note: this is a
work in progress begun on 8-16, last updated 8-19. Please give me
time to
flesh this out before adding to it.
Heat
At the heart of weather is the sun. Without it we wouldn't have weather at all. So the next time you don't like the conditions outside, just blame it on the annoying orange orb in the sky. Sol adds heat to the earth through its emmision of electromagnetic radiation in many ways:
Heat transfer between the
land,
waters,
atmosphere, and
space occurs via
radiation,
conduction, and
convection (this will be added to).
The earth would have rather boring weather if it weren't for the differential heating of the earth by the sun - it's round, and the sun is effectively a point source of EMR, so surfaces perpendicular to the source recieve more radiation than surfaces at an angle.
Hadley Cells
In a broad sense, this means the closer you get to the equator, the more heat the earth is recieving. If the earth weren't rotating, we would have a single atmospheric convective current: warm air would rise in the equatorial regions, pulling in surface air to replace it. The warm air has to go somewhere - it heads towards the poles, cooling as it goes, and then finally sinking at the poles as it gets cool enough. There's more air sinking behind it, so it gets pushed back to the equatorial regions - repeat.
The earth's rotation , though it follows that things on its surface move at the speed of the surface, does not affect the levels of the atmosphere the same way - the lower levels of atmosphere are drug along behind the surface, while the upper levels are less affected. Not only that, but because the spherical shape of the earth means that the closer to the poles you get, the slower you're rotating, you get something called the coreolis effect (someone check my spelling here).
This basically means that air that's moving north or south will also be imparted an east or west vector as well, depending on whether it's in the northern or southern hemisphere.
The end product of the differential heating and the coreolis effect is not one rotational convective air cell in each hemisphere, but three - and they are called Hadley cells.
Hadley cells, while having definate boundaries of latitude (each one is roughly 33 degrees of latitude long), do not necessarily occupy a full hemispheric section of the globe (i.e., they don't wrap all the way around the globe in actuality - theoretically, if you could create a copy of earth and then start time, you would see it start this way, and then things would, well, get interesting as chaos takes effect).
The cells nearest the poles rotate such that air is sinking near the poles and rising at 66 degrees north and south latitude. The ones in the middle (they all have names, but I'll have to look them up) rotate such that air is rising at 66 degrees north and south latitude, and sinking near 33 degrees north and south latitude. The ones nearest the equator rotate such that air is sinking near 33 degrees north and south, and rising at the equator - so all six hadley cells, spanning the globe from north to south, operate like gears.
Where the air is collectively sinking, there are areas of high pressure. Where it is rising, there are areas of low pressure. This would be all nice and uneventful if it weren't for the coreolis effect again - it bends these northerly and southerly winds at the tops of the Hadley Cells in such a way that in between the areas of high and low pressure, the winds aloft are moving east or west! This is what creates the prevaling winds and jet streams - notice that weather systems, in the US, (between 33 and 66 degrees north latitude), move from west to east.
Rossby Waves and Open-Wave Cyclones
So, in the US, the weather systems are pushed along by the jet streams. In Canada, which is for the most part above 66 degrees north latitude, the jet streams move from east to west (these particular jet streams may be way farther north than mid-Canada...I'm going to have to do some more research here). Some basic fluid dynamics happens here - one thing going one way, the other going the opposite - and right in between them, about where the 66 degree north low-pressure areas are, you start to get cylconic movement - counter-clockwise.
At the center of the cyclone is a low-pressure area (aka a low). Wind flows into the low from the bottom from all directions, spirals up into it, and then out. The coreolis effect is what gives it the cyclonic motion. Now, assuming that this low is somewhere on the US-Canada border, the air north of it will be cooler than the air south of it. Once the winds start to swirl counter-clockwise around the low, you will get cold air to the northwest of the low moving south, and warm air to the southeast of the low moving north - note that this takes the cold northern air into the warm southern air, and vice-versa; the first on the west side of the low and the second on the east side. Presto; you now have a cold front and a warm front. The whole thing taken together is an open-wave cyclone. These also occur in the southern hemisphere, but the circulations of highs and lows are reversed.
The term "open-wave" is used because the cold front typically runs from southwest to northeast up to the low, and the warm front runs from the low sort of down to the southeast (though a lot of the times it runs straight out east, and the cold front will run due south). So it lookes like a wave with the low at the crest.
A series of open-wave cyclones usually occur in a band around the globe, pushed along by the jet streams. The warm front of one will connect at its far end to the far end of the cold front of the next low east of it. These series of waves are called Rossby waves.
This is going to take some heavy reading on my part to finish this, and this semester's classes have picked up to the point that I may not finish this until after Christmas. But thanks for your patience.