The student continues following the main thread: weather and wind are parts of the process by which the atmosphere disposes heat absorbed from the ground, which is warmed by sunlight. This section is qualitative, and much of it can be viewed as optional.
- On the average, Solar heating of the ground gets more intense the closer one gets to the equator, because sunlight falls more vertically, spreading over a smaller "footprint."
- In addition to vertical motions discussed in #S-1A, the atmosphere has large-scale horizontal flows, which help disperse solar heat from near the equator, distributing it more evenly across the globe.
- Hadley in 1735 guessed that warm air rises near the equator, flows poleward in higher layers, then descends and returns near the ground. Such flow does exist, but only within about 20° of the equator. The descending dry air is responsible for desert belts around that latitude.
- Poleward and equatorward flows, on a global scale, are diverted by the Coriolis force, which shifts winds eastwards at middle latitudes, so they blow mainly from the west
- At middle latitudes (e.g. the "lower 48" states of the US) the dominant wind is still from the west, but the process is more complex. The poleward flow and equatorward return, instead of being at different heights, are spread out horizontally--poleward at one location, equatorward at a neighboring one to the east. This produces large-scale north-south "Rossby waves" which advance eastwards. The wind from the west ("westerlies") is fastest at high altitudes, where its core is known as the jet stream.
- . The return flow towards the equator may have lost some of its eastward momentum, leading to "trade winds" blowing from the east closer to the equator. In earlier times, these patterns determined the routes followed by sailing ships.
- The "El Niño" is a shift in the global weather, first noticed off Peru, where warmer sea currents caused fish to become scarce. It can last 1-3 years and is caused by gradual warming of the water of the eastern Pacific Ocean.
Terms: tropic of cancer, tropic of Capricorn, tropical region, polar region, "Hadley cell" flow, Coriolis force, jet stream, belt of deserts [optional: Rossby wave, El Niño].
Starting the lesson:
In thunderstorms and other convection, the driving motion is up and down: hot, humid air rises, cooler, dry air returns. The source of energy which drives all that is the heating of the ground by sunlight.
However, most winds we know are horizontal. We know that they are part of large systems, as we can see on weather maps. How can the heating due to sunlight drive horizontal winds?
-   Heating by sunlight is greatest near the equator. Elsewhere the Sun's rays usually arrive in a slanted direction, which spreads them over a wider area. Near the equator they are always close to vertical and therefore the heating is most concentrated.
(A drawing on the blackboard may help. See also "The angle of the Sun's rays.".)
Horizontal winds help spread the heat across the globe, allowing a larger area of the atmosphere to participate in returning it to space
Hadley in 1735 was the first to propose a process for such spreading of heat. What did he propose?
-   He thought warm air rose near the equator, flowed polewards high up while it cooled, then descended equatorwards again and returned at low level.
Suppose this happens: the flow would be in the north-south direction. Yet actual winds flow mostly east-west, and we understand this is caused by the rotation of the Earth around its axis. What causes the shift in direction?
-   The Earth is a sphere, so the speed of rotation is greatest at the equator, at points most distant from the axis of rotation. The speed is about 400 meters per second, eastwards.
[Satellites are launched eastwards from Cape Canaveral, which is about as close as one can get to the equator in the mainland USA, to include the rotation velocity as part of the launch velocity. The European Space Agency launches eastwards from Kourou, in French Guiana, on the north coast of South America, even closer to the equator.]
As one moves polewards from the equator, points get closer and closer to the axis, and the rotation speed drops--until at the poles, it is zero. Any air traveling poleward keeps much of its eastward momentum, which gives it an extra eastward velocity relative to the ground below.
This is an example of motion appearing different when viewed in a rotating frame of reference. Not only is a "centrifugal force" added there, but motions of an object from one rotation radius to another seem to get an extra push in a perpendicular direction. What is this called?
-   The Coriolis effect (after Gaspard Coriolis, who deduced it in 1835).
And what about air returning equatorwards?
- It loses its extra eastward velocity. Of course, if friction with the ground took away energy from the eastward motion, it ends up moving westward, relative to the ground.
[Optional] Suppose you have a region of low pressure, say, above the middle of USA. Air rushes in from all sides to fill it. The Coriolis force causes that air to swirl. Will it swirl clockwise or counterclockwise?
- Always clockwise. Details are described in the section on rotating frames of reference.
Students can figure out what will be the swirl out of a high pressure region, or else into a low pressure region south of the equator, say above Argentina. (Answer: clockwise, in both cases.)
Does the "Hadley Cell" circulation exist?
-   Only close to the equator, at latitudes below 20°
Hadley's prediction was that the circulation is between high and low altitudes. Instead, in middle latitudes, the motions are mostly horizontal: warm air flows poleward at one location, equatorward at neighboring ones.
The Coriolis force still makes the air moving poleward add an eastward motion, which is why most winds in the continental US come from the west. But the flow is horizontal, and includes large waves ["Rossby waves"], extending appreciably north south.
At what altitudes is this flow fastest?
-   At high altitudes, around 10,000 meters or 30,000 feet.
And it is called there...
(Because of the jet stream, a flight from California to New York may be noticeably shorter than one from New York to California.)
Moving air may lose some of its eastward flow, say, to friction with the ground. As it returns equatorward (even in Rossby waves), it rotates more slowly than the ground below. What is the result?
-   Winds which on the average blow from the east.
-   Trade winds, because sailing ships from Europe to America took advantage of them. They returned home on a more northerly path, where winds blow from the west.
What is "El Niño"?
-   A world-wide shift in weather patterns, when the waters of the western Pacific get too warm.
This disrupts cold ocean currents near Peru, and since cold water holds more nutrients, the ecology is temporarily changed. Fishermen found their catch was much smaller, and since the shift tended to occur around Christmas, they gave it this name--"Niño" in Spanish means child, in this case, the Christmas child. The shift may last one or more years. Other effects extend world-wide.