Lesson: Atmospheric Dynamics · Lesson: Atmospheric Dynamics By Keith Meldahl Corresponding to...

© 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Lesson: Atmospheric Dynamics

By Keith Meldahl

Corresponding to Chapter 8: Atmospheric Circulation

Our atmosphere moves

(circulates) because of

uneven solar heating of

the earth’s surface,

combined with the earth’s

rotation. The jet stream

shown in this figure is one

example of air movement

resulting from these

processes.


Summary of Important Concepts• The circulation of the atmosphere controls winds and weather,

moderates surface temperatures, creates waves, and drives ocean surface currents.

• The earth absorbs more solar energy near the equator than near the poles. This uneven solar heating causes the atmosphere to circulate by convection.

• Each hemisphere consists of three large, roughly circular vertical paths of moving air called cells. Going from the equator toward the poles, these are the Hadley Cells, Ferrel Cells, and Polar Cells.

• Air RISES near the equator and near 60 degrees latitude north and south. These areas of rising air have HIGH precipitation (rain and/or snow). Air SINKS near 30 degrees latitude north and south. These areas of sinking air have LOW precipitation.

• Earth’s rotation causes the Coriolis effect, which deflects moving air into right-curving paths in the Northern hemisphere and left-curving paths in the Southern hemisphere. The vertical cells of circulating air, combined with the Coriolis effect, produce the prevailing surface winds.


Summary of Important Concepts, continued

• Each hemisphere has three large regions of prevailing surface winds:1. The trade winds (or easterlies) blow from east to west in the region between the equator and 30 degrees latitude in both hemispheres. 2. The westerlies blow from west to east in the region between 30 and 60 degrees latitude in both hemispheres. 3. The polar easterlies blow from east to west in the region between 60 degrees latitude and the poles in both hemispheres.

• Prevailing wind patterns can be influenced by sea breezes and land breezes, and by monsoons.

• Most large storms are cyclonic systems; that is, they consist of large spinning masses of air spiraling into an area of low pressure.

Tropical cyclones (hurricanes) are spinning air masses that develop within the warm, humid air over tropical oceans.

Extratropical cyclones are spinning air masses that develop at the boundaries between warm and cold air masses away from the equator.


Composition and Properties of the Atmosphere

Important properties of the atmosphere.

The lower atmosphere consists mostly of nitrogen gas (N2; about

78%) and oxygen gas (O2; about 21%)

Water vapor (H2O molecules) can make up as much as 4% of the

volume of the atmosphere.

The density of air is influenced by temperature and water content.

• Warmer air is less dense than cool air, and will thus tend to rise

upward.

• Humid air is less dense than dry air, and will thus tend to rise

upward.

(Note: this last point may seem strange, but water molecules

(H2O) are lighter than oxygen molecules (O2) or nitrogen

molecules (N2), so air containing lots of water vapor is lighter!)



Note that this table shows the composition of dry air. Air can

contain up to 4% water vapor.



Rising air cools as it

expands. Cooler air can

hold less water, so water

vapor condenses into

clouds. Therefore, areas of

rising air tend to have

high precipitation (rain

and/or snow).

In contrast, sinking air

warms as it compresses.

Warmer air can hold more

water vapor, so the vapor

does not condense into

clouds. Therefore, areas of

sinking air tend to have

low precipitation (rain

and/or snow).


Uneven Solar Heating and Latitude

The main reason the atmosphere moves (circulates) is

because the earth gets uneven amounts of heat from the Sun.

The equator gets more solar radiation than the poles.

Why is this?

• Near the equator the

Sun hits from straight

overhead,

concentrating its

energy in a small area.

• Near the poles the

Sun hits at a low

angle, so the same

amount of energy is

spread out over a

larger area.


Uneven Solar Heating and Latitude

The result of this uneven

heating is that the equator

areas have net heat gain, and

the polar areas have net heat

loss.

Since heat tends to move from

areas with high amounts to

areas with less heat, the result

of this imbalance is that the

atmosphere and the oceans

both circulate, acting to even

out the imbalance of heat!!


Uneven Solar Heating & Atmospheric

Circulation

TWO factors govern the global circulation of air:

1. Uneven solar heating -- warm air near the equator rises; cold

air near the poles sinks.

2. The Coriolis effect -- the spin of the earth causes moving air

to change its direction.

As air warms, expands, and rises at the equator, it moves toward

the pole, but instead of traveling in a straight path, the air is

deflected eastward.

In the Northern Hemisphere the air follows right-curving paths.

In the Southern Hemisphere the air follows left-curving paths..


Atmospheric Circulation Cells


Uneven solar heating of the earth, combined with the Coriolis

effect, creates three large atmospheric circulation cells in

each hemisphere.

Hadley cells occur on either side of the equator. They are formed from

air rising at the equator and sinking at about 30 degrees latitude north

and south (see previous slide).

Ferrel cells occur at the mid-latitudes. They are formed from air sinking

at about 30 degrees latitude north and south and rising at about 60

degrees latitude north and south (see previous slide).

Polar cells occur near the poles. They are formed from air rising at

about 60 degrees latitude north and south and sinking at the poles (see

previous slide).



Areas of the three great circulation cells where air either rises

straight upward or sinks straight downward tend to have weak and

variable surface winds.

Two areas of weak surface winds are particularly apparent:

• The doldrums are the areas of weak surface winds along the equator

where Hadley cells air rises upward.

• The horse latitudes are areas of weak surface winds at about 30

degrees latitude north and south of the equator, where Hadley cell and

Ferrel cell air sink downward.

These areas were the nemesis of ancient sailors, whose sailing ships could be

stranded for weeks by calm winds. Often to lighten their loads ship captains

would order cargo thrown overboard -- and heavy horses were often the first

things to go! Hence the name “horse latitudes”. Sorry, you horse lovers out

there.



As air moves in the three great circulation cells, it’s direction

changes due to the Coriolis effect.

In the Northern Hemisphere the air follows right-curving paths.

In the Southern Hemisphere the air follows left-curving paths.

The result is that each circulation cell corresponds to a single direction

of prevailing surface wind.

• The trade winds (or easterlies) blow from east to west in the region between

the equator and 30 degrees latitude in both hemispheres. The trade winds are

surface winds of the Hadley cells.

• The westerlies blow from west to east in the region between 30 and 60

degrees latitude in both hemispheres. The westerlies are surface winds of

the Ferrel cells.

• The polar easterlies blow from east to west in the region between 60

degrees latitude and the poles in both hemispheres. The polar easterlies are

surface winds of the Polar cells.

Prevailing Winds


Prevailing WindsThe orange parts of the arrows show the prevailing surface trade winds and westeries. The third set of prevailing winds – the polar easterlies – are not well illustrated here.


The prevailing surface winds discussed above may be changed by

local conditions. One example is the sea breeze / land breeze situation.

Sea breeze: during the day, the land will heat up more than the ocean

(because water has higher heat capacity than land). The warm air over

the land rises, and the cool air over the ocean moves in to replace it. This

creates a wind blowing from the ocean toward the land, and is called a

sea breeze.

Land breeze: during the night, the land will cool down more than the

ocean. The cool air over the land sinks, and the relatively warmer air over

the ocean rises, creating a wind blowing from the land toward the ocean -

- a land breeze.

These are illustrated on the following slide:

Sea Breezes & Land Breezes


Top: Sea breeze

created during the day

by cool air sinking

over the ocean and

warm air rising over

the land.

Bottom: Land breeze

created during the

night by cool air

sinking over the land

and warm air rising

over the ocean.


Storms are regional atmospheric disturbances characterized by high

winds and often high precipitation.

Most major storms are cyclones, meaning that they are spinning

masses of air caused caused by air spiraling into an area of low

atmospheric pressure.

TROPICAL CYCLONES occur in tropical regions. These storms are

known as hurricanes to most of us, but go by other names in other

areas of the world.

The ultimate cause of hurricanes is the massive amount of heat that is

released by the condensation of huge volumes of water vapor. The air

over warm tropical oceans gets very humid, and as that vapor

condenses, it releases heat. The heat causes the air to begin rising,

which causes more water vapor to condense, releasing more heat. This

creates a runaway effect of massive amounts of rising air. Air gets

sucked into the area to replace the rising air, creating extreme winds!

Storms


Storms

The diagram of a hurricane

illustrates how air rises as

heat is released by

condensation of water

vapor. The rising air is

replaced by air rushing in

from the sides. This in-

rushing air creates the high

winds of a hurricane.

Hurricanes are

characterized by heavy rain

and extreme winds. Both

processes result from one

cause: the condensation of

huge amounts of water

vapor!!


The orange shaded areas show regions of warm tropical oceans

over which hurricanes form. The arrows show the paths of

hurricanes once they form. Notice that these storms follow right-

curving paths in the Northern hemisphere and left-curving paths in

the Southern hemisphere -- the Coriolis effect in action!

Storms


Changes in seasons are caused by the variations of incoming solar energy as

Earth makes its annual rotation around the Sun. The 23 1/2o tilt of the Earth’s axis

means the northern hemisphere gets less solar energy in the winter, and more in

the summer.

The Reason for the Seasons

Lesson: Atmospheric Dynamics · Lesson: Atmospheric Dynamics By Keith Meldahl Corresponding to...

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