101: Wind I

 

Winds of change...

A candle in the wind...

Throw caution to the wind...
 

Wind finds its way into about everything, literally and metaphorically. Perhaps the only component of weather that humans have noticed longer is temperature. Therefore, it's about time to dive into the concept of wind from a meteorological perspective.

To a meteorologist, there are many different types of wind and several ways wind data can be communicated. Before I get started, I should mention that some particulars of wind reporting change between weather centers and different countries. Also, for meteorological purposes wind is ALWAYS reported as the direction it is blowing FROM. Therefore, westerly wind is wind blowing from west to east.

Some simple concepts and definitions are needed in order to fully understand the different varieties of wind, so I'll start with those:

Forces:

Coriolis Force
Technically, the Coriolis Force is not a real force, it is an apparent force resulting from the rotation of the Earth. If one were to observe the Earth from a fixed point in space, they would not be aware this kind of force. The result of the Coriolis Force is that objects in motion are deflected, to the right in the northern hemisphere and to the left in the southern hemisphere, from traveling in a straight path. This force is very important because it is the primary source of rotation in both the atmosphere and the ocean. However, at small scales it is insignificant as other forces become much more dominant. Therefore, something like a single storm cloud is going to be relatively unaffected by this force. It goes with out saying that this force will not cause you to mess up your ball game and it is not the reason for the direction your toilet flushes. The magnitude of the Coriolis Force is proportional to the sine of latitude [sin(lat)], so it is at its maximum at the poles (90 degrees) and zero on the equator (0 degrees).

Centrifugal Force
This too is actually just an apparent force observed because we inhabit a revolving globe. Since the wind is also on this globe, the centrifugal force will be treated as a real force for this discussion. This is the same "force" that holds water in a bucket as it is swung around, or causes the contents of your car to slide when you go around a sharp turn. In the atmosphere, this force often becomes significant at times the Coriolis force becomes insignificant. For example, calculations on the wind inside a tornado completely ignore the Coriolis force, but are heavily dependent on the centrifugal force.

Pressure Gradient Force (PGF)
Air has weight, thus it exerts pressure on everything it is in contact with. However, the precise weight of air varies from place to place since the density of air varies ever so slightly. Therefore, some areas will have higher pressure, while others will have lower pressure. Like any fluid, air will try to redistribute in an attempt to make the amount of air uniform across the globe. The result is the pressure gradient force (PGF), in which higher pressure air is drawn to areas of lower pressure. The odd thing is: the pressure gradient's positive direction is toward high pressure, therefore, the PGF points in the gradient's negative direction (PGF= -1/ρ ∙ ∇p).

Frictional Force
The force exerted on air greatly depends on the environment it is in. Wind moving close to the surface will exhibit the greatest and most variable effects of friction due to differences in terrain. At upper levels, friction is considerably less, since the only source of friction is the air itself. Friction does not have a key role in major types of wind, instead, it is often a force that causes wind to deviate from those types of wind.

 

 

Coordinate Systems:

When meteorologists report or calculate wind, they must choose a manner in which to do so. Three major systems exist, each with a different use in mind (although there is some crossover). What all three have in common, however, is they all relate the speed of the wind, and its direction.

 

Polar Coordinates 

Decoded observations from Seattle, WA (KSEA). Notice the wind
direction and speed circled in yellow

A portion of a surface map. Circled is
 Chicago O'Hare airport (KORD) reporting 10kt
winds from about 200 degrees (from north).



When discussing wind using polar coordinates, the direction of the wind is given in degrees followed by the wind speed. North is by convention 0 degrees and that value increases clockwise around to 360 degrees. Wind speed is often displayed in knots or m/s, depending on the agency reporting the wind. This system is used for weather observation reports and on weather maps.


Cartesian Coordinates


The thin lines represent the flow of the wind, with the arrows on the right indicating the direction of the flow. The three black dots represent air parcels in the flow with the coordinate system shown as it applies at the point of the parcels. labeled are the i and j directions and the locations of north and east.

When performing calculations or running computer models, the direction and speed system is not very useful. The first alternative is the Cartesian coordinate system. This is essentially the same system used since grade school using an "x" and "y" plane. In contexts where objects in motion are concerned, these directions are often referred to as the "i" and "j" directions, respectively. In this system, north and east are the positive directions. Since the coordinates' orientation is fixed, wind must described by its components. The component of the wind that is oriented west-east (the i direction) is the "u" or "zonal" wind, while the south-north oriented wind (the j direction) is the "v" or "meridional" wind.

 

Natural Coordinates

This diagram follows the same basic premise as the one above, except now natural coordinates a shown. Notice how the t direction is tangent to flow at the air parcel and that the n direction is normal to the flow at the air parcel. It is important to note that the axes for the natural coordinates are different for each parcel.

Another type of coordinate system that is used when calculating wind properties are natural coordinates. Unlike Cartesian coordinates which are oriented relative to fixed directions on Earth, natural coordinates have different orientation at any given point. This system can be very useful for wind calculations because its orientation is determined by the direction of air flow. One component of the system is the "t" (for "tangent") direction, this component is positive when pointing downstream, and is tangent to the air flow at the point being considered. The other component is the "n" (for "normal") direction, this points perpendicular to the flow and is positive to the left of the flow by definition. Sometimes, when calculation concern a flow that is very tightly curved, or even a closed loop, the n direction will be referred to as the "r" (for "radius") direction. One great thing about this system is if the wind speed is not changing, then calculations needed for the t direction can be neglected, as will be done in this discussion since this post is only regarding the state of the wind.


Now that the background for wind has covered, in Wind II the properties of the various types of wind will be introduced.