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Blowin' in the Wind, Issue #002 - The weather systems that occur at high elvations. January 2, 2004
January 02, 2004
High altitude, low pressure weather mapsMeteorologists work with upper elevation maps and deal with these four variables:
Translation – movement across the map Deformation – change in shape, like squeezing a ball of dough without changing its size Divergence – increasing/decreasing the volume of part or all of the flow area Vorticity – rotation about a vertical axis, which may also be moving.
Pressure and systems are defined at upper levels by the altitudes above sea level one must go to reach the specified millibar level. The flow goes clockwise around a low and counter-clockwise around a high…lows are associated with increased cloud.
Relative vorticity deals with horizontal rotation of the air above the earth such as flow around some corner. The same effect is created by changes in speed over some horizontal distance (wind shear).
Meteorologists try to pinpoint high and low spots on the current vorticity map and determine whether they are stretched or transported intact by the upper level winds. We call these two processes vorticity shear and advection respectively
Long waves do not move very fast. Short waves go eastward along the long wave and show up as a bump on the larger wave. We also find that they are tied to maximum and minimum values in vorticity, and find that short wave trough really means an axis of maximum vorticity.
Air rises and clouds form ahead of a trough, while it descends and clouds dissipate in front of a ridge.
If you look at an upper air weather map, you can see many straight or meandering curves from the left side to the right. A map can show two or more streams (groups of curves) simultaneously on the same continent or ocean. Two streams become in phase when their high and low points (ridges and troughs) lie directly above (north and south of) one another.
Conservation of Angular Momentum. This is the concept that increases the rotational speed of a figure skater when she stands up straighter during a rapid spin. Imagine a fire hose that normally points to the right (east). Changing the direction of that hose changes the location and transport of the water’s momentum just as sending the high momentum (speed) winds toward the North Pole and returning low momentum winds equator ward has the net effect of transporting surplus angular momentum from the equatorial region towards the poles.
Tighter contour lines (more focussed) correspond to higher wind speeds and vice versa for curves that are more spread out. Southward momentum transport, less common, sometimes goes with sudden changes in weather.
If a deficiency becomes strong enough, streams can decouple and one or more new centres of high and/or low pressure can form between them, creating omega and other blocks.
Advection is simply the transport of something such as a blob of cold air by the wind. Where contour lines on an upper air map cross, they should form little four-sided boxes. Meteorologists look upstream on the wind contours and see if the incoming air is warmer or colder. To do this, remember that wind flow goes clockwise around a high pressure center and counter-clockwise around a low.
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