Measurement of wind speed and direction

a) Anemometer:

• It is a mechanical device which can measure spontaneous wind speed-average wind speed during 24 hours.
• Anemometer consists of three or four semispherical or conical cups mounted at the end of short arms which fits into a vertical spindle.
• Vertical spindle is firmly embedded in concrete base in the ground. The number of rotations per unit of time around this spindle is measure of wind speed.
• Hence 1 rotation=2πr= x distance, n rotation=nx distance. If t is time taken in second for the n rotation so cover distance is nx meter, then speed is nx/t in m/sec. A meter attached to spindle records movement of wind.
• Anemometer does not respond to very light wind of <0.5 m/sec. Accurate measurement of wind speed is obtained at velocities higher than 2m/sec.
• For agricultural purpose the height of the cup is maintained at 2 m above ground.

b) Wind vane:

• Wind vane is a horizontal metallic vane suspended to a frictionless vertical axis to show the direction from which wind is coming.
• The vane consists of two flat plates joined together at an angle.
• The rotating arm to which the plates are fitted is mounted on the wind vane stand (pillar) by means of ball bearing and brass sleeve.
• Below the rotating arms, four to eight permanent direction arms are fixed on brass bar to assist the reading.
• The position of arrowhead indicates the direction of wind.

c) Anemograph

• Recording anemometer or anemograph is used to record instantaneous wind speed and direction.
• Usually, the anemograph records the wind speed and direction on daily or monthly chart allowing calculation of wind velocity at any given period of time.
• These are expensive instrument and are not usually used in agrometeorological station for general measurement.