Download Pressure - KBCC Meteorology

Weather Map April 3
Atmospheric Pressure
Air Pressure Affects
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Wind movement
Cloud formation (convergence)
Precipitation
Development of storms
Pressure is one of the most properties for studying
the atmosphere and predicting weather!
• Pressure - force exerted per unit area
• Units of pressure
–Pascal (pa)
–Bar (b)
–Millibar (mb)
–1mb = 100Pa
• Surface pressure - pressure that the atmosphere
exerts on the surface.
Near sea level the average air pressure is about 1000mb =
1b = 100Pa = 1 atmosphere = 14.7 pounds/inch2.
1013mb is the true value.
• Pressure - force exerted per unit area
• In a sealed container, the air pressure is proportional
to rate of collisions between molecules and the walls
of the container.
• Two ways to increase P
–Increase the amount of gas molecules
–Increase temperature
• If the pressure in your container is less than the air pressure
outside the container, opening the container will cause air to
rush in.
• If the pressure in your container is greater than the air pressure
outside the container, opening the container will cause air to
rush out.
Air moves from areas of high pressure to areas of low pressure.
• Ideal Gas Law
• P=DRT
– P - pressure
– D - density
– R- constant
– T - temperature
This indicates a relationship between pressure,
temperature and density.
If D increases and T remains constant, P will
increase.
If T increases and D remains constant, P will
increase.
Measuring Pressure
• Barometer - any instrument used to measure
pressure.
• Two Common Types
– Mercury barometer
– Aneroid Barometer
Mercury Barometer
• Standard instrument for measuring pressure
• Consists of a reservoir of mercury and a graduated tube
marked in centimeters
• Air pressure is pushed down on the mercury in the
reservoir and forces it up the tube
• The barometric pressure is expressed in centimeters (cm)
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1 cm = 13.32 mb
Standard air pressure at sea level is 76 cm
1 inch = 33.84 mb
Standard air pressure at sea level is 29.2 inches
• Correction to the Mercury Barometer
– 1) elevation
– 2) Temperature
– 3) latitude (gravity differences)
Corrections for the Mercury Barometer
• 1) Elelvation
– When studying pressure distributions it is often useful
to look at horizontal distributions of pressure. Since
pressure changes with elevation, measurements of
pressure taken at different elevations are standardized
to”mean sea level”.
2) Temperature
since mercury also responds to temperature change, a
temperature correction must be made and mercury
barometers usually have thermometers too.
3) gravity
gravity variations are standardized to 45o N and S of
the equator.
• Aneroid Barometer
– Without liquid
– Contains a collapsible chamber (the aneroid cell)
– Weight of the atmosphere pushed down on the chamber
and compresses it by an amount proportional to the air
pressure.
– Corrections
• Elevation
• Location
• No temperature correction required.
Other Barometer types
• Barograph
• Microbarometer
Barograph
• Stacked aneroid cells
move a needle
• Needle movement is
traced out on a spool
of paper
• This creates a data-log
of variations in
atmospheric pressure
Microbarometer
• Extremely sensitive barometers
• Can measure with great precision
• Can measure in microbars or Pascals (vs.
hectopascals and millibars)
Microbarometer
• There is a global network of microbarographs
• They are used to monitor worldwide compliance
with the Comprehensive Nuclear Test Ban Treaty
• Detect minute changes in pressure associated with
nuclear explosions
• As a network can id the location and nuclear yield
of an explosion
Microbarometer
Image courtesy of: http://www.knmi.nl/~evers/infrasound/lofar/exloo/install1.jpg
Pressure distributions are important to map
across the globe because they influence winds.
Weather maps show distributions of sea level
pressure using lines of pressure called isobars.
Isobar - Map line connecting point of the same
pressure.
Points between isobars have pressures
between that represented by the lines.
Usually drawn at intervals of 4mb on a
surface map.
Isobars.
Spacing of isobars indicates the pressure
gradient.
Pressure gradient - rate of change in
pressure over a region.
Closely spaced isobars indicates a steep
pressure gradient.
Widely spaced isobars indicates a gentle
pressure gradient.
Air spreads from areas of high pressure to areas
of low pressure and the pressure gradient
produces a force called the pressure gradient
force (PGF)
Pressure gradient force (PGF) - a force that
arises from spatial variation in pressure.
Pressure Gradients
1) Horizontal pressure gradients
Changes in pressure gradient across the US (~
3000 km) are ~ 40mb
40mb/3000km or 1mb/75km
2) Hurricane pressure gradients
Hurricanes produce the most destructive
winds.
Hurricane pressure gradients =
1mb/6km
Vertical Pressure Gradients
Pressure Gradients
3) Vertical pressure gradients
Much greater
Surface e pressures are about 1000mb
5 km up into the troposphere pressures
decrease to 500mb
Vertical pressure gradients =
100mb/km
So why don’t the air blow away?
Hydrostatic equilibrium - vertical PGF
is balanced by the force of gravity.
5000m
500mb
500mb
4000m
3000m
2000m
1000m
0m
1000mb
1000mb
The PGF is proportional to
the slope of the 500mb pressure
surface.
5000m
500mb
500mb
700mb
4000m
3000m
2000m
1000m
0m
1000mb
1000mb
Contour Lines
• For the upper atmosphere contour lines are
used to show the height at which a specific
pressure occurs.
• Contour lines - lines connecting point of
the same elevation.
Weather Maps
• NOAA - National Oceanographic and Atmosphere
Administrations
• NWS - National Weather Service
– Produces a series of weather maps twice a day.
– Surface Maps - using isobar maps
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Using height contours:
300mb - Upper atmosphere
500mb - middle atmosphere
700mb -lower atmosphere
850mb - lower atmosphere
Forces Affecting Speed and
Direction of Wind
• If no other forces were present the wind would
flow in the direction of the pressure gradient force.
• Two other forces affect winds:
– Coriolis Force
– Friction
• Coriolis Force - force arising from Earth’s rotation
exerted on all freely moving objects.
First described by Gaspard de Coriolis (19th Century)
Coriolis Force
• Earth’s rotation causes two types of motion:
– Translational motion
– Rotational motion
• Rotational motion is greatest at the poles
Effects of the Coriolis Force
1) Objects moving in free space experience an
apparent deflection to the right of their path of
motion.
2) Coriolis force is strongest at the poles and weakest
at the equator.
3) Coriolis force is a deflective force affecting
direction only and not speed.
Friction
Air in contact with the surface experiences frictional
drag, which decreases wind speed.
Affects the lower 1.5 km of the atmosphere called
the planetary boundary later.
Planetary boundary layer - lower 1.5 km of the
atmosphere, which is affected by frictional drag.
Free atmosphere - part of the atmosphere above 1.5
km, which is not affected by frictional drag.
Therefore winds in the upper atmosphere are
simpler
Winds in the Upper Atmosphere
Geostrophic (Earth turned) flow - wind flows in a straight
path, at constant speed parallel to isobars or height
contours.
Isobars are straight, parallel and equally spaced- Ideal
Gradient Flow - wind blowing parallel to the
isobars and perpendicular to the pressure gradient.
Wind may change direction and speed.
Supergeostrophic flow - PGF is weaker than the
Coriolis force causing the air to rotate clockwise.
Occurs around high pressure systems called
anitcyclones
Winds in the Upper Atmosphere
Geostrophic (Earth turned) flow - wind flows in a straight path, at
constant speed parallel to isobars.
Isobars are straight, parallel and equally spaced- Ideal
Gradient Flow - wind blowing parallel to the isobars and perpendicular to
the pressure gradient.
Wind may change direction and speed.
Supergeostrophic flow - PGF is weaker than the Coriolis
force causing the air to rotate clockwise.
Occurs around high pressure systems called
anitcyclones.
Subgeostrophic flow - PGF is stronger than the Coriolis
force causing the air to rotate counterclockwise.
Occurs low pressure systems called cyclones.
Cyclones and Anticyclones
Cyclones and Anticyclones
• Sea level pressure across the globe are
divided into high and low pressure centers.
Anticyclones
• Enclosed areas of high pressure.
• Shown on weather maps by “H”
• In the Northern Hemisphere, upper level winds rotate
clockwise around H pressure systems.
In near surface H pressure systems air spirals out from the central H
in a clockwise rotation.
• In the Southern Hemisphere, upper level winds rotate
counterclockwise around H pressure systems.
In near surface H pressure systems air spirals out from the central H
in a counterclockwise rotation.
• Results:
– Inhibits cloud growth because of divergence
– Low wind speeds
– Clear skies
Cyclones
• Enclosed areas of low pressure.
• Shown on weather maps by “L”
• In the Northern Hemisphere, upper level winds rotate
counterclockwise around L pressure systems.
In near surface L pressure systems air spirals in toward the central L in a
counterclockwise rotation.
• In the Southern Hemisphere, upper level winds rotate
clockwise around L pressure systems.
In near surface H pressure systems air spirals in toward the central L in a
clockwise rotation.
• Results:
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cloud growth because of convergence
high wind speeds
Cloudy skies
Major producers of stormy weather in our region
Convergence in the upper atmosphere leads to divergence
near the surface
Divergence in the upper atmosphere leads to convergence
near the surface
Other weather indicators
• Storm Glass
• Tempest Prognosticator
Storm Glass
• Developed by Admiral
Robert FitzRoy
(captain of the HMS
Beagle )
• Mixture of:
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distilled water
Ethanol
potassium nitrate
ammonium chloride
Camphor
Image after: http://www.wikipedia.com
Tempest Prognosticator
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Developed by Dr. Merryweather of England.
Exhibited at “The Great Exhibition” In 1851
The Crystal Palace
Also called the Leech Barometer
Image after: http://www.britannica.com
Tempest Prognosticator
• Noted that leeches
become agitated
when storms
approach
• Leeches in the
device crawl to
the top of their
chambers, where
they trigger the
ringing of a small
bell
Image after: http://www.whitbymuseum.org.uk
Pressure Maps
• http://www.usairnet.com/weather/maps/curr
ent/barometric-pressure/
• http://ww2010.atmos.uiuc.edu/(Gl)/guides/
maps/sfc/slp/irslpvec.rxml