2 Atmospheric pressure Molecular view of atmospheric pressure
- Slides: 9
2. Atmospheric pressure
Molecular view of atmospheric pressure gravity random motion • Weight of all air molecules is propagated to surface by random motion of molecules • Random motion of molecules causes pressure to be applied in all directions EARTH SURFACE
Measurement of atmospheric pressure with the mercury barometer vacuum h atmospheric pressure (weight of atmosphere per unit area of surface) A Atmospheric pressure p = p. A = r. Hg gh SI unit for pressure is the Pascal (Pa): 1 Pa = 1 kg m-1 s-2 Mean sea-level pressure: p = 1. 013 x 105 Pa = 1013 h. Pa = 1013 mb = 1 atm = 760 mm Hg (torr)
“Sea level” pressure map 7 -day forecast https: //www. wpc. ncep. noaa. gov/basicwx/day 0 -7 loop. html
Sea-level pressure has narrow range: 1013 ± 50 h. Pa everywhere Consider a pressure gradient at sea level operating on an elementary air parcel dxdydz: p(x) p(x+dx) Pressure-gradient force Vertical area dydz Acceleration where ρa is air density For Dp = 10 h. Pa over Dx = 100 km, a 10 -2 m s-2 a 100 km/h wind in 3 h! Wind transports air to from high to low pressure, decreasing Dp On mountains, however, the surface pressure is lower, and the pressure-gradient force along the Earth surface is balanced by gravity: p(z+Dz) p-gradient gravity p(z) a. This is why weather maps show “sea level” isobars even over land; the fictitious “sea-level” pressure assumes an air column to be present between the surface and sea level
Total mass ma of the atmosphere Radius of Earth: 6380 km Mean pressure at Earth's surface: 984 h. Pa Total number of moles of air in atmosphere: Mol. wt. of air: 29 g mole-1 = 0. 029 kg mole-1
Vertical profiles of pressure and temperature mean values for 30 o. N, March Stratopause Tropopause
Barometric law describes the decrease of pressure with altitude • Consider elementary slab of atmosphere: p(z+dz) p(z) unit area Ideal gas law: Assume T = constant, integrate: barometric law hydrostatic equation
Application of barometric law: the sea-breeze effect
- High pressure and low pressure
- Pressure at different altitudes
- Denture retention definition
- Pressure head meaning
- Usairnet temperature map
- What measures atmospheric pressure
- What is a covalent bond simple definition
- Giant molecular structure vs simple molecular structure
- Zinc oxide + nitric acid → zinc nitrate + water
- Multi view projection