Chapter Nine:
The wind bloweth where it listeth...43

Let’s take a look again at one of the diagrams from the last chapter.44

Blue circles to the left: vertical patterns atmospheric mixing.
Red arrows: the major world prevailing wind patterns.
Right side of the diagram: wind patterns named.

The last chapter was an introduction to atmospheric mixing. An important concept, vital (as are the myriad of inter-linked Earth systems) to life. The result of this mixing, however, is a series of dynamic air pressure regions on the blue planet which regulate general wind patterns. These are basic patterns. Wind at any location is rarely so easily determined.

Pressure systems regulate general wind patterns. In the diagram, “H” is found in 4 general locations (blue letters). The “H” refers to high air pressure. There are 3 “L”s (red letters), showing where dynamic low pressure systems tend to be located. Notice that the red arrows, which represent general wind patterns, directed toward the “L”s and out from the “H”s. Low pressure regions draw air from high pressure regions.
The most important notion of the “L”s is that air rises in a low pressure setting.
The most important notion of the “H”s is that air subsides in a high pressure setting.

Low pressure regions tend to be well watered, because rising air cools and moisture in the air can condense. Therefore, you see and “L” in the equatorial region... where tropical rainforests are found. You also see an “L” in both the northern and southern hemispheres at about 60o of latitude. The 60th parallel runs right along the coast of southern Alaska. Did you know that it rains a lot there? The low pressure region tends, on the west coast of north America to reach all the way to Seattle. Check a map and you’ll find Seattle near to the 50th parallel. Is that a rainy place?
High air pressure regions tend to be dry, because subsiding air is warming by compression as it falls. Notice on the map that “H”s are found mid-way between the equatorial “L” and the 50o “L”... at about 25o of latitude (both north and south). Check out the Sahara Desert... that’s where you’ll find it! Ditto with the Kalahari Desert in southern Africa! The interior of Australia and the Sonoran Desert of Mexico and Arizona are also found straddling the southern and northern 25th parallels. No coincidence! Descending air is dry air!

The red arrows on the diagram show the basic wind patterns that I want to share in this chapter (do you want me to share wind?).

Look again at the red “L” in the center of the diagram. The pressure is low because it’s tropical and therefore warm! The warmed Earth heats the air which rises, leaving behind low pressure! Air rushes into the low along the Equator (more accurately, along the inter-tropical convergence, as referred to in the last chapter). Air rushes northward into the ITCZ in the southern hemisphere and air rushes southward into the ITCZ in the northern hemisphere.

Notice that the red arrows that move toward the ITCZ are not moving directly toward the equator (in the diagram), but are bending toward the west. Those arrows represent the easterly Trade Winds. The bend (because of the Coriolis Effect) is to the right in the northern hemisphere and to the left in the southern hemisphere.

Once again: the winds bend as they rush into the ITCZ low pressure region, curving toward the west. They are called EAST winds! The reason is that, in Meteorology (study of weather), wind is always identified by it’s source rather than its destination. Thus, a SEA-breeze is coming FROM the sea. An EAST wind in Southern California is a hot, dry Santa Ana wind because it’s coming FROM the high altitude, dry region to the north-east. A MOUNTAIN-breeze is one that descends down from the mountains. In the Rockies a powerful mountain wind is called a Chinook.
The EAST winds in the tropics are called the TRADE WINDS because in 1492 Columbus sailed the ocean blue! Yeah! How about a look at the north Atlantic?

Microsoft Encarta Interactive World Atlas, 2001

Remember the wind directions from the previous diagram? In the midlatitudes (The United States and most of Europe are in the mid-latitudes, generally between 30o and 60o of latitude) the prevailing wind is WESTERLY. Why? Because the high pressure system in the subtropics (about 25o north latitude) is descending air which moves both northward and southward when it bunches up at the Earth’s surface. The northern moving air will bend to the right (northern hemisphere)... which means toward the east... which means FROM THE WEST! WESTERLY!

So, when Columbus attempted to cross the Atlantic, using wind to move him along, he couldn’t! What he did was travel southward (toward the Equator) because there is an excellent ocean current which moves southward from Spain along the coast of Africa. In the subtropics, Columbus’ sails would have died. The air was likely to be still. Why? Because the air in the subtropics is subsiding... descending from great heights in the troposphere and not moving horizontally at all! A region of calm much of the time, called the horse latitudes. Why “horse latitudes?” Because some sailors, after Columbus, ventured too far out into the Atlantic, away from the coast of Africa and lost the south moving ocean current. In so doing they also lost the wind! And they had to choose whether to throw their cargo overboard or eat it! Their cargo was horses! TRADING (as in, TRADE WINDS) horses from Europe for the mineral and agricultural wealth of the new world.

Now Columbus (and mariners who followed him) wanted to get to Miami because of the girls on the beach there. So he let the current take him southward until the sails filled up with the blue planet’s most consistent winds! The trade winds, blowing from Africa, pushing him all the way to America!

Why are they called ‘east’ winds? Because the subtropical high bunches up and the air that moves southward bends to the right, caused by the Coriolis effect! Right of a south heading is west. Air that moves toward the west is called an EAST wind (because of it’s source).

Why are they called ‘trade winds?’ Because they carried mariners from Europe to America carrying horses and slaves (which were picked up in west Africa... convenient because ships had to move southward to Africa before catching the trade winds, anyway). In America, they picked up gold and silver and molasses. The trade winds facilitated centuries of trade!

That’s physical Geography!
How’d they get home from America? Well, as the trade was refined and as population in America developed, mariners caught the Gulf Stream (north heading ocean current) off Puerto Rico and were carried to Boston where the molasses they had picked up in the Caribbean was converted to Rum! Note that Boston is in the midlatitudes! That means WEST wind! And wind FROM the west was perfect to get them back to England (or Spain or France, depending on the particular trade).
Note that off Europe there was an ocean current to carry them south and off North America the Gulf Stream could carry them north. It’s that way off every continent on the blue planet. The motion of such currents results from the same effect that causes the winds to bend toward the right (in the northern hemisphere). The Coriolis Effect. Let’s see if I can explain this thing. It’ll help if you squint... this is a tad confusing!

Coriolis was the name of the fellow who figured out that the wind and ocean currents move, to a definable extent, independent of the continental surfaces as the Earth rotates on it’s axis. Because the Earth’s atmosphere is surrounded by vacuum, there is no resistance which might cause the atmosphere to slow relative to the Earth’s rotation. So why should the air mass above the continents and the currents of the oceans (which are mostly caused by the winds blowing over the seas) move in any way differently than the continents do as they make their daily rotation?
In fact, they don’t move in any way different from their source region! The confusion of wind direction comes when they leave their source region and change latitude.

High pressure over the subtropics generates air movement toward low atmospheric pressure over the warm Equator. Visualize this: relatively high pressure air begins it’s movement from Jacksonville, Florida toward Manaus, Brazil (South America).

Now the Earth rotates at the Equator (Brazil), at 1040 miles per hour. The Earth’s circumference is about 25,000 miles and it takes 24 hours to make a circuit. 25,000 divided by 24 is about 1050! So far, so good. But the distance around the Earth at the 25th parallel north is less (near Jacksonville, Florida). The east-west circle of the Earth reduces in length with distance from the Equator (whether moving north or south from the Equator). Which is really obvious; it’s 25,000 miles around the Earth at the Equator, but the North Pole at 90o north latitude (which is a geometric point) is 0 miles around! Each degree north or south from the Equator represents a smaller circle around the Earth.

So what? So the speed of rotational travel changes by latitude! If the Earth is spinning at 1050 miles per hour at the Equator, it is (theoretically) spinning at 0 miles per hour at the poles! Miami, Florida (25o north latitude), it is spinning at about 950 miles per hour (about 23,000 miles / 24 hours). At Quito, Ecuador (located nearly due south of Miami) the Earth is spinning at about 1050 miles per hour because it’s so close to the Equator.

But the wind which is blowing down out of Florida doesn’t know that the Earth’s rotational speed beneath it’s movement is changing! Thus the wind, while moving southward, continues to rotate with the Earth at Miami’s latitude at about 950 miles per hour. As the wind progresses southward, the rotational speed of the Earth increases! And the Earth (at a lower... more southward... latitude) leaves the wind behind! The appearance on a flat map (which cannot move with the rotation of the Earth) is that the wind has deflected off toward the west, or to the right (clockwise from) of it’s path of travel.

Feeling dizzy, yet? This can be very confusing, and is very difficult to verbalize.
But it’s not really so complex. It’s somewhat like being on a merry-go-round. It’s more complicated than a merry-go-round because that is a 2 dimensional rotating object while the Earth is a 3 dimensional rotating object... but let’s follow the example as far as it will take us.
Standing in the center of the merry-go-round you toss a softball to your child who is standing at the outer row of up-and-down moving play-horses. If you do not account for the speed that your child’s position on the merry-go-round is spinning, the soft ball will pass behind your child. On a flat, motionless merry-go-round map it will look as though the ball curved to the right away from your child. The speed of rotation at the center of the merry-go-round is very slow. The speed at the outside of the merry-go-round is very fast. The softball that is tossed from the center will travel outward at the rotational speed of it’s source.
If you are still confused, I’m sorry. It’s the best I can do!

Let me conclude with a spiffy story. In equatorial Africa (I’ve read) there is a tourist bus stop at a village located exactly on the Equator. One enterprising fellow claims to own a home which actually crosses the Equator. He has a kitchen sink south of the Equator and a bathroom sink located north of the Equator. Tourists pay to watch water swirl down the drain in a clockwise direction in the southern sink and in a counter-clockwise direction in the northern sink! Creative proof of the Coriolis effect?

No way! Ironically, this is used in some Earth Science classes as evidence of Coriolis. In fact, however, Coriolis only works over significant distances, the greater the distance the more effective the effect! Tornadoes, which are rarely more than a quarter-mile in diameter, do not always spin the correct way, according to Coriolis... although they more commonly do.

How intricately the atmosphere is formed. Coriolis works to filter the atmosphere through both latitudinal and longitudinal movement. It works to provide variety of environment on Earth. It works to differentially water the continents. It provides creative transportation options for creative people. Columbus used Coriolis to his advantage! Airline pilots can also use resultant wind patterns to speed long distance travel... and they do! Hot air balloonists can have more fun in the ongoing effort to circumnavigate the globe! Ain’t it great?

Before we call it quits, let’s apply Coriolis to ocean currents.
The wind is caused because the sun heats some Earth surfaces more than others. You know that. The winds blow over the ocean surfaces, which cover nearly 3/4 of total Earth surfaces. The energy in the wind... solar energy originally... transfers into ocean waters, causing ocean currents.
When energy is transferred to water, it has found an exceptionally effective energy conductor. Although winds are easily deflected and obstructed and tend to come and go, to change direction and intensity, often, ocean currents are different. Once an ocean current is formed, it is very likely to continue in strength and dimension with relatively little changeableness. And the energy is neither easily nor quickly depleted. Thus, once an ocean current begins, it is likely to far out-distance the winds that spawned it. Indeed, once formed, currents are likely to cross an ocean, whether Pacific or Atlantic or Indian. Even then they tend not to die out. When a current crosses an ocean and slams into a continental coast, the will consistently turn... You guessed! The turn will always be to the right (counter-clockwise) in the northern hemisphere and to the left (clockwise) in the southern hemisphere!

So it is that the great ocean currents are influenced by the Coriolis effect.
The effect of this is that there is a poleward moving current along almost every eastern continental coast. If the current is moving toward the pole (whether north pole or south pole) then it is coming from the tropics! Thus, this is a warm ocean current. An excellent example is the Gulf Stream current along the eastern coast of the United States. It carries so much warm tropical Atlantic water so far that it warms virtually all of Europe!

For fun, look how far north England is... London is farther north than Montreal, Canada! But it’s much warmer than Montreal! Partly because the Gulf Stream distributes tropical water so far northward.

And there is an ocean current moving equatorward along almost every western continental coastlines. An excellent example is the California current. It’s from the north and headed south! It’s origin is the Gulf of Alaska and it’s cold! Wanna go swimming in San Francisco on a pleasant July day? If you do, just try to keep your teeth from chattering! It’s, like, COLD! Although Coney Island (New York City)
is farther north than San Francisco, you can have a toasty warm swim there on a pleasant July day. Because there’s a cold current off San Francisco and a warm current off New York.

Source: PHYSICAL GEOGRAPHY, Tom McKnight, 7th edition, Prentice-Hall

Chapter 10


43-John 3:8, King James Version. The entire verse, as translated in the New International Version: “The wind blows wherever it pleases. You hear its sound but you cannot tell where it comes from or where it is going. So it is with everyone born of the Spirit.”

44- Understanding Weather and Climate, Edward Aguado and James Burt, Prentice Hall, 1999. Page 186.