Discussion: A home weather diary: what a garden or balcony sensor records and why it differs from the official station
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'The difference series is the siting' is too clean, and the diary as described cannot tell the three parts of the difference apart. Part of it is exposure, as the article says; part is the instrument (a consumer sensor's own error, which for temperature is often of the same order as the siting effect); and part is a real difference between two places: elevation, distance from the coast, an urban night-time surplus, a valley that pools cold air. The last part is climate, not a mistake, and it is what the diary was supposed to record. The protocol can separate at least the instrument part cheaply: put the home sensor in a ventilated shaded spot beside a second thermometer for a week before installing it, and repeat annually, so a sensor-to-sensor offset is known; and record the station's elevation and distance in the exposure description, so that a standing offset can be checked against the expected lapse-rate difference before it is called siting. Without that, a diary keeper who moves the sensor to 'fix' the difference is chasing a number that was partly correct.
Two siting and timing facts that decide whether the home-minus-station column means anything. The Met Office's daily maximum and minimum are taken over a 24-hour period running from 09:00 GMT to 09:00 GMT, so a diary that resets at midnight is comparing different days; setting the reset to the station's climatological day, and stating it, removes that. For rain, the WMO Guide to Instruments and Methods of Observation (WMO-No. 8) asks that no obstacle stand closer to a gauge than twice its height above the gauge rim, preferably four times, which is the reason a balcony gauge under-reads even when it is not under cover. Finally, part of the difference is real rather than siting: the standard atmosphere lapse rate is 6.5 K per kilometre, so a house 150 m above its reference station is expected to read about 1 K cooler on that account alone (arithmetic), and urban sites read warmer at night than rural stations.
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