AMS Weather Studies

Introduction to Atmospheric Science, 5th

Edition

Chapter 5Air Pressure

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What is the significance of horizontal and vertical variations in air pressure?

This chapter covers: The properties of air pressure Air pressure measurement Spatial and temporal variations in air pressure Gas Law Expansional Cooling and compressional warming

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Driving Question

Case-in-PointAir Pressures on Mount Everest

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Mount Everest World’s tallest mountain – 8850 m (29,035 ft) Same latitude as Tampa, FL Due to declining temperature with altitude, the summit is always cold January mean temperature is -36 °C (-33 °F) July mean temperature is -19° C (-2 °F) Shrouded in clouds from June through September due to monsoon

winds November through February – Hurricane-force winds

Due to jet stream moving down from the north Harsh conditions make survival at the summit difficult Very thin air Wind-chill factor Most ascents take place in May

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Air exerts a force on the surface of all objects it contacts As a gas, air molecules in constant motion Air molecules collide with a surface area in contact with air

The force of these collisions per unit area is pressure Dalton’s Law

Total pressure exerted by mixture of gases is sum of pressures produced by each constituent gas

Air pressure Depends on mass of the molecules and kinetic molecular energy Thought of as the weight of overlying air acting on a unit area

Weight is the force of gravity exerted on a massWeight = (mass) x (acceleration of gravity)

Average sea-level air pressure 1.0 kg/cm2 (14.7 lb/in.2) Air pressure acts in all directions

Structures do not collapse under all the weight

Defining Air Pressure

Barometer – instrument used to measure air pressure and monitor changes

Mercury barometer – employs air pressure to support a column of mercury in a tube Air pressure at sea level supports mercury

to a height of 760 mm (29.92 in.) Height of the mercury column changes

with air pressure Adjustments required for:

Expansion and contraction of mercury with temperature

Gravity variations with latitude and altitude

Air Pressure Measurement

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Aneroid barometer More portable/less precise Chamber with a partial vacuum

Changes in air pressure collapse or expand the chamber

Moves pointer on scale calibrated equivalent to mm or in. of mercury

New versions depend on the effect of air pressure on electrical properties of crystalline substance

Home-use aneroid barometers often have a fair, changeable, and stormy scale These should not be taken literally

Air Pressure Measurement

Air Pressure Measurement

Air pressure tendency – change in air pressure over a specific time interval Important for local forecasting

Barographs – Barometer linked to a pen that records on a clock-driven drum chart Provides a continuous trace of

air pressure variations with time

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Units of length Millimeters or inches

Units of pressure Pascal – worldwide standard

Sea-level pressure: 101,325 pascals (Pa) = 1013.25 hectopascals (hPa) = 101.325 kilopascals (kPa)

Bars – US Bar is 29.53 in. of mercury Millibar (mb) standard on weather maps (mb = 1/1000

bar) Usual worldwide range is 970-1040 mb Lowest ever recorded – 870 mb (Typhoon Tip in 1979) Highest ever recorded – 1083.8 mb (Agata, Siberia)

Air Pressure Units

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Overlying air compresses the atmosphere The greatest pressure at the lowest elevations

Gas molecules closely spaced at Earth’s surface Spacing increases with altitude At 18 km (11 mi), air density is only 10% of sea level

Because air is compressible Drop in pressure with altitude is greater in the lower

troposphere, Becomes more gradual aloft.

Vertical profiles of average air pressure and temperature are based on the standard atmosphere State of atmosphere averaged for all latitudes and seasons.

Variations in Air Pressurewith Altitude

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Even though density and pressure drop with altitude, it is not possible to pinpoint a specific altitude at which the atmosphere ends. ½ the atmosphere’s mass is below

5500 m (18,000 ft) 99% of the mass is below 32 km (20

mi) Denver, CO, average air pressure is

83% of Boston, MA

Variations in Air Pressurewith Altitude

Average air pressure variation with altitude expressed in mb.

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Horizontal Variations in Air Pressure

Horizontal variations much more important to weather forecasters than vertical Local pressures at

elevations adjusted to equivalent sea-level values Shows variations of

pressure in horizontal plane

Mapped by connecting points of equal equivalent sea-level pressure, producing isobars

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Horizontal Variations in Air Pressure

Horizontal changes in air pressure accompanied by changes in weather

In middle latitudes, continuous procession of different air masses brings changes in pressure and weather Temperature has more

pronounced affect on air pressure than humidity

In general, falling air pressure brings storms; rising air pressure brings clear or fair weather

Air pressure in Green Bay, WI, while under the

influence of a very intense low pressure system

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Influence of temperature and humidity Rising air temperature = rise in the average kinetic

energy of the individual molecules In a closed container, heated air exerts more pressure on

the sides Density in a closed container does not change No air has been added or removed

The atmosphere is not like a closed container Heating the atmosphere causes the molecules to space

themselves farther apart due to increased kinetic energy Molecules placed farther apart have a lower mass per unit

volume (density) The heated air is less dense and lighter

Horizontal Variations in Air Pressure

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Horizontal Variations in Air Pressure

Hot air balloons ascend within the atmosphere because the heated air within the balloons is less dense than the cooler air surrounding

the balloon.

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Influence of temperature and humidity Air pressure drops more rapidly with altitude in cold air

Cold air is denser, has less kinetic energy, molecules are closer together

500 mb surfaces represent where half of the atmosphere is above and half below, by mass This surface is at a lower altitude in colder air than warmer

Increasing humidity decreases air density Greater the concentration of water vapor, the less dense the

air; due to Avogadro’s Law. Muggy air reffered to as ‘heavy’ air, but it is lighter than dry air

Muggy air weighs heavily on personal comfort

Horizontal Variations in Air Pressure

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Influence of temperature and humidity Cold, dry air masses are the densest

Generally produce higher surface pressures Warm, dry air masses exert higher pressure than warm,

humid air masses Pressure differences create horizontal pressure gradients

Causes cold and warm air advection Air mass modifications also produces changes in surface

pressures

Conclusion: local conditions and air mass advection can influence air pressure.

Horizontal Variations in Air Pressure

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Influence of diverging and converging winds Diverging winds blow away from a

column of air Converging winds blow towards a

column of air Causes

Horizontal winds blowing toward/away from a location

Wind speed changes in a downstream direction (Chap 8)

If more air diverges at the surface than converges aloft Air density, surface air pressure

decrease If more air converges aloft than

diverges at the surface Density and surface pressure

increase

Horizontal Variations in Air Pressure

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Isobars on a map U.S. convention at every 4-mb intervals (996 mb, 1000

mb, 1004 mb) High – an area where pressure is higher than

surrounding air Usually fair weather systems Sinking columns of air

Low – an area where pressure is lower than the surrounding air. Usually stormy weather systems Rising columns of air

Rising air necessary for precipitation formation

Highs and Lows

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Variables of state Variability of temperature, pressure, and density

Magnitudes change from another across Earth’s surface, with altitude above Earth’s surface, and with time

Related through the ideal gas law, a combination of Charles’ law and Boyle’s law

Ideal gas law: pressure exerted by air is directly proportional to the product of its density and temperaturepressure = (gas constant) x (density) x (temperature)

The Gas Law

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Conclusions from ideal gas law Density of air within a rigid, closed container remains

constant Increasing the temperature leads to increased pressure

Within an air parcel with a fixed number of molecules Volume can change, mass remains constant Compressing air increases density because volume

decreases Within the same air parcel

With a constant pressure, a rise in temperature is accompanied by a decrease in density

Expansion due to increased kinetic energy increases volume At a fixed pressure, temperature is inversely proportional to

density

The Gas Law

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Expansional Cooling and Compressional Warming

Expansional cooling When an air parcel expands, temperature of the gas

drops

Compressional warming When the pressure on an air parcel increases, parcel is

compressed and temperature rises

Conservation of energy Law of energy conservation/1st law of thermodynamics

Heat energy gained by an air parcel either increases the parcel’s internal energy or is used to do work on parcel

Change in internal energy directly proportional to change in temperature

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A. If the air is compressed, energy is used to do work on the air.

B. If air expands, the air does work on the surroundings.

Expansional Cooling and Compressional Warming

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Expansional Cooling and Compressional Warming

Adiabatic process No heat is exchanged between an air parcel and

surroundings Temperature of an ascending or descending unsaturated

parcel changes in response to expansion or compression only

Dry adiabatic lapse rate: 9.8 C°/1000 m (5.5 °F/1000 ft) Once a rising parcel becomes saturated, latent heat

released to the environment during condensation or deposition partially counters expansional cooling.

Moist adiabatic lapse rate (averaged): 6 C°/1000 m (3.3 °F/1000 ft)

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Adiabatic Processes

Illustration of dry and moist adiabatic lapse rates.

Dry adiabatic lapse rate describes the expansional cooling of ascending

of unsaturated air parcels.