NAS 125: Meteorology

1. NAS 125: Meteorology Air Pressure

2. 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. Air Pressure

3. 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. Air Pressure

4. 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. Air Pressure

5. 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. Air Pressure

6. 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. Air Pressure

7. Air Pressure

8. 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. Air Pressure

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10. Air Pressure

11. 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. Air Pressure

12. Air Pressure

13. 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. Air Pressure

14. 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. Air Pressure

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16. 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). Air Pressure

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18. 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. Air Pressure

19. 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. Air Pressure

20. Air Pressure

21. 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. Air Pressure

22. Air Pressure

23. 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. Air Pressure

24. Air Pressure

25. 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. Air Pressure

26. 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. Air Pressure

27. Air Pressure

28. 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 Air Pressure