSPHERIC FORCES
or
Why Does the Wind Blow?
Air motion is well described by the Navier-Stokes equation,


dVh
1

 1
ˆ
  p  fk  V  Fr    
dt



1
2
3
1. Acceleration =
2. Pressure gradient force +
3. Coriolis force +
4. Friction force.
4
The change in pressure measured across a given distance is
called a "pressure gradient".
PG = change in pressure / distance
The pressure gradient results in a net force that is directed
from high to low pressure and this force is called the
"pressure gradient force".
PGF points at a right angle to the local isobar or height
contour line. It also points from high pressure to low
pressure (or high heights to low heights).
The closer the isobars or
height contours, the greater
the PGF magnitude. Larger
PGF result in stronger winds,
so strong winds are
associated with closely
spaced contour lines.
Reasons for pressure variation:
Pressure differences are ‘forced’ by uneven absorption of solar radiation at
the Earth's surface. (remember P=ρ*R*T, pressure = density x constant x
temperature)
 warmer air is less dense and typically exerts less pressure than cooler air
 however, for a given surface pressure, a column of cold air will decrease
in pressure with height more rapidly than will a warm column of air (see next
slide for example)
Moisture content of air also affects pressure
 the greater the amount of water vapor, the less dense the air will be, and
the less pressure it will exert
Wind develops as a result of spatial (i.e. horizontal) differences in
atmospheric pressure.
 Wind speed tends to be at their greatest during the daytime when the
greatest spatial extremes in atmospheric temperature and pressure exist.
Coriolis Force - an
artifact of the earth's
rotation.
F cor  fkˆ  V
f  2 sin  
V = wind speed
Ω = earth angular
velocity, 7.27 x 10-5
φ = latitude
Once air has been set in motion by the
pressure gradient force, it undergoes an
apparent deflection from its path, as
seen by an observer on the earth. This
apparent deflection is called the
"Coriolis force" and is a result of the
earth's rotation.
The Coriolis force is zero at the equator.
An air parcel initially at rest will accelerate from high pressure to low
pressure because of the pressure gradient force (PGF).
As the air parcel begins to move, it is deflected by the Coriolis force (to the
right in the northern hemisphere, to the left in the southern hemisphere).
Coriolis force always acts at 90° to the wind motion.
As the wind gains speed, the deflection increases until the Coriolis force
equals the pressure gradient force. This balance is called geostrophic
balance. The geostrophic wind blows parallel to contours. (Note: friction is
not a component!)
What
hemisphere
is depicted
in this
figure?
FRICTION
 The surface of the Earth exerts a frictional drag on the
air blowing just above it.
 This friction can act to change the wind's direction and
slow it down -- keeping it from blowing as fast as the wind
aloft.
 The difference in terrain conditions directly affects how
much friction is exerted.
 For example, a calm ocean surface is pretty smooth,
so the wind blowing over it does not move up, down,
and around any features.
 By contrast, hills and forests force the wind to slow
down and/or change direction much more.
Net result is that surface winds blow across isobars!
This cross-isobar flow allows the surface wind to spiral into
low pressure and out of high pressure.
Ekman turning has
two results:
1. Surface winds
around high and low
pressure systems are
determined: winds
spiral cyclonic into
Low Pressure and
anticyclonic out of
High Pressure
Surface wind crossing isobars is known as “Ekman
turning” or “Ekman pumping”. It is due to friction.
Du
1  p  u ' w'
 fv 

Dt
 x
z
2. Surface winds are
oriented
approximately 45° to
right of geostrophic
wind at surface. Top
of “Ekman layer” is
defined where v wind
component is zero.
(And thus wind is now
geostrophic).
Dv
1  p  v' w'
  fu 

Dt
y
z
Another consequence of Ekman turning is the response of the upper-atmosphere to
the low-atmosphere surface layer.
Low pressure system: convergence at surface, divergence aloft
High pressure system: divergence aloft, convergence at surface
upper level
surface
We have now seen three very important force balances in the
atmosphere. We will see a fourth soon.
PGF = Coriolis force  Geostrophic balance (Earth turning)
PGF = Centripetal force  Cyclostrophic balance (Tornado)
PGF = gravity  Hydrostatic balance
PGF = CO + Friction  Ekmann Balance (PBL)
Summary: What is wind? Simply a balance of forces acting on air
parcels
Geostrophic wind: case
where pressure gradient
force is equally and
oppositely balanced by
coriolis force. Resulting wind
blows parallel to isobars /
height contours
At the surface, friction force
induces a drag and creates
imbalance between PGF &
CF. Resulting wind crosses
isobars, diverging from high
and converging into low
pressure
Local wind circulations
During day, land and ocean
heat up differently (why?)
By afternoon, land is
warmer than ocean
Warm air has lower pressure
than cold air (assume
density remains constant)
Sea Breeze
A local pressure gradient
sets up and air moves from
H to L, resulting in a sea
breeze
At night, land cools down
faster than ocean, reversing
the pressure gradient
Wind blows off shore,
resulting in land breeze
Land Breeze
Land Breeze
Valley Breeze
• During the day the air close to the mountain sides becomes
warmer than the surrounding air, thus lowering pressure
relative to the cooler air at the same height over the valley.
• This warm air rises up the mountains and is replaced by air
from within the valley.
• So during the day valley air moves up the mountain sides.
Warmer, local
low pressure
Cooler, local
high pressure
This process leads to clouds and rain in the mountains in
the summer, particularly in the late afternoon.
Mountain Breezes
• At night the sides of the mountain cool down and cool air is
pulled down by gravity.
• In the morning, the coldest air is, therefore, often found
within the valley.
The large volcanoes on the Big Island of Hawaii are the source of
strong nighttime mountain breezes
General Circulation, Monsoons
Circulation scales
• We know that weather is essentially nothing more than moving air (&
the accompanying heat & moisture transport)
• We also know that air parcels are acted on by several forces, the net
result being motion:
– pressure gradient, Coriolis, friction, gravity
• To help us understand air motion, we categorize “disturbances” by size
and time
 Understanding global-scale atmospheric motion is
important because it shows us how and why the planet is
divided into different zones, distinguished from one another
by their typical weather and climate
 These divisions of the global climate system have had a
significant impact on the development of civilization on Earth,
because climate has been more or less the same for the last
10,000 years or so
 However, there are climatic fluctuations on timescales
of years to decades that can have significant effects
 On even longer timescales, we find that the
atmospheric general circulation and climate have
undergone very large fluctuations.
 In a simplistic world model,
heated air in the equatorial
regions becomes less dense,
more buoyant and rises.
 At the same time, cold air in
the Polar Regions becomes
denser, heavier and sinks.
 In the lower levels of the
atmosphere, cold dense air
from the Polar Regions flows
towards the equator. It replaces
the heated equatorial air.
 A return flow of warm air at
higher levels of the atmosphere
completes this simple
circulation model.
 This type of “direct”
circulation is called a Hadley
Cell
•Convergence is coming together.
•Divergence is going apart.
•Subsidence is sinking air.
The planet Venus rotates very slowly on its axis (a “day” on
Venus is longer than a “year”!)
So the general circulation of the lower atmosphere of Venus is
essentially large Hadley cells on either side of the equator
The circulation in the
northern and southern
hemispheres is broken
into 3 bands in each
hemisphere
The large-scale rising
or sinking of air tells us
(at least in general)
where the rainiest and
driest latitudes are:
Global (visible + infrared) satellite image
Tropical Rainforest
A dense evergreen forest occupying a tropical region with an
annual rainfall of at least 2.5 meters (100 inches). Cover
about 6% of earth’s land area
Temperate rainforest is defined as any forest in the mid-latitudes that receives
more than 50-60 inches of rainfall a year. They are found along the Pacific
coast of the USA and Canada (from northern California to Alaska), in New
Zealand, Tasmania, Chile, Ireland, Scotland and Norway. They cover less area
than tropical rainforests.
Westerlies and trade winds blow away from the
30 degrees latitude belt. Over broad regions
centered at 30 degrees latitude, surface winds
are light or calm. Air slowly descends to replace
the air that blows away. Descending air warms
and is dry. The subtropical deserts, such as the
Sahara of Africa and the Sonoran of Mexico,
occur under these regions of descending air.
A jet stream is a current of fast moving air found in the upper levels of the
atmosphere. This rapid current is typically thousands of kilometers long, a few
hundred kilometers wide, and only a few kilometers thick. Jet streams are usually
found somewhere between 10-15 km (6-9 miles) above the earth's surface. The
position of this upper-level jet stream denotes the location of the strongest SURFACE
temperature contrast
Consequences of three
large circulation scales:
- Jet streams & fronts
 There are two main jet
streams at polar latitudes,
one in each hemisphere,
and two minor subtropical
streams closer to the
equator.
 In the Northern
Hemisphere the streams are
most commonly found
between latitudes 30°N and
70°N for the polar jet stream,
and between latitude 20°N
and 50°N for the subtropical
stream.
Technically the wind speed has to
be higher than 90km/h or 55 mph to
be called a jet stream.
Jet streams can be explained as follows: If two air masses of different temperatures
meet, the resulting pressure difference is highest at altitude. If one of the air masses
lies north of the other one, then the wind will not flow directly from the warm to
the cold area, but is deflected by the Coriolis force and flows along the boundary of
the two air masses.
The wind speeds
vary according to the
temperature gradient,
averaging 55km/h or
35 mph in summer
and 120km/h or 75
mph in winter,
although speeds of
over 400km/h or 250
mph are known.
The general circulation serves to transport heat energy from warm
equatorial regions to colder temperate and polar regions. Without
such latitudinal redistribution of heat, the equator would be much
hotter than it is whilst the poles would be much colder.
So far we have used a simple rotating earth with a smooth uniform
surface to describe the general circulation of the atmosphere.
 However, the earth does not have a uniform surface, and this fact
plays an important role in modifying the pattern of airstreams.
January wind patterns. Note location of ITC(Z)
July wind patterns. Note location of ITC(Z)
Land and oceans absorb and reradiate heat differently. Consequently,
land heats up rapidly and loses heat just as rapidly. In contrast, oceans
distribute thermal energy through a much greater depth and take longer
to change temperature.
Northern Hemisphere
Southern Hemisphere
Atmospheric circulation does not account for all the energy
that needs to be redistributed to maintain the global energy
balance.
 Winds drive surface ocean circulation, and the cooling and
sinking of waters in the polar regions drive deep circulation.
 Surface circulation carries the warm upper waters poleward
from the tropics.
 Heat is disbursed along the way from the waters to the atmosphere.
 At the poles, the water is further cooled during winter, and
sinks to the deep ocean.
 This is especially true in the North Atlantic and along Antarctica.
 Deep ocean water gradually returns to the surface nearly
everywhere in the ocean.
 Once at the surface it is carried back to the tropics, and the
cycle begins again.
 The more efficient the cycle, the more heat is transferred,
and the warmer the climate.
Large-scale ocean currents
The Monsoon Circulation
Monsoon is used to describe seasonal reversals of wind direction,
caused by temperature differences between the land and sea. They
occur in a number of countries around the world.
The Monsoon climate in Southeast Asia and India is most well known
and most potent.
 This circulation is known to bring about long drought periods from
September to March. It is hard to grow crops during this period due to
the lack of water.
 The summer monsoon is known to do the opposite, bringing huge
amounts of moisture from the Indian Ocean causing massive flooding
in many areas.
During summer, the Tibetan Plateau acts like a gigantic exposed brick,
absorbing summer heat and heating the atmosphere above it. Hot air
rises, and cool, moist air—drawn in from over surrounding oceans—
rushes in to replace it. That moist air is the source of monsoon rains.
During the winter dry period, the airflow comes from high
pressure to the north—the dry Himalayas and Siberia.
The two key ingredients for the Asian Monsoon are a large land mass
and a large ocean – namely, southern Asia and the surrounding
Arabian Sea and the Indian Ocean.
About 60% of the earth's population live under the influence of the
Asian monsoon climate.
The Mexican Monsoon (also known as the North American
monsoon, the Southwest monsoon or Arizona monsoon) is a welldefined meteorological event
•The Mexican Monsoon is a regional-scale circulation that
develops over southwest North America during the months
of July through September.
• It is associated with a dramatic increase in rainfall
that occurs over what is normally an arid region of North
America.
Seasonal distribution of precipitation for southwestern North America. The largest peaks in
mid-to-late summer rainfall (i.e., monsoonal)are found in northwestern Mexico. Note that
northeastern Mexico and Texas display early summer/early fall precipitation peaks. Areas
south of the broken line receive 50% of their annual rainfall during July, August and
September (after Douglas et al. 1993).
Föhn wind
•Called “chinook” in N. America (native word for ‘snow eater’)
• The winds are caused by when a deep layer of prevailing wind
is forced up, over, and down a mountain range.
Graphic courtesy of USA Today
Black Hills of South Dakota are home to the world's fastest recorded rise in
temperature, a record that has held for six decades.
• On January 22, 1943, the northern and eastern slopes of the Black Hills were at
the western edge of an Arctic airmass and under a temperature inversion. A layer of
shallow Arctic air hugged the ground.
• At about 7:30am MST, the temperature in Spearfish, SD was –20o C. The chinook
kicked in, and two minutes later the temperature was 7.2o C above zero. The 27.2
degree rise in two minutes set a world record that is still on the books.
• By 9:00am, the temperature
had risen to 12.2o C.
• Suddenly, the chinook wind
ceased and the temperature
tumbled back to –20o.
• The 32.2 degree drop took
only 27 minutes.
Santa Ana Winds
Hot, dry winds that often
sweep through the L.A.
Basin in the fall and
winter.
•Develop when the desert
is cold, and are thus most
common during the cool
season stretching from
October through March.
•High pressure builds
over the Great Basin (e.g.,
Nevada) and the cold air
there begins to sink.
• Winds descend from the higher desert terrain down in to the L.A.
Basin - parcels become warmer and drier due to compressional heating
(about 10oC per kilometer of descent)
• As its temperature rises, the relative humidity drops; the air starts out
dry and winds up at sea level much drier still.
•The air picks up speed as it is channeled through passes and canyons.
• The fast, hot winds cause vegetation to dry out, increasing the danger
of wildfire. Once the fires start, the winds fan the flames and hasten
their spread.
Wind – a renewable resource for energy!
 Air has mass, and when it is in motion, it contains the energy of
that motion (kinetic energy)
 Some portion of that energy can converted into other forms —
mechanical force or electricity — that we can use to perform work
 Wind energy has been
used by people for hundreds
of years
Definition:
Kilowatt hour (kWh) = amount of electricity required
to light a 100 Watt light bulb for 10 hours
Average energy usage per person per year:
Alaska (highest in U.S.) = 340,000 kWh
Rhode Island (lowest in U.S.) = 6,200 kWh
Austria = 52,000 kWh
USA: 86% of energy comes from fossil fuels, 8% from
nuclear, 6% from renewable
Austria: 62% of energy comes from fossil fuels, 38% from
other (mostly renewable)
Equivalent oil consumption comparison:
kg/person/year
Austria
Canada
China
France
Germany
Norway
Sweden
Switzerland
United Kingdom
United States
AUT
CAN
CHN
FRA
DEU
NOR
SWE
CHE
GBR
USA
3,551.1
8,155.5
895.9
4,341.0
4,173.9
5,766.2
5,363.8
3,692.0
3,938.6
8,083.5
% of world population
% of world energy
U.S.
< 5%
~ 24%
China
~ 20 %
~ 10%
The considerable wind electric potential has not been tapped to its
fullest in modern times because wind turbine technology was not able to
utilize this resource.
However, during the past decade, increased knowledge of wind turbine
behavior has led to more cost-effective wind turbines that are more efficient
in producing electricity.
The price of electricity produced from wind by these advanced
turbines is estimated to be competitive with conventional sources of
power, including fossil fuels.