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
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.
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.
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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.
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.
So how does the Coriolis effect
act on wind?
• At right angles (perpendicular) to the
direction of air flow
• It is strongest at the poles and
nonexistent at the equator
• It affects only wind direction, not wind
speed
• However, the faster wind is moving, the
MORE it is deflected
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
So how important
is the Coriolis effect?
• For hurricanes, the deflection is about 40 km a
day.
• Over the large scale of most air masses, and the
days it takes them to travel, this adds up.
• In New Mexico a softball hit 100m down the right
field line will be deflected 1.5cm to the right.
• For the inertial circles, a wind speed of 10 m/s at
Hobbs’ latitude means a circle would be about
200 km (124 miles) across, with one rotation in
14 hours
Friction
Friction acts opposite to the direction of wind flow
Friction (cont.)
• Over the oceans, there
is little friction and
winds tend to flow
parallel to the isobars.
• 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’
• 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.
Jet Streams
The most important winds at
higher altitudes are the jet
streams, rivers of air traveling
120 to 240 km/h.
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.
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 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.
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
Notice that each cell has now
become THREE CELLS.
•Hadley cells: At the equator,
rising air produces a pressure
zone called the EQUATORIAL
LOW, an area with abundant
precipitation and little wind. 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 two
belts of wind between 30°
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.
At the poles, cold air sinks and
spreads towards the equator.
This is the POLAR HIGH.
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.
•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.
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.
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.
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.
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