Chapter 4: Pressure and Wind

Pressure, Measurement, Distribution

Hydrostatic Balance

Pressure Gradient and Coriolis Force

Geostrophic Balance

Upper and Near-Surface Winds

Slide 2

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.

Slide 6

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.

Slide 8

Slide 9

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 = P_s exp(-gz/RT)

   P = P_sexp(-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

Slide 26

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

Slide 30

Slide 31

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

Slide 35

Measuring Winds

Wind direction always indicates the direction from which wind blows.

An aerovane indicates both wind speed and direction.


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


	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.


	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 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.


	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.


	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)


	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)


	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:


	Since P= rRT (the ideal gas law), the hydrostatic equation becomes:
	dP = -P/RT x gdz
	è dP/P = -g/RT x dz
	P = P_s exp(-gz/RT)
	P = P_sexp(-z/H)
	The atmospheric pressure decreases exponentially with height


	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.


	Pressure gradient force
	Coriolis force (Earth’s Rotation)
	Friction (near Earth’s surface)
	Centrifugal 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.


	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.


	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.


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