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Sunday, April 27, 2014

The jet stream(s)

We keep saying that the Jet Stream pushes weather systems around the world.  It just ain't so.  Let me start with a similar situation that occurred in science.

Before the Kiwi scientist, Earnest Rutherford did his work, we had the "Plum Pudding" model of the atom. Lets call this model 1.   Atoms were solid round little bodies packed together with their component parts touching each other.  Rutherford fired alpha particles (Helium nuclei) at a thin gold foil and found that hardly any of the alpha particles bounced back.   Most of them flew straight through the foil with no impedance what so ever.  From this was born the "solar system" model of the atom.  That is to say, most of the atom was empty space and most of the mass of the atom was concentrated in a very small (in relation to the size of the atom) location in the middle.  Electrons orbited around the nucleus like little planets.

This model, lets call it model 2,  was far and away better than the Plum Pudding model but only went so far.  For instance, it didn't explain the peculiar properties of water.  From various other sources, it was realized that water should not be liquid at the temperatures and pressures we have on earth and yet it is.  It was realized that the hydrogen atoms in water are not "attached" on opposite sides of the oxygen atom but rather the water molecule is in a V shape with an angle of 104.5 degrees between the arms formed by the two hydrogen atoms.  This means that the water molecule is positively charged on the Hydrogen side and negatively charged on the oxygen side nd since the positive side is a "naked proton, it has the relatively powerful "Hydrogen Bond"  This explains why water condenses at a much higher temperature than expected, why it expands when it freezes, why it has such a high phase change energy and a number of other peculiar properties it has.

Apparently the orbits of the electrons are not circular as in the planets but, in oxygen, the "p" suborbitals are  like dumb bells at right angles to each other.  Incidentally, this model, model 3, is better than the solar system model but still left something to be desired and later models are even better.

So back to the Jet Stream.

First we have to have a look at the air circulation systems of the earth known as the Hadley cells.  At the equator, the greatest intensity of sunlight hits the earth because the surface of the earth at the equator is at right angles to the sun's rays.  One square meter of incoming sun energy hits one square meter of the earth.  The further north or south you go, the wider the footprint of one square meter of sun radiation.  For the mathematically inclined, you can find how far the sun spreads out by multiplying the one square meter of incoming sun light by the cosine of the latitude.  Cos0 = 1 (at the equator) while cos90 = 0 (at the poles).  Of course this neat picture is complicated by the tilt of the earth.  We could define something called the solar equator and this would be the latitude at which the sun is directly over head at that time of the year.  At the height of summer in the Northern hemisphere, our so called solar equator is at 23 degrees North.

Clear air has some interesting properties.  It is opaque to a little bit of the UV radiation from the sun and this heats up a band of air high in the atmosphere but by far the greatest part of the radiation from the sun passes right through the atmosphere and hits the surface of the earth.  This heats the earth and the earth in turn heats the atmosphere from the bottom.  This effect is greatest at what we have called  the "Solar Equator".

Quite a bit of solar energy falls on the poles at the height of their summer.  However, the poles are covered in snow an ice rather than forests, plowed fields and cities.  The poles reflect much of the incoming radiation right back into space without heating the ground and hence without heating the atmosphere above the poles.  So what results from this uneven heating.

At the equator, with the intense heating of the atmosphere from below, this expands the air and causes it to rise.  In addition, a lot of water is evaporated and as it reaches the dew point, it condenses and gives out it's latent heat.  Air is sucked in from North and South along the surface of the earth to replace this rising air and at altitude, the rising air spreads out North and South.  It moves away from the equator and sinks again at about 30 degrees North and 30 degrees South.   Let's just focus on the northern hemisphere from now on.

At the North Pole, the air isn't been heated at all in winter and not all that much in summer and the air is radiating heat into space.  (every substance which is above absolute zero - minus 273C - radiates heat).  The air cools, contracts, gets heavy and sinks.  At altitude  air is sucked from the south and as this descending air hits the ground it spreads out toward the south.  It rises again at about 60 degrees North.

You will have noticed that we have a gap between 30 degrees North and 60 degrees North.  Between the Equatorial Hadley cell and the Polar hadley cell there is the Ferrel cell which is probably powered by its larger cousins much as an idler gear is powered by the gears on either side of it.

You will notice in this diagram that the surface winds do not blow directly North and South but are scewed to the right in the Northern Hemisphere and to the Left in the southern hemisphere.  This is due to a phenomenon called Coriolis which we won't go into here.

Of interest, though, note that at the equator we have a wall of rising air, between the Northern Hemisphere and Southern hemisphere equatorial Hadley cells  Between the Equatorial cell and the Ferrel cell (not labeled in this diagram) there is a wall of descending air and between the Ferrel cell and the Polar cell, once more a wall of rising air.

The second point of interest, which we see on the following diagram, is that the border between cells does not follow a line of latitude around the globe but waves north and south like a demented sine wave.  These waves are quite irregular and progress from West to East.  They are called Rossby waves.

And note that the above diagram is showing us two of the jet streams.  This is also the border of the circulation cells.  Jet streams occur where Hadley cells meet.You can see where we are going with this.  It isn't the Jet stream that pushes weather patterns around the world,  It is the rising (upwhelling if you like) wall of air or the downwhelling wall of air that occurs between the Hadley cells.  This may sound like nit picking but just as Model three for the atom was more useful than model 2 and 2 better than one,  we are likely to get a far better insight into our earth with a more accurate model.  A more refined model would likely be even better.

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