Wind measurements Wind shear Vorticity Pilot weather balloons
Today we continue to talk about issues and concepts that pertain to the wind. We will first look at methods utilized by meteorologists to take wind measurements. Then we will discuss the concept of wind shear and vorticity, each of which play a very important role in the development of many different types of weather.
Wind measurements
Meteorologists employ many different instruments to take wind measurements. The most common method of taking wind speed measurements is via an anemometer. An anemometer is an instrument with 3 cups fastened to a ball-bearing apparatus. As win d enters the cups, it causes the unit to rotate. The instrument is equipped with a gauge that can calibrate the wind speed according to the rate of rotation of the cups. A cup anemometer is pictured below.
Anemometers should be strategically placed in a location that allows them full exposure to all wind directions. Anemometers are typically placed atop the highest buildings in the immediate area away from trees and other obstructions to the wind.
The counterpart instrument to the anemometer is the wind vane which measures wind direction. Wind vanes are weighted in such a way that they swing and point in the direction the wind is coming from. Below is a picture of a wind vane.
An aerovane is an instrument that combines features of both the wind vane and the anemometer. It is weighted like a wind vane such that it swings and points into the direction of the wind, and it has a propeller-like feature at its nose that ca librates the wind speed as an anemometer does.
An aerovane, in addition to another wind vane and anemometer, are pictured below.
There are other instruments besides aerovanes, wind vanes, and anemometers that measure the wind speed and direction. A more primitive instrument, called the wind sock, is pictured below.
Wind socks are often fixated to poles that allow them to swing into the direction of the wind. The higher the wind sock lifts off the ground, the stronger the wind. The speed of the wind is often calibrated by approximating the angle that the wind sock makes with the ground.
Satellite pictures can also help scientists determine the speed of the wind. If two satellite pictures are taken a known time interval apart, a cloud element can be analyzed and its displacement in that time period can be determined thus the velocity of the cloud and the wind it is being carried along by can be determined.
Scientists have developed other modern techniques for determining wind speed and direction. For instance, a highly sensitive heat measuring instrument can take a profile of the motions of the upper atmosphere. These instruments are called lidars.
Wind Shear
A wind shear refers to a wind speed or direction gradient. As we shall see, wind shear plays a significant role in the weather conditions.
There are basically two types of wind shear: Directional Shear and Velocity Shear. Below are examples of each.
Wind shear in the horizontal and vertical direction can both significantly contribute to lifting or subsidence (i.e., either upward or downward vertical motion). This is because wind shear can initiate atmospheric rotation which is referred to as vorticity.
Vorticity
Vorticity is the measure of the atmosphere's rotation at a given point. Shear (both directional and velocity) is just one of the ways that vorticity can be initiated. The best way to imagine how vorticity can be quantified is to consider the diagram below:
In the pinwheel example above, suppose you could determine how many radians the pinwheel would rotate every second. That measure would be the value of the vorticity. Counterclockwise rotations correspond with positive vorticities whereas clock wise rotations correspond with negative vorticities.
Vorticity and vertical motion
It is not the magnitude of vorticity that determines if there will likely be vertical motion, but it is the rate of change of vorticity at a given location that is important. To help understand this, consider a cylindrical parcel of air of a given volume. We will consider that our volume must remain constant but the shape of our cylinder is allowed to change. Now picture what happens when a figure skater pulls his or her arms closer to his or her body. Their rate of rotation increases signif icantly. This is due to a principle called the Conservation of Angular Momentum. In the atmosphere, when the rate of rotation of our cylindrical parcel increases, it will "pull its arms in". Since we said our volume is going to remain constant, the height of the cylinder must increase for the diameter to decrease. The top of the cylinder will be rising so the air at the top will be traveling upward. This is an example of how an increase in vorticity would lead to vertical motion.
The opposite is true as well. If the vorticity of our cylinder were to decrease, the diameter would increase and the height would fall thus corresponding to subsidence (sinking motion). In this way, changes in vorticity actually bring about either convergence or divergence.
Pilot Weather Balloons
A Pilot Balloon also called a pibal,is a balloon used to measure wind shear. Much smaller than its cousin the radiosonde, pibals are spherical and resemble large beach balls.
Pibals are filled with helium and are released into the atmosphere much the same way radiosondes are. Once released, a meteorologist on the ground tracks the balloon with a telescopic device called a theodolite. The theodolite has a lens that allows for easy tracking of the balloon and angular dials so that the angle of azimuth and elevation of the balloon can be tracked at regular time intervals. The rate of ascent of the balloon is predetermined before launch time and will depend on the cur rent atmospheric conditions. Since the rate of ascent is known, the distance away from the station can be determined at any given time and thus the wind speed and direction. Thus a profile of wind speed and direction can be determined vertically in the atmosphere. This is how vertical wind shear is determined.
The information from the pibal is plotted on radial graph paper. How the wind direction turns with height is a measure of the holicity of the atmosphere.
Holicity plays a major role in the development of thunderstorms and tornadoes.