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Ch05Pres-1 - AMS Weather Studies Introduction to...

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Unformatted text preview: AMS Weather Studies Introduction to Atmospheric Science, 4th Edition Introduction Chapter 5 Chapter Air Pressure 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 Due always cold always January mean temperature is -36 °C (-33 °F) January (-33 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 2 Driving Question What is the significance of horizontal and What vertical variations in air pressure? vertical – Air pressure is an element of weather we do not Air physically sense as readily as temperature and humidity changes humidity – This chapter examines: The properties of air pressure How air pressure is measured The reasons for spatial and temporal variations in air The pressure pressure 3 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 Dalton’s sum of pressures produced by each constituent gas sum Air pressure depends on: – Mass of the molecules and kinetic molecular energy Air pressure can be thought of as the weight of overlying air Air acting on a unit area acting – Weight is the force of gravity exerted on a mass Weight = (mass) x (acceleration of gravity) – 1.0 kg/cm2 (14.7 lb/in.2) (14.7 Average sea-level air pressure Air pressure acts in all directions – That is why structures do not collapse under all the weight 4 Air Pressure Measurement A mercury barometer employs air mercury pressure to support a column of mercury in a tube mercury Air pressure at sea level will Air support the mercury to a height of 760 mm (29.92 in.) 760 Height of the mercury column Height changes with air pressure changes Adjustments required for: – The expansion and contraction of The mercury with temperature mercury – Gravity variations with latitude and Gravity altitude altitude 5 An aneroid barometer is less An precise, but more portable than a mercury barometer mercury It has a chamber with a partial It vacuum vacuum Changes in air pressure Changes collapse or expand the chamber collapse This moves a pointer on a scale This calibrated equivalent to mm or in. of mercury in. New ones are piezoelectric – New depend on the effect of air pressure on a crystalline substance substance Home-use aneroid barometers Home-use often have a fair, changeable, and stormy scale and – These should not be taken These literally literally 6 Air Pressure Air Measurement Measurement Air Pressure Measurement Forecasting uses Forecasting air pressure and tendency values tendency – changes over time Barometers may Barometers keep a record of air pressure air – These are called These barographs barographs 7 Units of length – Millimeters or inches Inches typical for TV Air Pressure Air Units Units Units of pressure – Pascal – worldwide standard Metric scale Sea-level pressure = Sea-level – 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 millibar 1/1000 of a bar 1/1000 – Usual worldwide range is 970 – 1040 mb – Lowest ever recorded - 870 mb (Typhoon Tip in 1979) – Highest ever recorded – 1083.8 mb (Agata, Siberia) 8 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 Because altitude is greater in the lower troposphere altitude – Then it becomes more gradual aloft Vertical profiles of average air pressure and temperature Vertical are based on the standard atmosphere – state of atmosphere averaged for all latitudes and seasons atmosphere Even though density and pressure drop with altitude, it is Even not possible to pinpoint a specific altitude at which the atmosphere ends atmosphere – – – 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 9 Average Air Average Pressure Variation with Altitude Expressed in mb Expressed 10 10 11 11 Horizontal Variations in Air Pressure Horizontal variations are much Horizontal more important to weather forecasters than vertical differences differences – In fact, local pressures at In elevations are adjusted to equivalent sea-level values equivalent – This shows variations of This pressure in the horizontal plane plane – This is mapped by This connecting points of equal equivalent sea-level pressure, producing isobars pressure, 12 12 Horizontal Variations in Air Pressure Horizontal changes in pressure Horizontal can be accompanied by significant changes in weather significant In middle latitudes, a continuous In procession of different air masses brings changes in pressure and weather pressure – Temperature has a much more Temperature pronounced affect on air pressure than humidity pressure In general, the weather In becomes stormy when air pressure falls but clears or remains fair when air pressure rises rises Air pressure varies continuously 13 13 Horizontal Variations in Air Pressure Influence of temperature and humidity – Rising air temperature = rise in the average Rising kinetic energy of the individual molecules kinetic In a closed container, heated air exerts more In pressure on the sides pressure – 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 Heating themselves farther apart themselves – This is due to increased kinetic energy – Molecules placed farther apart have a lower mass per unit Molecules volume, or density volume, – The heated air is less dense, and lighter. 14 14 Horizontal Variations in Air Pressure Influence of temperature and humidity, continued – Air pressure drops more rapidly with altitude in a column Air of cold air of Cold air is denser, has less kinetic energy, so the molecules are Cold closer together closer – 500 mb surfaces represent where half of the 500 atmosphere is above and half below by mass atmosphere 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 the air due to Avogadro’s Law the We often refer to muggy air as heavy air, but the opposite is We true true – Muggy air only weighs heavily on our personal comfort Muggy factor factor 15 15 Horizontal Variations in Air Pressure Influence of temperature and humidity, Influence continued continued – Cold, dry air masses are the densest These generally produce higher surface pressures – Warm, dry air masses generally exert higher Warm, pressure than warm, humid air masses pressure – These pressure differences create horizontal These pressure gradients pressure Pressure gradients cause cold or warm air advection – Air mass modifications can also produce Air changes in surface pressures changes – Conclusion: local conditions and air mass Conclusion: advection can influence air pressure advection 16 16 Horizontal Variations in Air Pressure Influence of diverging and converging winds – Diverging = winds blowing away from a column Diverging of air of – Converging = winds blowing towards a column Converging of air of – Diverging/converging caused by : Horizontal winds blowing toward or away from some Horizontal location (this chapter) location Wind speed changes in a downstream direction Wind (Chapter 8) (Chapter 17 17 Influence of Temperature and Influence Humidity Humidity When air is heated, air When density usually decreases as a result in the increased activity of the heated molecules. Air pressure drops more Air rapidly with altitude in cold air than in warm air air Increasing humidity also Increasing decreases the density of air, because water vapor has a lower molecular weight than dry air weight 18 18 Influence of Diverging and Influence Converging winds Converging If more air diverges at If the surface than converges aloft, the air density and surface air pressure decrease pressure If more air converges If aloft than diverges at the surface, density and surface pressure increase increase 19 19 Highs and Lows Isobars are drawn on a map as previously Isobars discussed discussed – U.S. convention – these are drawn at 4-mb intervals U.S. (e.g., 996 mb, 1000 mb, 1004 mb) (e.g., A High is an area where pressure is relatively high High compared to the surrounding air compared A Low is an area where pressure is relatively low Low compared to the surrounding air compared 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 Lows columns of air. columns 20 20 Variables of State Temperature (T), Pressure (P) and density (d) At typical surface T and P: – Number density of dry, clean air is ~25 billion cm-3 – Mass density is 1.29 kg m-3 Temperature is proportional to the average kinetic Temperature energy of an air mass (function of mass and velocity of the constituent air molecules) the – K.E. = ½ mv2 Average speed of an air molecule at surface T and P Average is 460 m s-1 (~1000 mph)! is 21 21 The Gas Law We have discussed variability of temperature, We 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 altitude The three variables of state are related through the The ideal gas law, which is a combination of Charles’ law and Boyle’s law and – The ideal gas law states that pressure exerted by air is The directly proportional to the product of its density and temperature, i.e. pressure = (gas constant) x (density) x (temperature) (temperature) 22 22 The Gas Law Conclusions from the ideal gas law – Density of air within a rigid, closed container remains Density constant. Increasing the temperature leads to increased pressure 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 Compressing decreases decreases – Within the same air parcel: – With a constant pressure, a rise in temperature is With accompanied by a decrease in density. accompanied – Expansion due to increased kinetic energy increases volume – Hence, at a fixed pressure, temperature is inversely Hence, proportional to density proportional 23 23 Expansional Cooling and Expansional Compressional Warming Compressional Expansional cooling – when an air parcel Expansional expands, the temperature of the gas drops expands, Compressional warming – when the pressure on Compressional an air parcel increases, the parcel is compressed and its temperature rises and 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 parcel – A change in internal energy is directly proportional to a change 24 24 change in temperature change Conservation of Energy A. If the air is compressed, energy is used to do work on the air B. If we allow the air to expand, the air does work on the surroundings 25 25 Adiabatic Processes During an adiabatic process, no heat is exchanged During between an air parcel and its surroundings between – The temperature of an ascending or descending The unsaturated parcel changes in response to expansion or compression only compression – Dry adiabatic lapse rate = 9.8 C°/1000 m (5.5 °F/1000 °/1000 ft) ft) – Once a rising parcel becomes saturated, latent heat Once released to the environment during condensation or deposition partially counters expansional cooling deposition – Moist adiabatic lapse rate = 6 C°/1000 m (3.3 °F/1000 Moist °/1000 ft) → this is an average rate ft) 26 26 Adiabatic Processes Dry adiabatic lapse rate describes the expansional cooling of ascending unsaturated air parcels Illustration of dry and moist Illustration adiabatic lapse rates adiabatic 27 27 ...
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