atmospheric pressure and wind - forsiden...force exerted per unit of surface area is pressure. •...

© 2015 Pearson Education, Inc.

Atmospheric

Pressure and

Wind

Chapter 4 Lecture

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal, University of Oslo


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.


Molecular movement in a sealed container (a).

Pressure increased by increasing density (b) or temperature (c).

The Concept of Pressure


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.


• 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


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


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.


• 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.



• 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.


∆p/ ∆z = -ρg


• 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.



• 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


• 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



– 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.


of the Wind




of the Wind


• 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.


of the Wind


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.


• 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.



• Gradient Flow



• 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.



• 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


• Cyclones


Ridges



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.


• Anticyclones


Ridges


• 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.


Ridges


• Troughs and Ridges

– Ridges and troughs in the Northern Hemisphere.


Ridges

atmospheric pressure and wind - forsiden...force exerted per unit of surface area is pressure. •...

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