AMS Weather Studies Introduction to Atmospheric Science, 5th Edition Chapter 5 Air Pressure © AMS

Driving Question 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 © AMS

Case-in-Point Air Pressures on 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 © AMS

Defining Air Pressure 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 mass Weight = (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 © AMS

Air Pressure Measurement 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 © AMS

Air Pressure Measurement 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 © AMS

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 Easier to determine pressure tendency © AMS

Air Pressure Units Units of length Units of pressure 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) © AMS

Variations in Air Pressure with Altitude 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. © AMS

Variations in Air Pressure with Altitude 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 Average air pressure variation with altitude expressed in mb. © AMS

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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 © AMS

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 © AMS

Horizontal Variations in Air Pressure 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 © AMS

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. © AMS

Horizontal Variations in Air Pressure 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 © AMS

Horizontal Variations in Air Pressure 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. © AMS

Horizontal Variations in Air Pressure 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 © AMS

Highs and Lows 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 © AMS

The Gas Law 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 temperature pressure = (gas constant) x (density) x (temperature) © AMS

The Gas Law 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 © AMS

Expansional Cooling and Compressional Warming 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 © AMS

Expansional Cooling and Compressional Warming If the air is compressed, energy is used to do work on the air. If air expands, the air does work on the surroundings. © AMS

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) © AMS

Illustration of dry and moist adiabatic lapse rates. Adiabatic Processes Dry adiabatic lapse rate describes the expansional cooling of ascending of unsaturated air parcels. Illustration of dry and moist adiabatic lapse rates. © AMS