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BY GEORGE: The Complicated Wind
Submitted by George Elliott on Sat, 01/05/2013 - 6:49pm.
Sounds simple enough. We might not be able to see it, unless it carries “stuff” with it, but all it is, is a breeze. Right? Well, it is rather a complicated subject, and even this so-called more advanced summary only touches on the main elements of wind analysis, it easily gets much more complicated real quickly if we venture much farther from what I talk about here. Let’s not even talk about “geostrophic” flow and the “thermal wind.” We should, because they’re all related, but this isn’t a meteorology class after all.
Let’s start with something we know, and that being the standard atmospheric pressure is 14.7 pounds per square inch at the surface of Earth. We also know that air pressure decreases as we rise in the atmosphere. Since air is a gas, it responds to changes in temperature, elevation, and latitude (owing to a non-spherical Earth). Air pressure decreases naturally as we rise in the atmosphere, or up a mountain, so we must make correction to the air pressure owing to elevation above sea level. These corrections are easily made by adding the air pressure that would be exerted by the air column at that elevation.
Usually when you fly on a commercial airline, the pilot comes on the loudspeaker and announces, thank you for flying their airline, the estimated time of arrival and the height you'll be flying, e.g., 39,000'. Well, they are not exactly telling you the truth. Since pressure changes from place to place, owing to weather systems, temperature, and elevation, airliners will fly at a constant air pressure rather than constant altitude. So, for example, if the pilot sets the airline to fly at 7.83 inches of air pressure (at the surface the air pressure averages 29.92 inches), that should be approximately 32,800' in a “standard” atmosphere, but the actual elevation above sea level is variable, given the vicissitudes of the weather and thermal structure of the atmosphere.
Wind results from a horizontal difference in air pressure, and since the sun heats different parts of the Earth differently, causing pressure differences, the Sun is the driving force for most winds. The wind is a result of forces acting on the atmosphere:
1. Pressure Gradient Force (PGF) - causes horizontal pressure differences and winds.
2. Gravity (G) - causes vertical pressure differences and winds.
3. Coriolis Force (Co) - causes all moving objects, such as air, to diverge, or veer, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
4. Friction (Fr) - very little effect on air high in the atmosphere, but more important closer to the ground.
5. Centrifugal Force (Ce) - objects in motion tend to travel in straight lines, unless acted upon by an outside force.
The Net force = PGF + G + Co + Fr + Ce
The Pressure Gradient Force (PGF) is the direct result of different air pressures. The magnitude of the pressure difference and the distance between any two points in question will essentially determine the velocity of the PGF of the wind.
The vertical pressure gradient is much larger than the horizontal pressure gradient (about 100 times larger for that matter), yet winds don't blow straight up. Why? Gravity acts to stop, or slow, the vertical flow of air, so vertical winds are much less than horizontal winds. Most vertical winds are on the order of 1 mph, however some downdrafts and updrafts in thunderstorms can be up to 110 mph.
Since the Earth rotates, objects that are above the Earth “appear” to move or are deflected if they are already moving, owing to Earth’s rotation. This apparent motion is caused by the Coriolis Force (named after the scientist that discovered the principle). In the Northern Hemisphere, objects will be deflected to their right, while in the Southern Hemisphere, objects will be deflected to their left. The magnitude of the deflection is also a function of distance from the equator and velocity. So, the farther from the equator the object is (including wind), the greater the deflection, and the faster an object is moving, the greater the deflection. These "objects" can be anything from airplanes, to birds, to missiles, to parcels of air.
By the way, the Coriolis Force has nothing whatsoever to do with water direction that drains down sinks and toilets.
Winds near the surface are influenced by the ground. This influence is in the form of friction. Friction acts to retard the motion of the wind -- it is always in the direction opposite the wind velocity. Friction acts to oppose the flow of the air. The air will slow down, reducing the Coriolis force. This results in an imbalance of forces. The atmosphere adjusts, to regain a balance, by turning the wind from higher to lower pressure.
A new balance is achieved when the sum of the Friction and Coriolis forces balance the horizontal pressure gradient force.
Also, as basically states in one of nature's fundamental equations of motion, an object will tend to move in the general direction it is moving, unless acted upon by another force. The centrifugal force is essentially the law that governs this principle.
With all of the above forces acting as a large addition math problem, they eventually balance out to give us any particular surface wind as observed in the moment.
So, there you have it. The basics of a simple breeze.
Easy enough? Right?
By: George Elliott