Air Pressure and Wind

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Air Pressure and Wind
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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 doesnt your desk collapse?

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• Your desk doesnt 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.
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10 m high aquarium filled with water
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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
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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.
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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!
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• Wind is natures way of balancing inequalities in
  air pressure.
• Ultimately, the sun is responsible for wind
  unequal heating of the Earths surface generates
  pressure differences.

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So why doesnt wind flow straight from high to
low pressure?

• Two reasons
• The Earth rotates
• There is friction between moving air and Earths
  surface
• So there are THREE factors that control wind
  pressure differences, the Coriolis effect, and
  friction.

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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.
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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.
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Pressure Gradients

• Closely spaced isobars indicate a steep pressure
  gradient and high winds.
• Widely spaced isobars indicate a weak pressure
  gradient and light winds.

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• 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.

However, the other two factors (Coriolis effect
and friction) modify the wind direction, and
friction also modifies the wind speed.
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Coriolis Effect

• The Earths rotation affects freely-moving
  objects, by deflecting them to the right in the
  Northern Hemisphere and to the left in the
  Southern Hemisphere.
• Lets watch a brief movie about this effect.

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• 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
  Earths 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 objects
path, at a rate of 15 degrees per hour eastward.
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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 Earths 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, cant, and
  moves towards the poles instead.

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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

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• 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.
• Lets watch a bit more of that movie that
  demonstrates this on the merry-go-round

Inertial circles
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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

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FrictionFriction acts opposite to the direction
of wind flow
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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
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• 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.

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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.

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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 thats not what we mean here!
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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 .

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Southern Hemisphere

• The opposite is true in the Southern Hemisphere
  winds in lows circulate clockwise, while winds in
  highs circulate counterclockwise.

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Cyclonic and Anticyclonic Winds

• Friction was the other factor, and it causes air
  to flow
• INWARD around a LOW
• OUTWARD around a HIGH

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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.

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• In an anticyclone (high), surface air diverges
  (outflow), which means there must be convergence
  (inflow) and SUBSIDING air aloft.

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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.
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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.
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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.

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Exploration Lab

• Break here for Exploration Lab activity

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Global winds
Remember, its 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 heat- transfer
system by balancing these differences, moving
warm air toward the poles, and cold air toward
the equator.
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If the Earth didnt rotate

• 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.

Notice that there are TWO CELLS, one in the
Northern Hemisphere and one in the Southern
Hemisphere.
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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.
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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.
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• 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.
• Polar Cells, from 60 latitude to the poles. The
  POLAR EASTERLIES are winds that blow from the
  polar high to the subpolar low, but theyre not
  constant like the trade winds.

At the poles, cold air sinks and spreads towards
the equator. This is the POLAR HIGH.
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.
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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.
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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.

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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
  low-pressure cells, which permit air to flow onto
  the land. Look at the map above to see this
  general pattern.

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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.
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Regional Wind Systems

• Middle latitude circulation doesnt 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.

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Local winds

• Caused by either topographic effects or
  variations in surface compositionland and
  waterin 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
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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.

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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 (anemowind, metronmeasuring
  instrument)
• Beaufort wind scale

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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

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