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Instruction 5-1

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Circulation Patterns in the Earth's Atmosphere and Oceans | The Relationship Between the Rotation of the Earth and the Circular Motion of Ocean Currents and Air in Pressure Centers | The Origin and Effects of Temperature Inversion | Properties of Ocean Water | Location of Deserts and Rain Forests | Features of ENSO

Circulation Patterns in the Earth's Atmosphere and Oceans
CCSTD Earth Science 5.a.

There are two major reservoirs of water in the Earth's hydrologic (water) cycle:
the atmosphere and the oceans.

Here's how they work together.

The heat of the Sun causes water to evaporate from lakes, rivers and oceans -- as well as from the leaves of plants (which is called transpiration).

But this water doesn't just disappear -- it's absorbed into the atmosphere and carried around the world by currents of air.

Eventually the water in the air condenses to form clouds, which return it to Earth as rain or snow.

The water that collects on land flows back to the oceans in rivers or streams -- to begin the cycle all over again.

This interaction between atmosphere and ocean is what gives us our weather and climate.


Atmospheric Circulation / Convection

Weather, as you know, is the condition of the atmosphere at a specific time and specific place -- what's happening right here right now. Weather includes temperature, atmospheric pressure, humidity, precipitation and wind velocity.

The amount of heat in the atmosphere varies from place to place. This keeps it moving constantly (circulating).

Most atmospheric circulation takes place in the troposphere, the layer of the atmosphere closest to the Earth. The troposphere extends to an altitude of 10-15 km and contains about 90% of the mass of the atmosphere.

Circulation happens because of convection, a form of heat transfer in which a gas or fluid expands and rises as it gets warmer.

The Earth gets more of the Sun's heat at the Equator than it does at the poles. So as the air at the Equator gets warmer, it expands and rises. It continues to rise until it gets to the top of the troposphere, where it spreads out toward the poles and then comes back down to Earth again.

It all sounds straightforward – but it's not.

To understand atmospheric circulation, we need to start by looking at some simplified graphic models.

The first is called The Simple Model of Global Circulation.

The Simple Model of Global Circulation

To understand this model, we need to make three assumptions. For the sake of argument, let's assume that:

  • The Earth is not rotating in space
  • The entire surface of the Earth is covered by the same material, and
  • The air is hotter at the Equator and colder at the poles (thisis the only one of the assumptions that's true)

To see a Simple Model of Global Circulation, go to Figure 7p-1 at:
http://www.physicalgeography.net/fundamentals/7p.html

In this Model, surface air flows from the poles to the Equator.

When it reaches the Equator, it gets heated and rises to the top of the troposphere, where it spreads back toward the poles.

When it gets to the poles, it descends back to the surface again to complete the cycle.


Three-Cell Model of Global Circulation / Coriolis Effect

Now, to get a little closer to what really happens, we need to eliminate our first assumption – that the Earth is not rotating in space. Because since of course it is, our circulation patterns become more complicated.

That's because of a natural phenomenon called The Coriolis Effect, which was discovered by French mathematician Gaspard de Coriolis in the 19th century.

The Coriolis Effect causes moving objects or currents on the surface of a rotating planet to veer to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) in the Southern Hemisphere.

Rotation causes the development of three circulation cells in each hemisphere. These cells are the Hadley Cell, the Ferrel Cell and the Polar Cell.

To take a look at the Three-Cell Model of Global Circulation, go to Figure 7p-2 at
http://www.physicalgeography.net/fundamentals/7p.html

The Three-Cell Model demonstrates the formation of the northeast and southeast trade winds, the westerlies, the polar easterlies and the subtropical and polar jet streams.

Actual Global Surface Circulation

At last we get to a model that shows Actual Global Surface Circulation -- the real thing. This model is based on 39 years of record keeping.

The circulation patterns differ from those shown on the Three-Cell Model in two primary ways:

First, the entire surface of the Earth is not composed of the same material, which eliminates the second assumption we made in the Simple Model. The surface of the Earth is made up of both land and water (70% water) -- and land and water behave differently when it comes to heating and cooling.

And second, because of elevation. Different elevations (like high mountains) cause different pressure centers, which is what creates atmospheric circulation.

To see an animation of the Actual Global Surface Circulation, go to the following web site and access Figure 7p-3: http://www.physicalgeography.net/fundamentals/7p.html

Gyres

The Coriolis Effect affects the oceans, too. Global atmospheric circulation patterns create large circular ocean currents in each hemisphere. These currents are called Gyres.

Gyres move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

Wind

We've been talking about atmospheric circulation. A simple word for atmospheric circulation is wind, but it is not a simple subject.

At first glance, it should be -- since air always flows from High Pressure Centers to Low Pressure Centers. But there are complicating factors (like the Coriolis Effect).

We'll try to explain it as succinctly as possible, beginning with an explanation of pressure centers.

Low Pressure Centers

An area where air is rising is less dense than its surroundings. This creates a Low Pressure Center.

So, since winds blow from high pressure to low pressure areas, you might expect that surface winds would always blow toward a Low Pressure Center.

Because of the Coriolis Effect they don't.

In the Northern Hemisphere they are deflected to the right and in the Southern Hemisphere to the left. This means that they don't actually arrive at a Low Pressure Center -- they circulate around it.

These winds are called Cyclonic Winds.

High Pressure Centers

An area where air is descending is more dense than its surroundings. This creates a High Pressure Center. Therefore, you might expect that winds would blow away from it.

But – again because of the Coriolis Effect – winds are deflected toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere.

These winds, which circulate around High Pressure Centers, are called Anticyclonic Winds.

There are a number of different pressure belts around the Earth. We'll list them first and then explain a little about how they work.

Pressure Belts

At the Equator, there is a belt of low pressure called the doldrums. Thirty degrees north and south of the Equator, there are belts of high pressure called the horse latitudes.

Then, at about 60 degrees north and south, there are belts of low pressure where the polar fronts are located.

Ffinally there are high pressure areas at both the north and south poles. These high and low pressure belts are what cause our wind and weather.

You can see an excellent diagram of them at:
http://www.cmdowns.com/wind-zones.htm
(Scroll Down to Illustration)

From the Doldrums to the Jet Stream

There are a number of components in our wind/weather system. Here are brief descriptions of some of the most important of them – starting at the Equator and moving toward the poles.

The Doldrums / ITCZ

The doldrums are a belt of low pressure along the Equator where the Sun's heat is the hottest. As the hot air expands and rises, it leaves long periods of windless calm. Sailing ships used to be trapped here for days or weeks, giving rise to the name doldrums.

Today, the doldrums are usually known as The Intertropical Convergence Zone (ITCZ), since this is where the trade winds from each hemisphere converge (meet). The weather here is hot and humid, and this is the home of the Earth's major rain forests.

The Horse Latitudes

What goes up must come down. And the hot air that rises in the doldrums comes down again in two high-pressure areas called the horse latitudes (30 degrees north and 30 degrees south of the Equator).

The horse latitudes are also known as the mid latitudes or the subtropics. The winds are light here and the weather is hot and dry.

Air flows from the horse latitudes toward the Equator (as part of the trade winds) or toward the poles (as part of the easterlies).

During the days when Spanish sailing vessels carried horses to the West Indies, ships would sometimes be becalmed here. Legend tells us that if the sailors ran out of food or water, they would either eat the horses or throw them overboard, which is how the horse latitudes got their name.

Trade Winds

The trade winds are winds that blow from the horse latitudes toward the Equator. They blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.

These winds blow almost constantly. They are called the trade winds because they gave trading ships a steady dependable ocean route to the New World.

Prevailing Westerlies

A prevailing wind is the wind that blows most frequently across a particular region. Different regions on Earth have different prevailing winds.

At between thirty and sixty degrees latitude, the prevailing winds blow almost constantly toward the poles.

Because of the Coriolis Effect, these winds appear to curve to the east. This means that they come from the west. So, since winds are named for the direction they come from, these winds are called westerlies.

Westerlies are responsible for much of the weather across the United States and Canada.

Polar Easterlies

Polar easterlies are winds that form when the air over the poles cools and sinks to the surface.

As the air flows away from the poles, it is turned to the west by the Coriolis Effect. Since these winds come from the east, they are called easterlies.

The Polar Jet Stream

The polar jet stream is formed by the deflection of upper air winds by the Coriolis Effect. It resembles a stream of water moving from west to east at an altitude of from 13 to 19 kilometers.

Wind velocity is highest in its core -- about 130 kilometers per hour in the winter and 65 kilometers per hours in the summer (although wind speeds of 400 kilometers per hour are not unknown).

The polar jet stream is why you can fly from San Francisco to New York (west to east) in less time than you can fly from New York to San Francisco (east to west).

Polar Front

The polar jet stream is associated with the polar front (a front is where two air masses of different temperatures or humidity meet.)

The polar front is the zone where warm air from the subtropics meets cold air from the poles.

Subtropical Jet Stream

The subtropical jet stream is located approximately 13 kilometers above the subtropical high-pressure zone

Like the polar jet stream, it flows from west to east and is characterized by relatively fast uniform winds concentrated within the upper atmosphere in a narrow band. It does not exist in the midlatitudes.

In addition to the above phenomena, there are winds that are either highly seasonal or highly local.

Seasonal Winds

Some of the better-known seasonal or local winds are:

Monsoons

A monsoon occurs because of the different ways land and water retain heat. In the summer, winds flow from the water to the land, causing heavy rains over the land.

In the winter, the winds flow from land to sea, resulting in dry land conditions.

The most famous monsoons occur in India and Southeast Asia, although they can also occur in Australia or the American Southwest. The word monsoon comes from the Arabic word mausim (season).

Siroccos, Chinooks, Santa Anas

A Sirocco is a hot, dry, dust-laden wind that blows from the deserts of North Africa across the Mediterranean into southern Europe. It occurs mainly in the spring.

A Santa Ana is a hot, fast-moving wind that blows down the side of the Santa Ana Mountains toward the Pacific Ocean in Southern California. It occurs between October and February and can move at from 25 to 100 miles an hour. This wind be very dangerous in case of wildfire.

A similar wind, called a Chinook, blows down the east side of the Northern Rocky Mountains in the winter and spring. It produces a rapid temperature rise and thaw, which is important for agriculture. In Switzerland, a wind like this is called a snow eater.

A Sirocco is different from a Chinook or Santa Ana. First, because downslope flow is one of the causes of a Chinook or Santa Ana, while a Sirocco flows mostly over flat terrain in the North African deserts. And second, because a Sirocco is caused by a low pressure area while a Santa Ana is caused by a high pressure area.

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for Students, Parents and Teachers

Now let's do Practice Exercise 5-1 (top).

Next Page: The Relationship Between the Rotation of the Earth and the Circular Motion of Ocean Currents and Air in Pressure Centers (top)