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Atmospheric
Pressure and
Wind
Chapter 4 Lecture
Redina L. Herman
Western Illinois University
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal, University of Oslo
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The Concept of Pressure
• Earth contains a number of gas molecules that exert a force on all surfaces and the amount of
force exerted per unit of surface area is pressure.
• Pressure is measured in millibar or pascal.
• Sea-level pressure is about 1000 mb (1013.25 mb).
• Total pressure expressed through Dalton’s Law, the sum of partial pressures exerted by individual
gases.
• Pressure is exerted in all directions equally.
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Molecular movement in a sealed container (a).
Pressure increased by increasing density (b) or temperature (c).
The Concept of Pressure
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Vertical and Horizontal Changes in Pressure
• Pressure decreases with altitude.
• All recording stations are reduced to sea-level pressure in order to make horizontal comparisons.
• Compressibility of atmospheric gases results in a
nonuniform decrease of pressure with height.
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• Pressure does not decrease with height in a uniform rate.
• It decreases most rapidly at low elevations and
gradually tapers off at greater altitudes.
Vertical and Horizontal Changes in Pressure
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The Equation of State
• Pressure, temperature, and density are related to one another and their relationship can be described
through the equation of state (ideal gas law).
• The equation of state results in the following:
– At constant temperatures, an increase in air density will
cause pressure to increase.
– Under constant density, an increase in temperature will
also cause an increase in pressure.
p = ρ R T p Pressure
ρ Density
R Gas constant
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The Distribution of Pressure
• It is important to view pressure differences.
• Pressure maps depict isobars,
or lines of equal pressure.
• Pressure gradients depict the rate of change in pressure. They
are apparent on maps by the
spacing between the isobars.
– Steep pressure gradients are
indicated by closely spaced
isobars.
– Weak pressure gradients are
indicated by widely spaced isobars.
A weather map
depicting the sea-level
pressure distribution for
March 4, 1994.
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• Pressure Gradients – The pressure gradients provide the movement of air
commonly known as wind.
– The strength of the pressure gradient force determines
the horizontal wind speed.
• Horizontal Pressure Gradients – Typically, small gradients exist across large areas.
– Concentrated weather features, such as hurricanes and
tornadoes, display larger pressure gradients across small
areas.
• Vertical Pressure Gradients – Vertical pressure gradients are greater than extreme
examples of horizontal pressure gradients as pressure
always decreases with altitude.
The Distribution of Pressure
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• Hydrostatic Equilibrium – Gravity balances strong vertical pressure gradients to create
hydrostatic equilibrium.
– Local imbalances create various up- and downdrafts
• The Role of Density in Hydrostatic Equilibrium – Gravitational force is proportional to mass.
– A dense atmosphere needs greater gravitational force to remain
in balance.
• For warm air, this equates to smaller vertical pressure gradients leading to hydrostatic equilibrium.
• For cold air, this equates to larger vertical pressure gradients leading to hydrostatic equilibrium.
The Distribution of Pressure
∆p/ ∆z = -ρg
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• 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.
The Distribution of Pressure
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• 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.
Horizontal Pressure Gradients in the Upper
Atmosphere
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• The Coriolis Force
– Objects in the atmosphere are influenced by Earth’s rotation.
– Overall, the result is a deflection of moving objects to the right
in the Northern Hemisphere and to the left in the Southern
Hemisphere.
Forces Affecting the Speed and Direction
of the Wind
Fc = 2Ωsin(φ)v
Force/mass (acceleration) Ω Earth’s rotation rate
φ Latitude
v velocity
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• The Coriolis Force
– Coriolis deflection increases from zero at the equator to a
maximum at the poles.
– The deflective force also increases with the speed of the
moving object.
– Takes place regardless of the direction of motion.
Forces Affecting the Speed and Direction
of the Wind
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• The Coriolis Force
Forces Affecting the Speed and Direction
of the Wind
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• Friction
– A force of opposition which slows air in motion.
– Initiated at the surface and extends, decreasingly, aloft.
– Important for air within ~1.5 km of the surface, the planetary
boundary layer.
– Because friction reduces wind speed, it also reduces Coriolis
force.
– Friction above ~ 1.5 km (free atmosphere) is negligible.
Forces Affecting the Speed and Direction
of the Wind
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Winds Aloft and Near the Surface
• Gradient Flow
– Upper air moving from areas of higher pressure 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.
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• Winds in the Upper Atmosphere
– 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.
Winds Aloft and Near the Surface
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• Gradient Flow
Winds Aloft and Near the Surface
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• Near-Surface Winds
• Winds near the surface slow due to friction and therefore are not parallel to the isobars. They cross the
isobars.
• Coriolis deflection still occurs but it is reduced.
Winds Aloft and Near the Surface
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• Cyclones
– Air converges toward low pressure centers, called
cyclones.
– 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.
Anticyclones, Cyclones, Troughs, and
Ridges
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• Cyclones
Anticyclones, Cyclones, Troughs, and
Ridges
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Anticyclones, Cyclones, Troughs, and
Ridges
• Anticyclones
– High pressure areas, or anticyclones, have clockwise
airflow in the Northern Hemisphere and counterclockwise in
the Southern Hemisphere.
– This occurs as air diverges from the high pressure areas at
the surface and is deflected by Coriolis force.
– Characterized by descending air that warms and creates
clear skies.
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• Anticyclones
Anticyclones, Cyclones, Troughs, and
Ridges
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• Troughs and Ridges
– Low and high pressure systems occur as elongated areas
called troughs (low pressure) and ridges (high pressure).
– Pressure is distributed as cyclones and anticyclones at the
surface and gradually give way to ridges and troughs in
the upper atmosphere.
Anticyclones, Cyclones, Troughs, and
Ridges
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• Troughs and Ridges
– Ridges and troughs in the Northern Hemisphere.
Anticyclones, Cyclones, Troughs, and
Ridges