Since the wind is motion of air, it exerts a certain force. The speed of the wind can therefore, be measured in terms of the effect it produces on various objects in its path. It can either be measured by devising suitable instruments or it can be measured by using the effect it produces on natural objects in the surroundings and estimate the strength of the wind. The wind velocity is fully described by two quantities; its speed and direction. The speed may be indicated in any one of the following units:
(a) Knots (or nautical miles per hour)
(b) Metres per second
(c) Kilometres per hour
(d) Miles per hour
(e) Feet per second
1 kt = 1.863 KMPH
The direction of wind is specified relative to true North at the place of observation and refers to the direction from which the wind is blowing. It is expressed by points of compass or by degrees. For example if wind is blowing from West then the wind direction will be Westerly (Points of compass) or 270°.
In early part of 19th century, Admiral Beaufort of the Royal Navy developed the well-known scale of wind velocity that bears his name. The Beaufort system employs a series of numbers from 00 to 12. Each stands for a wind velocity range between certain limits in miles per hour.
|Sl No.||Description||Specification of land||Speed of the wind at 10 meters above the ground|
|Calm||Smoke rises vertically||< 01||< 01|
|1||Light Air||Smoke bends from the vertical|
and drifts slowly with the wind,
wind vane not affected
|01 – 03|
|03 – 05|
|2||Light Breeze||Wind felt on the face, leaves|
rustle, ordinary vane moved by
|04 – 06|
|06 – 11|
|3||Gentle Breeze||Leaves and small twigs in|
constant motion; wind extend light
|07 – 10|
|12 – 19|
|Raises dust and loose papers,|
small branches moved
|11 – 16|
|20 – 28|
|Small trees begin to sway;|
crested wavelets form on inland
|17 – 21|
|29 – 38|
|Large branches in motion|
whistling heard in telegraph wires;
umbrella used with difficulty
|22 – 27|
|39 – 49|
|7||Near Gale||Whole trees in motion;|
inconvenience felt while walking
against the wind
|28 – 33|
|50 – 60|
|8||Gale||Slight structural damage|
occurs (chimney, post, slabs
on roof removed)
|34 – 40|
|62 – 74|
|Breaks twigs of trees; gene|
rally impedes progress.
Difficulty felt in walking against
|41 – 47|
|75 – 88|
|10||Strom||Trees uprooted and|
considerable damages due to
|48 – 55|
|89 – 102|
|Widespread damage due to|
|56 – 63|
|12||Hurricane||Maximum damage due to wind||64 and|
It is made of fabric having length 3.6 m with a diameter 0.9 m. The colour is somewhere orange, and somewhere combination of two colours preferably orange and white or red and white. It should be visible from aircraft in flight or on the movement area. It should be free from the effect of disturbance caused by near by objects. It gives a clear indication of the direction of the surface wind and a general indication of the wind speed. It is just like a cone attached at one end to the top of a pole and other end opened, so that it blows like a flag to show which way the wind is blowing.
The direction of the wind is usually indicated by means of a wind vane. This is essentially a body mounted asymmetrically and free to rotate about a vertical axis. A pointer attached to it shows the direction of the wind. Wind vane, commonly used in IAF observatories, is same as the one India Met Department (IMD) uses. The special feature of the vane is the streamlined tail mounted on a long arm. At the end of the shorter arm is placed a weight to balance the long arm and the whole rod rotates about the vertical axis passing through its centre of gravity. The rotating arm is connected to a short spindle passing through the centre of the supporting base.
This design ensures a large turning movement about the axis and the vane is exceedingly sensitive even in light wind. Under this swayable system are four direction rods carrying the letter N, E, S and W. These arms are set properly to the original direction (North, East, South and West) of that particular place.
The observer must take the observation from a position vertically below the indicator to avoid the error due to parallax and estimate the wind direction nearest to the point of the compass. If the wind is variable and the vane is not steady, the average position should be noted. When the wind is dead calm the indicator will still point to some direction but the direction should be reported as “00” (calm). If the wind is too feeble to move the vane, the wind direction can be determined with the help of the drift of smoke. The direction also can be determined with the help of dust drift, swaying flag and windsock.
Anemometer is an instrument used to measure the speed of the wind. They are divided into two main types;
(a) Rotating cup anemometer.
(b) Pressure anemometer (no more in use in IAF).
Rotating Cup Anemometer
This cup anemometer normally consists of three or four cups mounted symmetrically about a vertical axis so that the diametrical plane of each cup is vertical. The cup assembly is free to rotate about an axis. As the force on the concave side of the cup due to the wind is greater than that on the convex side in a similar position, the cup wheel starts rotating. The Number of revolution of the cups in a given time is usually assumed to be proportional to the amount of wind, which passes them.
The rotation of the cups can be used to measure the speed of the wind. In one procedure an electrical contact is made after a known number of revolutions of any one cup. This may be used in conjunction with a buzzer and a stop watch or any electrical counting device.
In the second procedure the cup wheel is connected to a mechanical counting system which records the number of revolution of the cup wheel since the record was set. In the third procedure the cup wheel is linked to a small electrical generator and the instantaneous speed of rotation is determined by the electromotive force generated. The systems commonly used for wind measurement are:
(a) IMD Cyclometer Pattern Anemometer.
(b) Distant Reading Anemometer.
Distant Reading Wind Equipment (DRWE)
In a DRWE, the instantaneous value of speed and direction of wind at a particular place is indicated on a dial at a remote place. This equipment consists of:
(a) A cup anemometer generator.
(b) A wind vane for direction.
(c) A wind panel for indicating wind speed and direction.
Note: wind vane is connected to a 12 V supply source for the operation of the instrument.
The wind vane is provided with a set of two slides at the bottom of the spindle. The slides make contact at two diametrically opposite points on an electrical transmitter. The transmitter is provided with five (5) terminals marked (+), 1, 2, 3, and (-). The indicators are provided with three terminals marked 1, 2, and 3. The positive (+) and negative (-) terminals of the 12 Volts DC supply is connected to the positive and negative terminals of the transmitter. Terminals 1, 2, and 3 are connected to the corresponding terminals of the indicators.
Operation of the Instrument
(a) Put the switch of the supply unit to “ON” position.
(b) Watch the indicators in both the panels (speed and direction) for about 15 seconds and estimate the mean value about which the speed and direction are fluctuating.
Care and Maintenance
The wind vane generally does not need any maintenance during normal life. However the following are to be checked.
(a) Change the transmitter if the indicating needle fluctuates erratically.
(b) Check the proper installation of the connecting cables.
(c) Clean and lubricate sparingly wherever required.
Exposure of Wind Instrument
The standard exposure for wind instrument is 10 meter above ground level on a mast or tower away from tall buildings or trees. The distance between the anemometer and any obstruction must be 10 times the height of obstruction above the anemometer. If such a site is not available, the instrument should be installed on a building at least three meters higher than any obstacle, such that eddies caused by the obstacle both vertically and horizontally do not vitiate the readings. If the anemometer and the wind vane are installed on the same platform or mast, they should be 2 meter apart.