Gary Archibald, Weathercaster

This is the first blog installment of several to come – the topic: weather devices. Let’s go with it shall we? And first up…let’s examine the anemometer (it’s a word that defies proper pronunciation – at least for me…). But is nonetheless a useless weather-measuring tool. Let’s take a look.

An anemometer is a device for measuring the velocity or the pressure of the wind, and is one instrument used in a weather station. The term is derived from the Greek word, anemos, meaning wind. Anemometers can be divided into two classes: those that measure the velocity of the wind, and those that measure the pressure of the wind, but as there is a close connection between the pressure and the velocity and a suitable anemometer of either class will give information about both these quantities.

There are just a few different types of anemometers.

Cup anemometers
The simplest type of anemometer is the cup-anemometer, invented (1846) by Dr. John Thomas Romney Robinson, of Armagh Observatory. It consists of three or four hemispherical cups, mounted one on each end of a horizontal arm, which lie at equal angles to each other.

A hemispherical cup anemometer of the type invented in 1846 by John Thomas Romney Robinson

A vertical axis round which the cups turn passes through the centre of the arms; flow of air past the cups in any horizontal direction turns the cups in a manner that is proportional to the wind speed. Therefore counting the turns of the cups over a set time period gives the average wind speed for a wide range of speeds.

Windmill anemometers
The other forms of mechanical velocity anemometer may be described as belonging to the windmill type. In the Robinson anemometer the axis of rotation is vertical, but with this subdivision the axis of rotation must be parallel to the direction of the wind and therefore horizontal. Furthermore, since the wind varies in direction and the axis has to follow its changes, a wind vane or some other contrivance to fulfill the same purpose must be employed.

A windmill style of anemometer

An aerovane combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument. In cases where the direction of the air motion is always the same, as in the ventilating shafts of mines and buildings for instance, wind vanes, known as air meters are employed, and give most satisfactory results.

Sonic anemometers
Sonic anemometers, first developed in the 1970s, use ultrasonic sound waves to measure wind speed and direction. They are capable of measuring wind velocity in all directions. The spatial resolution is given by the path length between transducers, which is typically 10 to 20 cm (approx. 5-10”).

3D ultrasonic anemometer

Sonic anemometers can take measurements with very fine temporal resolution, 20 Hz or better, which make them well suited for turbulence measurements.

Wind speed and direction – they are important data points that we in the weather business need to know to better understand active weather developments and effects. Anemometers are as you have just seen, an integral part of data essential weather data-collecting apparatus… next up, in a future blog…I’ll look into BAROMETERS.

Cheers!

GA