A few Southern Hemisphere Weather Analysis Topics

We live in a Northern Hemisphere-biased world, because much of our early population resided there. Thus, North Pole at the top of a map, South Pole at the bottom; which is fine until you consider things with no such preference - e.g., processes such as weather. Looking down on the North Pole, our earth rotates counterclockwise (as many things do - almost everything except clocks ) Among consequences of this rotation is apparent deflection of moving objects relative to our quasi-spherical, rotating Earth. The reason for this is angular momentum conservation. Such can be illustrated doing the following experiment : Quickly spin a wheel in the middle of a rod holding it, with the wheel rotating vertically. Sit on a stool which can easily rotate. Turn the wheel sideways, such that it spins horizontally. If the wheel spins clockwise, the stool spins counterclockwise (much slower, with a heavy person on it); and vice versa - conserving angular momentum. Similarly, an object moving poleward in the Northern Hemisphere is deflected eastward (magenta), because our earth moves fastest relative to its rotation axis at the equator, decreasing to no motion at the poles. More angular momentum further south (b) than north (c), thus when the object moves north, it deflects east because it has additional angular momentum our earth does not have. Similarly, an object moving eastward (green) deflects south, again conserving angular momentum.

An object deflects to the right of its direction of motion in the Northern Hemisphere for all situations. Similar arguements illustrate that moving objects in the Southern Hemisphere deflect left of their direction of motion. An apparent force can be imagined causing such, which is often called the Coriolis force.

When considering air motion, the only 3 real forces on air are gravity, pressure, and friction forces. The Coriolis force is imaginary, and a centrifugal force can also be imagined because of a tendency for air to remain moving straight in curved flow. Neglecting friction and assuming gravity is exactly balanced vertically with bouyancy (hydrostatic balance), consequence of balance of the remaining forces is the gradient wind meteorologists often speak of, which is illustrated for the Northern and Southern Hemisphere.

A consequence of the 'Coriolis' deflection is a balance such that winds flow in an opposite sense around High and Low pressure areas in either hemisphere. For similar pressure gradients, an anticyclonic gradient wind is faster than a cyclonic one because pressure gradient and centrifugal forces balance Coriolis force (proportional with gradient wind speed) for that situation.

Because of the opposite rotation, weather maps appear different for each hemisphere. For example, Southern and Northern Hemisphere surface analyses. The weather is basically same wherever on this planet you may go, only the mathematical magnitudes changes (regarding vorticity, etc.). If we place the Pole at the top of Northern Hemispheric maps, then why not do so for Southern Hemispheric ones also ? Such is a more natural depiction, but you'll see that standard Southern Hemispheric maps are upside-down. Because of the opposite air flow, circulation is illustrated similarly to the Northern Hemisphere, except for the fact that mid-latitude weather mainly moves from right to left instead of left to right (which many people read anyway), because upper air winds tend to blow from the west in either hemisphere, with low pressure north and high pressure south :

With the South Pole on top (example map for 10 JUN, 12 UTC from Australian Bureau of Meteorology :
pressure systems, fronts, etc. all appear same as for the Northern Hemisphere. So why isn't this done ? If the South Pole is defined as 0 degrees, positive counterclockwise (opposite the Northern Hemisphere), winds from a specific direction have similar meteorological significance, as illustrated.

Some maps include the Pole in the center, which is unbiased, but impractical for local, non-polar regions. So everyone should now begin printing 'upside-down' maps

Text and embedded graphics are copyright of Joseph Bartlo, though may be used with proper crediting.