Weather Studies Introduction to Atmospheric Science American Meteorological Society

1. Weather StudiesIntroduction to Atmospheric ScienceAmerican Meteorological Society Chapter 5 Air Pressure Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. Case-in-Point • 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

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 thermometer 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 • 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 • 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

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 Variations in Air Pressure • 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

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 • 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. Air pressure varies continuously

16. 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

17. 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 (this chapter) • Wind speed changes in a downstream direction (Chapter 8)

18. Horizontal Variations in Air Pressure

19. 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 • A Low is an area where pressure is relatively low compared to the surrounding air • Highs are usually fair weather systems • Lows are usually stormy weather systems • Rising air is necessary for precipitation formation • Lows are rising columns of air. Highs are sinking columns of air.

20. 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)

21. The Gas Law, Continued • 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

22. 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

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