Download Understanding Weather and Climate Ch 4

Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 1. Energy and Mass
Chapter 4.
Atmospheric Pressure and Wind
Introduction
Atmospheric pressure = force per unit area exerted by
atmospheric gases
Expressed in millibar or pascal
Surface pressure is approximately 1000 mb (1013.25 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
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
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)
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
Aneroid barometer (left)
and its workings (right)
A barograph continually
records air pressure
through time
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
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
A surface pressure chart
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
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.
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
Upper air heights decrease with latitude
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
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
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
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
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
Supergeostrophic flow
Subgeostrophic flow
Cyclones, Anticyclones, Troughs, and Ridges
Global air pressure is typically divided into a number of smallscale 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
Coriolis deflection into
a low pressure system
in the northern hemisphere
Ridges and troughs in the northern hemisphere
Maps depicting troughs, ridges, cyclones, and anticyclones
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
An aerovane
End of Chapter 4
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt