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.

A typical instable days' weather;

Usually the day starts with a clear sky, but when the sun gains some strength, the first clouds can be spotted.

These are cumulus clouds (usually abbreviated to Cu). They form above thermic bubbles (convection). This is air that rises from the now warmed earth. But when air rises up, and thus gets colder, it also loses it's capability to hold moisture and at some point becomes saturated. The moisture condensates and drops of water are formed around some core that it clings to (a grain of salt, dust, etc).

The first clouds are usually a bit 'shapeless' as the develop the level at which the water condensates becomes more visible. Because clouds usually form far apart from eachother, it looks like they're on a different height, but this in fact, is an optical illusion. Every cloud in the same category (cirrus, stratus, cumulus etc) is formed roughly on the same level.

As the morning-thermic fades, and the sun gains more strength, the earth warms up a lot quicker, which in turn causes more air to rise, condensate etc. Clouds develop quickly during this time of the day and begin to shield sunlight from the earth. As a result, the warming of the earth slows down, and so does cloud developement.

Sometimes, these Cu clouds join and form bigger structures. This way cumulonimbus (abbrev: Cb) clouds can form. These are massive cloud structures. And because they are so immense, they block a lot of light, which makes the base of the structure look dark-grey. These are the clouds lightning can form in.


So, when will it rain?

Clouds, Planes and Umbrellas

This is really hard to tell, but when you see a Cb or two, and jet propelled airplanes leave long white trails, you'd better pack your umbrella.

The wind factor

You might also want to have a look at a barometer, like everything else air wants to float down, from high pressure to low pressure areas. This is what we call wind.

Air usually drifts towards a depression in a swirl (much like water in a sink), when a depression is near (high wind speeds), this means the air is cold (and unable to hold a lot of water), this increases the chance that it'll rain.

Did you know: If you're on the northern hemisphere, and the wind is in your back, the depression (or low pressure area) is at your left. The center of the low pressure area is about thirty degrees before that. The high pressure area is at your right. Reverse this for the southern hemisphere. This little fact is validated by the 'Buys-Ballot' law.

Weath"er (?), n. [OE. weder, AS. weder; akin to OS. wedar, OFries. weder, D. weder, weer, G. wetter, OHG. wetar, Icel. ve&edh;r, Dan. veir, Sw. vader wind, air, weather, and perhaps to OSlav. vedro fair weather; or perhaps to Lith. vetra storm, Russ. vieter', vietr', wind, and E. wind. Cf. Wither.]

1.

The state of the air or atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness, or any other meteorological phenomena; meteorological condition of the atmosphere; as, warm weather; cold weather; wet weather; dry weather, etc.

Not amiss to cool a man's stomach this hot weather. Shak.

Fair weather cometh out of the north. Job xxxvii. 22.

2.

Vicissitude of season; meteorological change; alternation of the state of the air.

Bacon.

3.

Storm; tempest.

What gusts of weather from that gathering cloud My thoughts presage! Dryden.

4.

A light rain; a shower.

[Obs.]

Wyclif.

Stress of weather, violent winds; force of tempests. -- To make fair weather, to flatter; to give flattering representations. [R.] -- To make good, ∨ bad, weather Naut., to endure a gale well or ill; -- said of a vessel. Shak. -- Under the weather, ill; also, financially embarrassed. [Colloq. U. S.] Bartlett. -- Weather box. Same as Weather house, below. Thackeray. -- Weather breeder, a fine day which is supposed to presage foul weather. -- Weather bureau, a popular name for the signal service. See Signal service, under Signal, a. [U.S.] -- Weather cloth Naut., a long piece of canvas of tarpaulin used to preserve the hammocks from injury by the weather when stowed in the nettings. -- Weather door. Mining See Trapdoor, 2. -- Weather gall. Same as Water gall, 2. [Prov. Eng.] Halliwell. -- Weather house, a mechanical contrivance in the form of a house, which indicates changes in atmospheric conditions by the appearance or retirement of toy images.

Peace to the artist whose ingenious thought Devised the weather house, that useful toy! Cowper.

-- Weather molding, or Weather moulding Arch., a canopy or cornice over a door or a window, to throw off the rain. -- Weather of a windmill sail, the obliquity of the sail, or the angle which it makes with its plane of revolution. -- Weather report, a daily report of meteorological observations, and of probable changes in the weather; esp., one published by government authority. -- Weather spy, a stargazer; one who foretells the weather. [R.] Donne. -- Weather strip Arch., a strip of wood, rubber, or other material, applied to an outer door or window so as to cover the joint made by it with the sill, casings, or threshold, in order to exclude rain, snow, cold air, etc.

 

© Webster 1913.


Weath"er (?), v. t. [imp. & p. p. Weathered (?); p. pr. & vb. n. Weathering.]

1.

To expose to the air; to air; to season by exposure to air.

[An eagle] soaring through his wide empire of the air To weather his broad sails. Spenser.

This gear lacks weathering. Latimer.

2.

Hence, to sustain the trying effect of; to bear up against and overcome; to sustain; to endure; to resist; as, to weather the storm.

For I can weather the roughest gale. Longfellow.

You will weather the difficulties yet. F. W. Robertson.

3. Naut.

To sail or pass to the windward of; as, to weather a cape; to weather another ship.

4. Falconry

To place (a hawk) unhooded in the open air.

Encyc. Brit.

To weather a point. (a) Naut. To pass a point of land, leaving it on the lee side. (b) Hence, to gain or accomplish anything against opposition. -- To weather out, to encounter successfully, though with difficulty; as, to weather out a storm.

 

© Webster 1913.


Weath"er, v. i.

To undergo or endure the action of the atmosphere; to suffer meteorological influences; sometimes, to wear away, or alter, under atmospheric influences; to suffer waste by weather.

The organisms . . . seem indestructible, while the hard matrix in which they are imbedded has weathered from around them. H. Miller.

 

© Webster 1913.


Weath"er, a. Naut.

Being toward the wind, or windward -- opposed to lee; as, weather bow, weather braces, weather gauge, weather lifts, weather quarter, weather shrouds, etc.

Weather gauge. (a) Naut. The position of a ship to the windward of another. (b) Fig.: A position of advantage or superiority; advantage in position.

To veer, and tack, and steer a cause Against the weather gauge of laws. Hudibras.

-- Weather helm Naut., a tendency on the part of a sailing vessel to come up into the wind, rendering it necessary to put the helm up, that is, toward the weather side. -- Weather shore Naut., the shore to the windward of a ship. Totten. -- Weather tide Naut., the tide which sets against the lee side of a ship, impelling her to the windward. Mar. Dict.

 

© Webster 1913.

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