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NAS 125: Meteorology Air Pressure Mount Everest, part 1 • Mount Everest is 8,850 m (29,035 ft) above sea level. • Local peoples revered it as sacred and traditionally did not try to ascend it. • The mountain was first scaled on 28 May 1953 by Sir Edmund Hillary of New Zealand and Sherpa Tenzing Norgay of Nepal. – More than 4,000 have attempted the summit, of those, more than 140 have died. Rev. 2 March 2006 Air Pressure 2 Mount Everest, part 2 • Mount Everest is at roughly the same latitude as Tampa, Fla. – Because of its tremendous elevation, however, the upper reaches of the mountain are never above freezing. • Mean January temperature at the summit is -36 °C • Mean July temperature at the summit is -19 °C – Clouds enshroud the peak through much of June through September as the Indian monsoon buffets the subcontinent. – The jet stream brings hurricane-force winds to the summit from November through February. – Hypothermia is a constant threat on the mountain. Rev. 2 March 2006 Air Pressure 3 Mount Everest, part 3 • The atmosphere becomes thinner – there are fewer molecules per unit volume – at higher elevations. • The decrease in oxygen levels with increasing elevation further increases the risk of attempting the summit. – The pressure of oxygen at the surface is only one-third of oxygen at sea level. • Although some may reach the summit without supplemental oxygen, most need an extra oxygen supply. Rev. 2 March 2006 Air Pressure 4 Atmospheric pressure • Pressure is the force a gas (or liquid) exerts on some specified area of the container walls. • Atmospheric pressure is the force exerted by gas molecules in the atmosphere. – It affects Earth’s surface as well as any other body on Earth. – It is an omnidirectional force, a force exerted equally in all directions. – The force drops with increasing altitude because actual number of gas molecules also drops. Rev. 2 March 2006 Air Pressure 5 Measuring pressure, part 1 • A barometer is used to measure pressure and monitor its changes. • Two types of barometers: – Mercury barometer: Cumbersome; accurate; uses a glass tube, sealed at one end but open at the other; and filled with mercury; the open end of the tube is inserted into a reservoir of mercury; the mercury fills the tube until the pressure of the mercury in the tube is equalled by the pressure of the atmosphere pressing down upon it. – Average pressure at sea level is 760 mm. Rev. 2 March 2006 Air Pressure 6 Rev. 2 March 2006 Air Pressure 7 Measuring pressure, part 2 • Two types of barometers (continued): – Aneroid barometer: Does not use liquid; more portable than a mercury barometer; consists of a flexible chamber that compresses and expands with changes in air pressure. – Aneroid barometers can be modified to measure altitude; such modified instruments are called altimeters. • Air pressure tendency is the change in air pressure with time. – Rising pressure generally indicates fair weather. – Dropping pressure generally indicates stormy weather. Rev. 2 March 2006 Air Pressure 8 Rev. 2 March 2006 Air Pressure 9 Rev. 2 March 2006 Air Pressure 10 Measuring pressure, part 3 • Sometimes an aneroid barometer is attached to a pen that traces a line on a chart on a clock-driven drum; this instrument is called a barograph. • Units of air pressure: – For civilian use, pressure is often reported in millimeters or inches of mercury (760 mm or 29.92 inches). – Physicists use Pascals (101,325 Pa) – U.S. meteorologists use millibars (1013.25 mb) – Worldwide range 970 mb to 1040 mb. Rev. 2 March 2006 Air Pressure 11 Rev. 2 March 2006 Air Pressure 12 Vertical variations • Density: Mass per unit volume • Number density: Number of molecules per unit volume • Air thins with increasing altitude. – At 16 km, air density is about 14 percent of density at sea level. – Pressure decreases as well with increasing altitude. • Air is compressible. Rev. 2 March 2006 Air Pressure 13 Standard atmosphere, part 1 • The standard atmosphere is a model of the atmosphere averaged for all latitudes and seasons. – – – – Fixed sea-level temperature (15 °C) Fixed sea-level pressure (1013.25 mb) Fixed vertical profiles of pressure and temperature Actual values vary, of course • Upper-air weather patterns are plotted as isobaric surfaces – contour lines in which the air pressure is the same everywhere. – Typically 200-mb, 500-mb, and 850-mb levels. Rev. 2 March 2006 Air Pressure 14 Rev. 2 March 2006 Air Pressure 15 Standard atmosphere, part 2 • The Earth’s atmosphere grades imperceptibly with interplanetary space. – Half the atmosphere’s mass lies below 5,500 m. – About 99 percent of the atmosphere’s mass lies below 32 km. – Above the homosphere (80 km) the relative proportions of atmospheric gases change markedly. – About 1,000 km, the atmosphere merges with interplanetary gases (hydrogen and helium). Rev. 2 March 2006 Air Pressure 16 Rev. 2 March 2006 Air Pressure 17 Standard atmosphere, part 3 • With uniform pressure and temperature (at average sea-level value) the top of a uniform density atmosphere would be 8 km. • Low density at high altitudes affects air temperature and heat transfer. – Despite high temperatures of thermosphere (1,200 °C), the low density of the air prevents efficient heat transfer. Rev. 2 March 2006 Air Pressure 18 Horizontal variations, part 1 • Mapping pressure with isobars – An isobar is a line joining points of equal atmospheric pressure. – “High” and “low” pressures are relative conditions, with the distinction depending on the pressure of the adjoining areas. – On weather maps, pressure measured at the surface is adjusted to sea-level pressure to make comparisons easier. – A pressure gradient, the horizontal rate of pressure change, representing the “steepness” of the pressure slope, directly affects the speed of wind. Rev. 2 March 2006 Air Pressure 19 Rev. 2 March 2006 Air Pressure 20 Horizontal variations, part 2 • Mapping pressure with isobars (continued): – Despite the fact that horizontal pressure variations are of relatively lower magnitude that vertical pressure variations, the horizontal variations may be associated with important changes in weather. • Pressure also varies day by day and hour by hour. Rev. 2 March 2006 Air Pressure 21 Rev. 2 March 2006 Air Pressure 22 Temperature and humidity, part 1 • Cold air is more dense than warm air. – Warm air rises. • As air density increases, volume decreases, while pressure and temperature increases. • Temperature of air affects the rate of pressure change with change in altitude. – Pressure drops more rapidly in cold air than warm air. • Dry air is more dense than moist air. – Molecular weight of water is less than that of oxygen and nitrogen. Rev. 2 March 2006 Air Pressure 23 Rev. 2 March 2006 Air Pressure 24 Temperature and humidity, part 2 • Cold, dry air masses are more dense and usually produce higher surface pressures than warm, moist air masses. • Warm, dry air masses typically exert higher surface pressures than equally warm, but more humid, air masses. • Changes in surface pressure are typically accompanied by replacement of one air mass by another – advection. • Air masses are modified by the Earth’s surface. Rev. 2 March 2006 Air Pressure 25 Divergence and convergence • Diverging winds blow away from an area. – Diverging winds, accompanied by lead to increasing pressure at the surface. • Converging winds blow toward an area. – Converging winds lead to decreasing pressure at the surface. Rev. 2 March 2006 Air Pressure 26 Rev. 2 March 2006 Air Pressure 27 Ideal gas law • Temperature, pressure, and density are known as variables of state. • Ideal gas law: Pressure is proportional to the product of density and temperature. – Pressure = constant * density * temperature Rev. 2 March 2006 Air Pressure 28