AMS Weather Studies Introduction to Atmospheric Science, 5 th Edition

1. AMS Weather StudiesIntroduction to Atmospheric Science, 5th Edition Chapter 5 Air Pressure

2. 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

3. Case-in-PointAir Pressures on Mount Everest • 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

4. 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

5. 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

6. 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

7. 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

8. Air Pressure Units • 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)

9. Variations in Air Pressurewith 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.

10. Variations in Air Pressurewith 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.

12. 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

13. 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 Horizontal Variations in Air Pressure Air pressure in Green Bay, WI, while under the influence of a very intense low pressure system

14. 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

15. 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.

16. 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

17. 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.

18. 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

19. 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

20. 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)

21. 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

22. 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

23. 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.

24. 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)

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