ch_6 Atmosphere

Copyright © 2013 Pearson Education, Inc.

The Atmosphere:

An Introduction to

Meteorology, 12th

Lutgens • Tarbuck

Lectures by:Heather Gallacher,Cleveland State University

Chapter 6: Air Pressure and Winds



Unequal heating of the Earth’s surface

creates unequal air pressure.

These pressure differences create wind,

but the direction and strength of the wind

depends upon three important factors:

1. pressure gradient force,

2. the Coriolis force,

3. and friction.

It Is All About the Wind


Consider a "unit area"

of 1 square inch.

At sea level, the weight of the air

above this unit area is

(on average) 14.7 pounds!

That means pressure

applied by this air on

the unit area would be

14.7 pounds per square inch.

Air Pressure is the Weight of the Air

Meteorologists use

a metric unit for pressure

called a millibar and

the average pressure at sea level is

1013.25 millibars.


2. Describe the operating principles of the mercury barometer.

A glass tube filled with mercury is inverted into a dish of mercury.

The mercury flows out of the tube until the pressure

at the base of the mercury column

is balanced by the pressure exerted

on the surface of the mercury by the air above.

When air pressure rises, the height of the column increases

and when air pressure drops, so does the height of the column.

Concept Check 6.2


Measuring Air Pressure


2. Describe the operating principles of the aneroid barometer.

An aneroid (without liquid) barometer

consists of evacuated metal chambers

that compress as air pressure increases,

and expand when pressure decreases.

Concept Check 6.2


1. What is average sea-level pressure when measured in millibars?

Average sea level pressure is measured as 1013.25 millibars.

In inches of mercury?

29.92 inches, of mercury.

What is standard sea-level pressure, in pounds per square inch?

Standard sea level pressure is 14.7 pounds per square inch.

Concept Check 6.2


1 What is wind and what is its basic cause?

Wind is the horizontal movement of air and is the result of horizontal differences in atmospheric pressure.

Concept Check 6.1


Air Pressure Round the Planet Varies All the Time

Why?


Why Does Air Pressure Vary?

Temperature is a measure of

the average molecular motion

(kinetic energy) of a substance.

Temperature influences air pressure.

Air pressure drops more rapidly

with altitude in a column of cold (dense) air

than in a column of warm (less dense) air.

We assume that both columns of air exert

the same surface pressure,


At sea level a column of air weighs 14.7 pounds persquare inch and therefore exerts that amount of pressure.

As we ascend through the atmosphere, we find that the air becomes less dense

because of the continual decrease in the amount (weight) of air above.

Therefore there is a corresponding decrease in pressure with an

increase in altitude.

14.7 lbs/in2

7.35 lbs/in2



We assume that both columns of air exert

the same surface pressure, and

(although greatly exaggerated)

differences in the spacing of air molecules

represent differences in density.

Looking at the line drawing halfway up

notice that there are

more air molecules above this altitude

in the warm column than in the cold column

13 16



At the red line

16 molecules weigh more

then 13 molecules

so the pressure at the altitude

of the red line is greater

in the warm air.

This creates a “HIGH PRESSURE” aloft

While in the cold air we have a “LOW PRESSURE” aloft.

13 16

HL


Concept Check 6.4

1 Explain why a cold, dry air mass produces a higher surface pressure than a warm, humid air mass.

Cold, dry air is more dense and therefore heavier than warm, humid air. Because it is heavier, the cold, dry air will tend to sink, creating a high-pressure center.

Because it is lighter, warm, humid air will tend to rise and leave a low-pressure center.

2 If all other factors are equal, does a dry or moist air mass exert more air pressure? Explain.

Dry air mass exerts more pressure because the molecular weight of water vapor is less than the molecular weight of nitrogen and oxygen. In a moist air mass, the water vapor molecules will displace some of the nitrogen and oxygen molecules, lowering the molecular weight of the air massand its overall density.


Why does atmospheric pressure vary daily,

and why is that important?

Recall that variations in air pressure

cause the wind to blow,

which in turn causes changes in temperature and humidity.

In short, difference in air pressure

create global winds that

become organized into the systems

that bring us our weather.

Summary


The movement of air can also cause variations in air pressure.

Where there is a net flow of air into a region, a phenomenon called convergence, air accumulates.

As it converges horizontally, the air is squeezed into a smaller space, which results in a more massive air column that exerts more pressure at the surface.

By contrast,in regions where there is a net outflow of air, a situation referred to as divergence, the surface pressure drops.

In summary, the pressure at the surface will increase when there is a net convergence in a region and

the surface pressure will

decrease when there is

a net divergence.

HL

Horizontal Air Movement Changes Surface Pressure

convergencedivergence


Factors Affecting Wind

Pressure gradient force

Coriolis force

Friction



Pressure gradient force:

Isobars are lines connecting places of equal pressure.

If isobars are close together a steep gradient and high winds result.

If isobars are far apart, the result is a low gradient and lower wind

speeds.



Find a station with high winds. What is the spacing of the isobars?

Find a station with low winds. What is the spacing of the isobars?



The horizontal pressure gradient is the driving force of wind.

The magnitude of the pressure gradient force is shown by the spacingof isobars.

The direction of force is always from areas of higher pressure toward areas of lower pressure and

at right angles to the isobars.

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Concept Check 6.5

2 What force is responsible for generating wind?

Winds are only generated by horizontal pressure differences, that is, by the pressure gradient force.

3 Write a generalization relating the spacing of isobars to wind speed.

Closely spaced isobars indicate a faster wind, while widely spaced isobars indicate a slower wind.



But the wind does not blow directly from high to low pressure.

Why




Coriolis force

Friction


Air moves out of the regions of higher pressure and into the regions of lower pressure.

However, the wind rarely crosses the isobars at right angles, as the pressure gradient force directs.

This deviation is the result of Earth’s rotation and

has been named the Coriolis force, after the French scientist Gaspard-Gustave Coriolis, who first expressed its existence.

It is important to note that the Coriolis force

cannot generate wind;

rather, it modifies airflow.

Coriolis force


The Coriolis force acts

to change

the direction of a moving body

to the right in

the Northern Hemisphere

and to the left

in the Southern Hemisphere.

Coriolis force

http://www.youtube.com/watch?v=i2mec3vgeaI


This deflecting force

is always directed

at right angles to the direction of airflow.

Affects only wind direction, not wind speed.

Is affected by wind speed the stronger the wind,

the greater the deflecting force.

Is strongest at the poles and weakens equatorward,

becoming nonexistent at the equator.

Coriolis force


Concept Check 6.5

5 Which two factors influence the magnitude of theCoriolis force?

The two factors that influence the magnitude of the Coriolis force are

wind speed (the stronger the wind, the greater the deflecting force)

and latitude (strongest deflection at the poles

and zero at the equator).




Coriolis force

Friction

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Friction significantly

influences airflow

near Earth’s surface.

Its effect is negligible

above a height of a

few kilometers.

Friction Effect on Wind


The only force acting on a stationary parcel of air is the pressure gradient force.

Once the air begins to accelerate, the Coriolis force deflects it to the right in the Northern Hemisphere.

Greater wind speeds result in a stronger Coriolis force (deflection) until the flow is parallel to the isobars.

At this point the pressure gradient force and Coriolis force are in balance,

and the flow is called a geostrophic wind

Geostrophic Wind


Winds AloftCurved airflow and the gradient wind

Low-pressure systems (L)

that occur in the middle latitudes

are called midlatitude cyclones.

High-pressure systems (H)

are called anticyclones,


Winds Aloft

Curved airflow and the gradient wind

Wind flow aloft can create ridges (high) and troughs of (low)pressure


Concept Check 6.6

3 Describe the direction of cyclonic and anticyclonic flow in both the Northern and Southern Hemispheres. Draw diagrams to answer the question.


Surface Winds

Surface winds travel at an angle across isobars, toward low pressure.


How Winds Generate Vertical Air Motion

Vertical airflow is associated with cyclones and anticyclones.



Factors that promote vertical airflow:

Friction

Increased friction causes a drop in wind speed

resulting in a pileup of air upstream from the ocean to land.

Convergence can result in cloudy weather.

Decreased friction causes an increase in wind speed

from the land to the ocean.

Subsidence and divergence results in clearing weather.



Vertical airflow is associated with cyclones and anticyclones.


Concept Check 6.8

2 What general weather conditions are to be expected when the pressure tendency is rising?

When the pressure tendency is rising, it means that a high-pressure center is approaching. Because highs are associated with descending air, adiabatic warming precludes cloud formation. Thus, clear, fair weather is foretold by a rising barometer.

When the pressure tendency is falling?

Conversely, when the pressure tendency is falling, a low-pressure center is approaching. Because lows are associated with rising air, cloud formation is likely and precipitation is possible. Thus, cloudy and possibly rainy weather is foretold by a falling barometer.


Wind Measurement

Wind direction is always measured

from the direction it is coming.

A North wind blows north to south.

A prevailing wind consistently blows

more often than not

from one direction.


Wind Measurement


Wind Measurement

A wind vane shows wind direction.

Wind speed is often measured with a

cup anemometer, which has a dial

that shows wind speed.

An aerovane is a combination of wind vane

and anemometer.


End Chapter 6

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