December in Reigate 2013 summary stats:
A quick summary of an historically stormy and wet December in the SE.
Average temp 6c
Tmax 11.4c 16-17Dec
Tmin -2c 5 Dec
Max wind gust 39mph 27 Dec
Total rainfall 110mm of which 47mm fell 23-24 Dec (or 70mm from manual rain gauge)*
21 days with rain
For the RGS weather station there were some interesting developments regarding rainfall measurements which are worth sharing. During the early storms in December, when wind speeds gust 35mph in Reigate, the VP2 rain gauge and ISS (integrated sensor suite), which houses all the thermometer instruments and the tipping bucket rain gauge, was blown over! The standard tripod had been pegged down but the wind had pulled out the pegs on one side and rocked it over. This was noticed remotely almost straight away by the lack of rainfall rate turning up whilst it was clearly raining outside! The unit was rescued within an hour, checked thoroughly and secured with guys with no adverse effects.
This prompted a comparison of rainfall between our VP2 and our standard rain gauge and some comparison checks with other neighbouring weather stations (including Kenley and Charlwood, two Metoffice “official” stations). Through studying the rainfall totals of several subsequent storms, It appears that the VP2 rain gauge is sometimes recording significantly less in comparison to our standard rain gauge, which is located a few feet away at ground level. The most extreme example of this disparity was the 23-24 Dec storm when 70mm was recorded on the standard rain gauge at ground level, while the VP2 recorded 47mm. This initially looks like the VP2 must be incorrectly calibrated because the standard rain gauge (collecting an “absolute” total rainfall amount in a graduated collecting jar rather than a 0.2 mm resolution of counted tipped buckets on the VP2) must be the more accurate… but hang-on! Our VP2 measurements match other local weather station and the rainfall rate correlates well with other local stations such as Kenley, Smallfield and Charlwood on a daily record resolution. Such comparisons would never be exactly the same of course because rainfall varies a lot even across a small local area (see pics below), so relying on comparisons with other stations would certainly be incorrect in establishing the accuracy of our own figures. Nevertheless, it does at least appear we are all in agreement to a close degree regarding rain-rate trends as known storms progress across our region.
So rainfall is notoriously tricky to measure and all sorts of differences can arise between neighbouring gauges due to tiny differences including height above ground. Height of rainfall instrumentation above the ground is especially important in turbulent gusty weather when rain can appear from all angles and get swept in all directions. The VP2 rain collector is set at the standard thermometer height of 1.2m as it is attached as a single unit. This means it might have been more susceptible to collecting less rainfall during the very stormy December weather due to being more exposed to the wind at over 1m. In less gusty conditions the two gauges might agree far more, so a re-calibration at this stage would make our measurements of our more commonly occurring vertically descending rainfall less accurate!
Rainfall comparisons are on-going but evidence remains insufficient as yet to attempt a rushed re-calibration of the VP2 rain gauge immediately as it appears to be working well or, indeed, to go through any awkward resetting of the rain gauge to a location at ground level. The conclusion is that …
- Our rainfall measurements and trends in rainfall rate match our sister station in Reigate and rainfall rate follows the nearest official weather station at Kenley precisely enough to show broadly accurate rainfall measurements.
- The VP2 rain gauge was calibrated last year and currently measures rainfall at 1.2m above the ground.
- As more automatic rain gauges become the norm the height issue will be standardized.
- Our standard manual rain gauge measures rainfall at ground level.
- The two measure rainfall correctly but illustrate the vagaries of data recording and the caution we must use when quoting and comparing figures about the weather from different weather stations, even ones that are close together.
- We will continue to use un-adjusted VP2 tipping bucket rain gauge readings for the time being but quote our standard rain gauge readings to use as comparison when appropriate and available.
- A full calibration will be conducted in due course 🙂
Please let me know if you have had similar experience of rainfall data collection.
December will, of course be remembered for some extraordinary stormy weather, but with some spectacularly crystal sunny days in between and few frosts and no snow at all. Previous posts cover flooding and some individuals storm events but here is a round-up of December pics from around Reigate as they happened. Comparisons with last December will be analysed with the 2013 summary in due course.