Notes

Outline

Chapter 4: Pressure and Wind  Pressure, Measurement, Distribution  Hydrostatic Balance  Pressure Gradient and Coriolis Force  Geostrophic Balance  Upper and Near-Surface Winds

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Units of Atmospheric Pressure  Pascal (Pa): a SI (Systeme Internationale) unit for air pressure.      1 Pa = force of 1 newton acting on a surface of one square meter      1 hectopascal (hPa) = 1 millibar (mb)  [hecto = one hundred =100] Bar: a more popular unit for air pressure.      1 bar = 1000 hPa = 1000 mb One atmospheric pressure = standard value of atmospheric pressure at lea level = 1013.25 mb = 1013.25 hPa.

Measurement of Atmos. Pressure Mercury Barometers Height of mercury indicates downward force of air pressure Three barometric corrections must be made to ensure homogeneity of pressure readings First corrects for elevation, the second for air temperature (affects density of mercury), and the third involves a slight correction for gravity with latitude Aneroid Barometers Use a collapsible chamber which compresses proportionally to air pressure Requires only an initial adjustment for elevation

Pressure Correction for Elevation Pressure decreases with height. Recording actual pressures may be misleading as a result. All recording stations are reduced to sea level pressure equivalents to facilitate horizontal comparisons. Near the surface, the pressure decreases about 100mb by moving 1km higher in elevation.

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Isobar It is useful to examine horizontal pressure differences across space. Pressure maps depict isobars, lines of equal pressure. Through analysis of isobaric charts, pressure gradients are apparent. Steep (weak) pressure gradients are indicated by closely (widely) spaced isobars.

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Pressure Gradients Pressure Gradients The pressure gradient force initiates movement of atmospheric mass, wind, from areas of higher to areas of lower pressure Horizontal Pressure Gradients Typically only small gradients exist across large spatial scales (1mb/100km) Smaller scale weather features, such as hurricanes and tornadoes, display larger pressure gradients across small areas (1mb/6km) Vertical Pressure Gradients Average vertical pressure gradients are usually greater than extreme examples of horizontal pressure gradients as pressure always decreases with altitude (1mb/10m)

Hydrostatic Balance in the Vertical

What Does Hydrostatic Balance Tell Us?  The hydrostatic equation tells us how quickly air pressure drops wit height. èThe rate at which air pressure decreases with height (DP/ Dz) is equal to the air density (r) times the acceleration of gravity (g)

The Ideal Gas Law An equation of state describes the relationship among pressure, temperature, and density of any material. All gases are found to follow approximately the same equation of state, which is referred to as the “ideal gas law (equation)”. Atmospheric gases, whether considered individually or as a mixture, obey the following ideal gas equation:

Hydrostatic Balance and Atmospheric Vertical Structure Since P= rRT (the ideal gas law), the hydrostatic equation becomes:            dP =  -P/RT x gdz è   dP/P =  -g/RT x dz    P =  Ps exp(-gz/RT)    P = Ps exp(-z/H) The atmospheric pressure decreases exponentially with height

Temperature and Pressure

Wind Changes with Height

Pressure Gradient Force  PG = (pressure difference) / distance Pressure gradient force goes from high pressure to low pressure.  Closely spaced isobars on a weather map indicate steep pressure gradient.

Balance of Force in the Horizontal

Force that Determines Wind  Pressure gradient force  Coriolis force (Earth’s Rotation)  Friction (near Earth’s surface)  Centrifugal force

Coriolis Force

Coriolis Force  Coriolis force causes the wind to deflect to the right of its intent path in the Northern Hemisphere and to the left in the Southern Hemisphere. The magnitude of Coriolis force depends on (1) the rotation of the Earth, (2) the speed of the moving object,  and (3) its latitudinal location. The stronger the speed (such as wind speed), the stronger the Coriolis force. The higher the latitude, the stronger the Coriolis force. The Corioils force is zero at the equator. Coriolis force is one major factor that determine weather pattern.

Coriolis Force Change with latitudes

Upper Atmospheric Winds

Geostrophic Balance

Upper Atmosphere Geostrophic Flow

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Frictional Force A force of opposition which slows air in motion. Initiated at the surface and extend, decreasingly, aloft. Important for air within 1.5 km (1 mi) of the surface, the planetary boundary layer. Because friction reduces wind speed it also reduces Coriolis deflection. Friction above 1.5 km is negligible. Above 1.5 km = the free atmosphere.

Surface Winds

Surface Geostrophic Flow

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Centrifugal Force

Gradient Wind Balance  The three-way balance of horizontal pressure gradient, Coriolis force, and the centrifugal force is call the gradient wind balance.  The gradient wind is an excellent approximation to the actual wind observed above the Earth’s surface, especially at  the middle latitudes.

Super- and Sub-Geostrophic Wind

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Measuring Winds Wind direction always indicates the direction from which wind blows. An aerovane indicates both wind speed and direction.