About wind speed, air
pressure and rain
Measurements
Wind speed is measured with a Vector A101M Pulse Output Anemometer
(pictured below). It consists of a wind-driven rotor that creates
an electric pulse every complete revolution. The data-logger counts
the pulses and calculates wind speed every 20 seconds. The highest
of these is the maximum gust, and the average of all of them over
five minutes is the average speed. Wind direction is measured by
a Vector W200P/L windvane. The position of the vane determines the
resistance across a potentiometer. The data-logger calculates the
average wind bearing based on these resistance readings.
Air pressure is measured with a Vaisala PTA 427 Barometric Pressure
Transducer. It utilises the varying capacitance of a sliver of silicon
crystal caused by it flexing due to pressure fluctuations.
Interpretation - Wind
Wind speed is given in kilometres per hour. Interpretation of
wind speed is conventionally done using the Beaufort scale, which
equates ranges of wind speed with observable effects.
The scale on the chart goes up to 120 km/hr, which equates to
a Beaufort force number of 11 (a violent storm). Note that the weather
station is considerably elevated, meaning it is likely to get marginally
stronger winds than at ground level. Check out the Beaufort
scale.
Wind direction is given in degrees of arc, as with a normal compass.
The bearing indicated is the direction the wind is coming from.
Note that because the scale is circular and 0° and 360°
both equate to north, points are likely to occur at the top and
bottom of the chart when the wind is coming from approximately that
direction. Also note that when the conditions are quite still, wind
direction points will be scattered randomly as there will be no
strong directional influence on the windvane.
Interpretation - Pressure
Mean sea level atmospheric pressure is 1013 hPa. Pressure readings
below this indicate a low pressure air-mass is in the region, and
readings above indicate a high pressure air-mass. Low pressure is
associated with wet, cold weather and high pressure with fine, warm
weather. Rapid change in the pressure reading, along with rapid
changes in temperature and wind direction indicates a frontal system
is moving in. Note that pressure also decreases with increasing
altitude, and elevated as it is, the pressure transducer will read
approximately 5 hPa lower than it would at sea-level.
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