Transcript 05_AirPressure - davidmlawrence.com

NAS 125: Meteorology
Air Pressure
Mount Everest, part 1
• Mount Everest is 8,850 m (29,035 ft) above sea level.
• Local peoples revered it as sacred and traditionally
did not try to ascend it.
• The mountain was first scaled on 28 May 1953 by Sir
Edmund Hillary of New Zealand and Sherpa Tenzing
Norgay of Nepal.
– More than 4,000 have attempted the summit, of those, more
than 140 have died.
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Mount Everest, part 2
• Mount Everest is at roughly the same latitude as
Tampa, Fla.
– Because of its tremendous elevation, however, the upper
reaches of the mountain are never above freezing.
• Mean January temperature at the summit is -36 °C
• Mean July temperature at the summit is -19 °C
– Clouds enshroud the peak through much of June through
September as the Indian monsoon buffets the subcontinent.
– The jet stream brings hurricane-force winds to the summit
from November through February.
– Hypothermia is a constant threat on the mountain.
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Mount Everest, part 3
• The atmosphere becomes thinner – there are fewer
molecules per unit volume – at higher elevations.
• The decrease in oxygen levels with increasing
elevation further increases the risk of attempting the
summit.
– The pressure of oxygen at the surface is only one-third of
oxygen at sea level.
• Although some may reach the summit without
supplemental oxygen, most need an extra oxygen
supply.
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Atmospheric pressure
• Pressure is the force a gas (or liquid) exerts on some
specified area of the container walls.
• Atmospheric pressure is the force exerted by gas
molecules in the atmosphere.
– It affects Earth’s surface as well as any other body on
Earth.
– It is an omnidirectional force, a force exerted equally in all
directions.
– The force drops with increasing altitude because actual
number of gas molecules also drops.
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Measuring pressure, part 1
• A barometer is used to measure pressure and monitor
its changes.
• Two types of barometers:
– Mercury barometer: Cumbersome; accurate; uses a glass
tube, sealed at one end but open at the other; and filled with
mercury; the open end of the tube is inserted into a
reservoir of mercury; the mercury fills the tube until the
pressure of the mercury in the tube is equalled by the
pressure of the atmosphere pressing down upon it.
– Average pressure at sea level is 760 mm.
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Measuring pressure, part 2
• Two types of barometers (continued):
– Aneroid barometer: Does not use liquid; more portable than
a mercury barometer; consists of a flexible chamber that
compresses and expands with changes in air pressure.
– Aneroid barometers can be modified to measure altitude;
such modified instruments are called altimeters.
• Air pressure tendency is the change in air pressure
with time.
– Rising pressure generally indicates fair weather.
– Dropping pressure generally indicates stormy weather.
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Measuring pressure, part 3
• Sometimes an aneroid barometer is attached to a pen
that traces a line on a chart on a clock-driven drum;
this instrument is called a barograph.
• Units of air pressure:
– For civilian use, pressure is often reported in millimeters or
inches of mercury (760 mm or 29.92 inches).
– Physicists use Pascals (101,325 Pa)
– U.S. meteorologists use millibars (1013.25 mb)
– Worldwide range 970 mb to 1040 mb.
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Vertical variations
• Density: Mass per unit volume
• Number density: Number of molecules per unit
volume
• Air thins with increasing altitude.
– At 16 km, air density is about 14 percent of density at sea
level.
– Pressure decreases as well with increasing altitude.
• Air is compressible.
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Standard atmosphere, part 1
• The standard atmosphere is a model of the
atmosphere averaged for all latitudes and seasons.
–
–
–
–
Fixed sea-level temperature (15 °C)
Fixed sea-level pressure (1013.25 mb)
Fixed vertical profiles of pressure and temperature
Actual values vary, of course
• Upper-air weather patterns are plotted as isobaric
surfaces – contour lines in which the air pressure is
the same everywhere.
– Typically 200-mb, 500-mb, and 850-mb levels.
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Standard atmosphere, part 2
• The Earth’s atmosphere grades imperceptibly with
interplanetary space.
– Half the atmosphere’s mass lies below 5,500 m.
– About 99 percent of the atmosphere’s mass lies below 32
km.
– Above the homosphere (80 km) the relative proportions of
atmospheric gases change markedly.
– About 1,000 km, the atmosphere merges with
interplanetary gases (hydrogen and helium).
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Standard atmosphere, part 3
• With uniform pressure and temperature (at average
sea-level value) the top of a uniform density
atmosphere would be 8 km.
• Low density at high altitudes affects air temperature
and heat transfer.
– Despite high temperatures of thermosphere (1,200 °C), the
low density of the air prevents efficient heat transfer.
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Horizontal variations, part 1
• Mapping pressure with isobars
– An isobar is a line joining points of equal atmospheric
pressure.
– “High” and “low” pressures are relative conditions, with
the distinction depending on the pressure of the adjoining
areas.
– On weather maps, pressure measured at the surface is
adjusted to sea-level pressure to make comparisons easier.
– A pressure gradient, the horizontal rate of pressure change,
representing the “steepness” of the pressure slope, directly
affects the speed of wind.
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Horizontal variations, part 2
• Mapping pressure with isobars (continued):
– Despite the fact that horizontal pressure variations are of
relatively lower magnitude that vertical pressure variations,
the horizontal variations may be associated with important
changes in weather.
• Pressure also varies day by day and hour by hour.
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Temperature and humidity, part 1
• Cold air is more dense than warm air.
– Warm air rises.
• As air density increases, volume decreases, while
pressure and temperature increases.
• Temperature of air affects the rate of pressure change
with change in altitude.
– Pressure drops more rapidly in cold air than warm air.
• Dry air is more dense than moist air.
– Molecular weight of water is less than that of oxygen and
nitrogen.
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Temperature and humidity, part 2
• Cold, dry air masses are more dense and usually
produce higher surface pressures than warm, moist air
masses.
• Warm, dry air masses typically exert higher surface
pressures than equally warm, but more humid, air
masses.
• Changes in surface pressure are typically
accompanied by replacement of one air mass by
another – advection.
• Air masses are modified by the Earth’s surface.
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Divergence and convergence
• Diverging winds blow away from an area.
– Diverging winds, accompanied by lead to increasing
pressure at the surface.
• Converging winds blow toward an area.
– Converging winds lead to decreasing pressure at the
surface.
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Ideal gas law
• Temperature, pressure, and density are known as
variables of state.
• Ideal gas law: Pressure is proportional to the product
of density and temperature.
– Pressure = constant * density * temperature
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