Chapter 5 Air Pressure

1. Chapter 5 Air Pressure

2. Case-in-Point • Mount Everest • World’s tallest mountain – 8848 m (29,029 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

3. Driving Question • What is the significance of horizontal and vertical variations in air pressure? • Air pressure is an element of weather we do not physically sense as readily as temperature and humidity changes • This chapter examines: • The properties of air pressure • How air pressure is measured • The reasons for spatial and temporal variations in air pressure

4. Defining Air Pressure • Air exerts a force on the surface of all objects it contacts • The air is a gas, so the molecules are in constant motion • The 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 • Air pressure can be 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 • That is why structures do not collapse under all the weight

5. Air Pressure Measurement • A mercury barometer employs air pressure to support a column of mercury in a tube • Air pressure at sea level will support the mercury to a height of 760 mm (29.92 in.) • Height of the mercury column changes with air pressure • Adjustments required for: • The expansion and contraction of mercury with temperature • Gravity variations with latitude and altitude

6. Air Pressure Measurement • An aneroid barometer is less precise, but more portable than a mercury barometer • It has a chamber with a partial vacuum • Changes in air pressure collapse or expand the chamber • This moves a pointer on a scale calibrated equivalent to mm or in. of mercury • New ones are piezoelectric – depend on the effect of air pressure on a crystalline substance • Home-use aneroid barometers often have a fair, changeable, and stormy scale • These should not be taken literally

7. Air Pressure Measurement • Forecasting uses air pressure and tendency values • changes over time • Barometers may keep a record of air pressure • These are called barographs

8. Air Pressure Units • Units of length • Millimeters or inches • Inches typical for TV • Units of pressure • Pascal – worldwide standard • Metric scale • Sea-level pressure = • 101,325 pascals (Pa) • 1013.25 hectopascals (hPa) • 101.325 kilopascals (kPa) • Bars – U.S. • A bar is 29.53 inches of mercury • A millibar (mb) is the standard used on weather maps, meaning 1/1000 of a 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 Pressure w/Altitude • Overlying air compresses the atmosphere • the greatest pressure is at the lowest elevations • Gas molecules are closely spaced at the surface • Spacing increases with altitude • At 18 km (11 mi), air density is only 10% of that at sea level • Because air is compressible, the drop in pressure with altitude is greater in the lower troposphere • Then it becomes more gradual aloft http://atmo.tamu.edu/class/atmo202/Pressuredir/pressure-pg11.html • Vertical profiles of average air pressure and temperature are based on the standard atmosphere – state of atmosphere averaged for all latitudes and seasons • Even though density and pressure drop with altitude, it is not possible to pinpoint a specific altitude at which the atmosphere ends

10. Average Air Pressure Variation with Altitude Expressed in mb

11. Horizontal Variations in Air Pressure • Horizontal variations are much more important to weather forecasters than vertical differences • In fact, local pressures at elevations are adjusted to equivalent sea-level values • This shows variations of pressure in the horizontal plane • This is mapped by connecting points of equal equivalent sea-level pressure, producing isobars

12. Horizontal changes in pressure can be accompanied by significant changes in weather In middle latitudes, a continuous procession of different air masses brings changes in pressure and weather Temperature has a much more pronounced affect on air pressure than humidity In general, the weather becomes stormy when air pressure falls but clears or remains fair when air pressure rises Horizontal Variations in Air Pressure Air pressure varies continuously

13. 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 • This is due to increased kinetic energy • Molecules placed farther apart have a lower mass per unit volume, or density • The heated air is less dense, and lighter.

14. Horizontal Variations in Air Pressure • Influence of temperature and humidity, continued • Air pressure drops more rapidly with altitude in a column of cold air • Cold air is denser, has less kinetic energy, so the 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 cold air vs. in warm air • Increasing humidity decreases air density • The greater the concentration of water vapor, the less dense is the air due to Avogadro’s Law (amu H20 is 18, for air is 29 amu) • We often refer to muggy air as heavy air, but the opposite is true • Muggy air only weighs heavily on our personal comfort factor

15. Horizontal Variations in Air Pressure • Influence of temperature and humidity, continued • Cold, dry air masses are the densest • These generally produce higher surface pressures • Warm, dry air masses generally exert higher pressure than warm, humid air masses • These pressure differences create horizontal pressure gradients • - Pressure gradients cause cold or warm air advection • Air mass modifications can also produce changes in surface pressures • Conclusion: local conditions and air mass advection can influence air pressure

16. Horizontal Variations in Air Pressure • Influence of diverging and converging winds • Diverging = winds blowing away from a column of air • Converging = winds blowing towards a column of air • Diverging/converging caused by : • Horizontal winds blowing toward or away from some location

17. If more air diverges at the surface than converges aloft, the air density and surface air pressure decrease If more air converges aloft than diverges at the surface, density and surface pressure increase Influence of Diverging and Converging winds

18. Highs and Lows • Isobars are drawn on a map as previously discussed • U.S. convention – these are drawn at 4-mb intervals (e.g., 996 mb, 1000 mb, 1004 mb) • A High is an area where pressure is relatively high compared to the surrounding air (usually fair weather systems; sinking columns of air). • A Low is an area where pressure is relatively low compared to the surrounding air (usually stormy weather systems; Lows are rising columns of air). • Rising air is necessary for precipitation formation

19. The Gas Law • We have discussed variability of temperature, pressure, and density → these properties are known as variables of state; their magnitudes change from one place to another across Earth’s surface, with altitude above Earth’s surface, and with time • The three variables of state are related through the ideal gas law, which is a combination of Charles’ law and Boyle’s law • The ideal gas law states that pressure exerted by air is directly proportional to the product of its density and temperature, i.e. pressure = (gas constant) x (density) x (temperature)

20. The Gas Law • Conclusions from the 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 the air increases density because its 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 • Hence, at a fixed pressure, temperature is inversely proportional to density

21. Expansional Cooling and Compressional Warming • Expansional cooling – when an air parcel expands, the temperature of the gas drops • Compressional warming – when the pressure on an air parcel increases, the parcel is compressed and its 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 the parcel • A change in internal energy is directly proportional to a change in temperature

22. Conservation of Energy • If the air is compressed, energy is used to do work on the air • If we allow the air to expand, the air does work on the surroundings

23. Adiabatic Processes • During an adiabatic process, no heat is exchanged between an air parcel and its surroundings • The 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 = 6 C°/1000 m (3.3 °F/1000 ft) → this is an average rate

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