1. Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt
Anthony J. Vega

2. Chapter 4. Atmospheric Pressure and Wind
Part 1. Energy and Mass
Chapter 4.
Atmospheric Pressure and Wind

3. Introduction
Atmospheric pressure = force per unit area exerted by atmospheric gases
Expressed in millibar or pascal
Surface pressure is approximately 1000 mb ( mb)
Total pressure expressed through Dalton’s Law, the sum of partial pressures exerted by individual gases
Or, the weight of the overlying air (sea level weight = 14.7lbs/in2)
Pressure is exerted in all directions equally, not just downward
Pressure is a function of density and temperature

4. Vertical and Horizontal Changes in Pressure
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
Compressibility of atmospheric gases causes a non-linear decrease in pressure with height
Pressure will be less
at P2 than at P1 due to
pressure decreasing with
height

6. The Equation of State
Relationships between pressure, temperature, and density are described through the equation of state (ideal gas law)
Indicates that at constant temperatures, an increase in air density will trigger a pressure increase
Under constant density, an increase in temperature will be accompanied by an increase in pressure
Molecular movement in a sealed container.
Pressure increased by increasing density (b)
or temperature (c)

7. Measurement of Pressure
Mercury Barometers, invented by Torricelli
Utilizes an inverted tube filled with mercury
Height of mercury indicates downward force of air pressure
Measurement frequently given as a distance
Average Sea Level Pressure = 76 cm (29.92 in) of mercury
Corrections to Mercury Barometer Readings
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

8. Aneroid barometer (left)
and its workings (right)
A barograph continually
records air pressure
through time

9. The Distribution of Pressure
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
A weather map depicting
the sea level pressure
distribution for March 4, 1994

10. Horizontal Pressure Gradients
The pressure gradient force initiates movement of atmospheric mass, wind, from areas of higher to areas of lower pressure
Horizontal wind speeds occur relative to the strength of the pressure gradient
Horizontal Pressure Gradients
Typically only small gradients exist across large spatial scales
Smaller scale weather features, such as hurricanes and tornadoes, display larger pressure gradients across small areas
Vertical Pressure Gradients
Average vertical pressure gradients are usually greater than extreme examples of horizontal pressure gradients as pressure always decreases with altitude

11. A surface pressure chart

12. Hydrostatic Equilibrium
The downward force of gravity balances strong vertical pressure gradients to create hydrostatic equilibrium
Forces balance and the atmosphere is held to Earth’s surface
Local imbalances initiate various up- and downdrafts
The Role of Density in Hydrostatic Equilibrium
Gravitational force is relative to mass
A dense atmosphere experiences greater gravitational force
A vertical pressure gradient must increase to offset increased gravitational force
Higher temperature columns of air are less dense than cooler ones
For warm (cold) air this equates to smaller (larger) vertical pressure gradients leading to hydrostatic equilibrium

13. Heating causes a density
decrease in a column of air.
The column contains the
same amount of air, but has
a lower density to
compensate for its greater
height.

14. Horizontal Pressure Gradients in the Upper Atmosphere
Upper air pressure gradients are best determined through the heights of constant pressure due to density considerations
Constant pressure surfaces of cooler air will be lower in altitude than those of warmer air
Height contours indicate the pressure gradient
Twice daily, heights are drawn at 60 m intervals for the 850, 700, 500, and 3000 mb levels
500 mb height contours
for May 3, 1995

15. Upper air heights decrease with latitude

16. Forces Affecting the Speed and Direction of Wind The Coriolis Force
Free moving objects in the atmosphere are influenced by Earth rotation
A resulting apparent deflective force
This causes two types of motion
Translational movement, movement of an object from one place to another
Caused by planet rotation
Proportional to latitude
Rotational movement, indicative of vorticity
Also related to latitude such that it is maximized at the poles and zero at the equator
Overall result is a deflection (if viewed from surface) of moving objects to the right (left) in the northern (southern) hemisphere

17. The deflective force also increases with speed of the moving object
Coriolis deflection increases from zero at the equator to a maximum at the poles
The deflective force also increases with speed of the moving object
Overall force is weak
Noticeable deflection only on objects with long periods of motion
Takes place regardless of the direction of motion
Coriolis
deflection

18. Friction 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

19. Winds in the Upper Atmosphere
Upper air moving from areas of higher to areas of lower pressure undergo Coriolis deflection
Air will eventually flow parallel to height contours as the pressure gradient force balances with the Coriolis force
This geostrophic flow (wind) may only occur in the free atmosphere

20. Supergeostrophic and Subgeostrophic Flow
Height contours and pressure distributions are frequently curved
Air flow remains parallel to the height contours
This flow is not truly geostrophic as forces are temporarily out of balance
Around high pressure areas, air undergoes rapid acceleration and the Coriolis force dominates the pressure gradient force producing supergeostrophic conditions
Around low pressure areas, subgeostrophic conditions occur as the pressure gradient force dominates a weaker Coriolis force
Both supergeostrophic and subgeostrophic conditions result in airflow parallel to curved height contours
Termed gradient flow

21. Supergeostrophic flow
Subgeostrophic flow

22. Cyclones, Anticyclones, Troughs, and Ridges
Global air pressure is typically divided into a number of small-scale high and low pressure areas
High pressure areas, or anticyclones, have clockwise (counterclockwise) airflow in the northern (southern) hemisphere
This occurs as air diverges from the high pressure areas at the surface and is deflected by Coriolis acceleration which turns air to the right (left)
Characterized by descending air which warms creating clear skies
Opposite conditions occur relative to low pressure areas, or cyclones
Air converges toward low pressure centers, cyclones are characterized by ascending air which cools to form clouds and possibly precipitation
In the upper atmosphere, ridges correspond to surface anticyclones while troughs correspond to surface cyclones

23. Coriolis deflection into
a low pressure system
in the northern hemisphere

26. Ridges and troughs in the northern hemisphere

27. Maps depicting troughs, ridges, cyclones, and anticyclones

28. Measuring Wind
Wind direction always indicates the direction from which wind blows
An azimuth indicates the degree of angle from due north moving clockwise from 0 to 360o
Wind vanes are devices which indicate wind direction while anemometers record wind speed
An aerovane indicates both wind speed and direction
Similar devices are attached to balloons to record upper air conditions
An azimuth

29. An aerovane

30. End of Chapter 4 Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt