Download Air Pressure and Wind

Air Pressure and Wind
Definition of Air Pressure
Air pressure is simply the pressure exerted
by the WEIGHT of the AIR ABOVE.
Average air pressure at sea level is
about 1 kg/cm2 or the weight of a column
of water 10 meters high! This is called
one atmosphere.
So the air pressure exerted on the top of
your desk is more than 5000 kg!!!
So why doesn’t your desk collapse?
Your desk doesn’t collapse because
AIR PRESSURE IS EXERTED IN ALL
DIRECTIONS
The air pressure pushing DOWN on an object EXACTLY
BALANCES the air pressure pushing UP on the object.
10 m high
aquarium filled
with water
• The desk on the left
collapses because
the pressure
downward exceeds
the pressure from the
sides
• The desk on the right
does not collapse
because the water
pressure is exerted in
all directions.
Measuring Air Pressure
• Barometer – a device for
measuring air pressure
(bar = pressure, metron
= measuring instrument)
• As air pressure goes up,
the mercury in the tube
rises.
• 1 atm = 1013.5 mb
Torricelli, a student of Galileo,
invented the mercury
barometer in 1643
Aneroid Barometer
Mercury barometers are neither small nor
portable, so the aneroid barometer was
developed.
It uses a partially empty metal
chamber that is very sensitive
to changes in air pressure
(expanding as pressure
decreases, and compressing
as pressure increases).
This type of barometer can be connected to
a recording device, but nowadays there are
digital barometers with built-in memory.
What Causes Wind?
Wind is the result of HORIZONTAL
differences in air pressure.
Air flows from
areas of HIGHER
pressure to areas
of LOWER
pressure.
Why?
The kinetic theory of
matter!
Wind is nature’s way of balancing
inequalities in air pressure.
Ultimately, the sun is responsible for wind:
unequal heating of the Earth’s surface
generates pressure differences.
So why doesn’t wind flow straight
from high to low pressure?
Two reasons:
• The Earth rotates
• There is friction between moving air
and Earth’s surface
So there are THREE factors that
control wind: pressure differences,
the Coriolis effect, and friction.
Pressure differences
The greater the
difference in pressure,
the greater the wind
speed will be.
Isobars are lines on a
weather map that
connect places of equal
pressure.
The spacing of isobars
indicates the amount of
pressure change
happening over a given
distance.
You are familiar with a similar type of
map, a CONTOUR map of elevation.
Recall that closely spaced lines mean
steep changes in elevation, while lines
that are far apart indicate a gentle
slope or flat land.
Pressure Gradients
Closely spaced
isobars indicate a
steep pressure
gradient and high
winds.
Widely spaced
isobars indicate a
weak pressure
gradient and light
winds.
•The MAGNITUDE of the pressure gradient is
shown by the spacing of the isobars. The closer
the spacing, the bigger the pressure
gradient.
•Its DIRECTION goes from higher pressure towards
lower pressure, and PERPENDICULAR to the
Isobars
isobars.
This is known as the
H
L
PRESSURE
GRADIENT FORCE.
However, the other two factors (Coriolis effect
and friction) modify the wind direction, and
friction also modifies the wind speed.
Coriolis Effect
The Earth’s rotation affects
freely-moving objects, by
deflecting them to the right in
the Northern Hemisphere
and to the left in the
Southern Hemisphere.
This affects weather systems
and airplane navigation.
Let’s watch a brief movie about
this effect…
• The horizontal path of an object is really a straight
line (as it would appear to someone looking down
from space), but to someone on Earth’s surface, it
appears that the object veers away from its
intended path.
Of course, what is really happening is that the
Earth is rotating ‘out from under’ the object’s
path, at a rate of 15 degrees per hour eastward.
But what about winds traveling east or west?
The Coriolis effect causes the
same deflection to the right or
left!
This is more complicated to explain as
it involves centripetal acceleration
(what keeps something moving in a
circular orbit). Air moving east is
going faster than the Earth’s rotation,
so it wants to move outward toward
space. Gravity holds it down though,
so instead the air moves towards the
equator. Air moving west is going
slower, so it wants to dive down,
can’t, and moves towards the poles
instead.
How does the Coriolis Effect
act on wind?
1) Deflects to the right in the Northern
Hemisphere and to the left in the Southern
Hemisphere
2) At right angles (perpendicular)
to the direction of air flow
3) It is strongest at the poles and nonexistent
at the equator
4) It affects only wind direction, not wind speed
5) However, the faster wind is moving, the
MORE deflection occurs
Inertial circles
• Another result of the
Coriolis effect is that a
moving air (or water)
mass travels in a circular
trajectory called an
'inertial circle'. The
circles are bigger at the
equator than at the poles.
• Let’s watch a bit more of
that movie that
demonstrates this on the
merry-go-round to see
this demonstrated.
Friction
Friction acts opposite to the direction of wind flow
• For higher altitudes, friction is
unimportant. For wind speeds high enough,
the Coriolis effect exactly balances the
pressure-gradient force, and the winds flow
parallel to the isobars. This flow is called a
geostrophic wind.
Friction (cont.)
• Over the oceans,
there is little friction
and winds tend to flow
parallel to the isobars
(geostrophic wind).
• Over rougher terrain,
winds can be deflected
as much as 45
degrees from the
isobars, toward lower
pressure areas.
PGF – pressure gradient force
CF – Coriolis ‘force’
Jet Streams
The most important winds at higher altitudes are the
jet streams, rivers of air at 9 -10 km (30,000 ft)
high traveling 120 to 240 km/h (75 to 150 mph).
The one you are most familiar with
travels west to east across the
U.S. at the polar front, the
boundary between cold, polar
air and moist subtropical air.
See also TD A 5-Day View of the Jet Stream
Effect of Jet Streams
on Climate embedded
Pressure Centers and Winds
When you look at a weather map, you see
highs and lows. The are pressure centers
known as ANTICYCLONES and
CYCLONES (from the Greek kyklon
meaning ‘moving in a circle’).
CYCLONES are centers of LOW pressure
(pressure decreases toward the center).
ANTICYCLONES are centers of HIGH
pressure (pressure increases toward the
center).
Yes, hurricanes are called cyclones in the Indian
Ocean, but that’s not what we mean here!
Cyclonic and Anticyclonic Winds
We know that winds are most affected by the
pressure gradient and the Coriolis effect.
These two factors cause winds in the Northern
Hemisphere to blow COUNTERCLOCKWISE
around a LOW
and CLOCKWISE around a HIGH .
Southern Hemisphere
The opposite is
true in the
Southern
Hemisphere:
winds in lows
circulate
clockwise, while
winds in highs
circulate
counterclockwise.
Cyclonic and Anticyclonic Winds
Friction was the other factor, and it causes
air to flow
INWARD around a LOW
OUTWARD around a HIGH
Weather and Air Pressure
Remember convergence as a way to lift air?
This happens as winds flow into a cyclonic
(low pressure) system: to balance the
inflow (CONVERGENCE), there must be
outflow (DIVERGENCE) aloft at the same
rate.
In an anticyclone (high), surface air diverges
(outflow), which means there must be
convergence (inflow) and SUBSIDING air
aloft.
Weather of Cyclones
Convergence at the
surface, divergence
aloft.
Because of this
upward movement of
air, cyclones (lows) are
often associated with
stormy weather and
unstable conditions.
Weather of Anticyclones
Anticyclones (highs)
have the opposite
pattern of flow, with
winds converging aloft
and subsiding air at the
surface.
Because of the
downward movement of
air, highs are usually
associated with clear
skies and stable air.
Weather Forecasting
So now you can see why weather reports
emphasize the locations and possible
paths of lows and highs, especially the
lows.
Lows move roughly west-to-east across the
contiguous US, taking days to do so.
Their paths are somewhat unpredictable
because surface conditions are linked
to the air above them, so meteorologists
need to understand the total atmospheric
circulation.
Exploration Lab
• Break here for Exploration Lab activity
Global winds
Remember, it’s the sun that ultimately causes
winds.
More solar radiation is received at the equator
than is lost back to space.
Less solar radiation is received at the poles than is lost
back to space.
The atmosphere acts as a giant heattransfer system by balancing these
differences, moving warm air toward
the poles, and cold air toward the
equator.
If the Earth didn’t rotate …
Notice that there are TWO
CELLS, one in the Northern
Hemisphere and one in the
Southern Hemisphere.
Global winds would be
simple, with heated air at
the equator rising to the
tropopause, flowing
toward the poles, sinking,
and flowing along the
surface back to the
equator.
There would be permanent
lows along the equator
and highs at the poles.
Rotating Earth Model
•Hadley cells: At
the equator, rising
air produces a
pressure zone
called the
EQUATORIAL
LOW, an area with
abundant
precipitation and
little wind.
Notice that each hemisphere now
has THREE CELLS.
• Air then flows aloft
northward and
southward to about 30°
latitude where the air
subsides and heats due
to compression. These
are the SUBTROPICAL
HIGHS.
• The stable, dry
conditions associated
with the subtropical
highs are responsible
for the great deserts of
Africa (the Sahara),
Australia, and Arabia.
At these subtropical
highs, air flows along
the surface, some
toward the equator
and some toward the
poles, deflected due to
the Coriolis effect.
The TRADE WINDS
are belts of wind
between 30° N and S
latitude and the
equator that blow
almost constantly from
easterly directions.
You will recall from your social studies
classes how important these winds
were for global exploration and trade.
•Ferrel Cells between
30° and 60° latitude.
The prevailing
WESTERLIES are two
belts of wind that
dominate the surface
weather patterns in this
cell.
At 60° latitude is the
SUBPOLAR LOW
where surface winds
converge and rise to
the tropopause.
Subpolar low
The interaction between the warm air masses from the Ferrell
Cell and the cool air masses from the Polar Cell produce a
storm belt called the POLAR FRONT.
• Polar Cells,
from 60°
latitude to the
poles.
• The POLAR
EASTERLIES
are winds that
blow from the
polar high to
the subpolar
low, but they’re
not constant like
the trade winds.
At the poles, cold air
sinks and spreads
towards the equator.
This is the POLAR
HIGH.
Here is another view
of the global
circulation that
shows what kinds of
biomes are
associated with each
cell and the location
of the primary jet
streams.
Jet streams
Influence of Continents
The only truly continuous pressure belt is the
subpolar low in the Southern Hemisphere.
Here the ocean is uninterrupted by landmasses.
At other latitudes, particularly in the Northern
Hemisphere where landmasses break up the
ocean surface, large seasonal temperature
differences disrupt the pressure pattern.
July average global air circulation
Large landmasses, particularly Asia, become cold in the
winter when a seasonal high-pressure system develops.
From this system, surface airflow is directed off the land.
In the summer, landmasses are heated and develop lowpressure cells, which permit air to flow onto the land.
Look at the map above to see this general pattern.
These seasonal changes in wind direction are known as
the MONSOONS. During warm months, areas such as
India experience a flow of warm, water-laden air from
the Indian Ocean, which produces the rainy summer
monsoon.
The winter monsoon is dominated by dry continental air.
A similar situation exists to a lesser extent over North
America.
Regional Wind Systems
• Middle latitude circulation doesn’t fit the
model for the tropics, being complex.
• A PREVAILING WIND is a wind that
consistently blows more often from one
direction than any other.
• The general west-to-east flow in the
contiguous United States (the ‘westerlies’)
is interrupted by migrating cyclones and
anticyclones.
Local winds
• Caused by either topographic effects or
variations in surface composition—land and
water—in the immediate (LOCAL) area
• Land and sea breezes affect areas on coasts
or near large lakes, causing small areas of high
or low pressure that drive short-range winds.
VIDEO
Valley and mountain breezes
In mountain regions, there is a similar effect. During the
day, air along the slopes of mountains is heated
more intensely than air in the valleys. This is
reversed at night, with cold air flowing down the
slopes into the valleys.
Mountain breezes are most common during winter, while
valley breezes are most common during summer.
Valley breeze
Mountain breeze
Wind Measurement
Direction
• Labeled by the direction from
which they blow
• Using a wind or weather vane (N, E, S, W,
or degrees with 0° at north, 90° at east,
etc.)
Speed
• Anemometer (‘anemo’ = wind,
‘metron’ = measuring instrument)
• Beaufort wind scale
Global Distribution of Precipitation
• The tropical region in the area of the
Equatorial Low is the rainiest region on
Earth, including the rain forests of South
America and Africa.
• The regions in the area of the subtropical
highs are deserts.
Average Annual Precipitation