What is the WMO Requirement for Data Reporting Time Interval of Weather Stations?

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QUESTION: What is the WMO requirement for weather station data reporting time-interval?

ANSWER: MeteoHelix IoT weather stations along with MeteoWind IoT and MeteoRain IoT rain gauges have been designed to comply with WMO and NWS data reporting standards as outlined in the WMO Guide To Meteorological Instruments And Methods Of Observation (The Cimo Guide), part Measurements of automatic weather stations as listed on page 540.

Since sensors of different types and manufacture may have varying response times, the averaging of measurements over a time interval greater than the longest sensor response time is desirable to enable direct comparison of meteorological data from various makes and construction of weather stations with the goal of reducing the uncertainty of reported data.

WMO Cimo Guide states that in order to standardize averaging algorithms it is recommended [1]:

(a) That atmospheric pressure, air temperature, air humidity, sea-surface temperature, visibility, among others, be reported as 1 to 10 minute averages, which are obtained after linearization of the sensor output. Termed INSTANTANEOUS values of meteorological variables.

(b) That wind, except wind gusts, be reported as 2 or 10 minute averages, which are obtained after linearization of the sensor output. Termed INSTANTANEOUS values of meteorological variables.

Meteorological Terminology

  • Raw instantaneous sensor samples which are averaged to produce 1 to 10 min averages are called RAW INSTANTANEOUS SAMPLES.

  • 1 to 10 minute averaged values as reported by automatic weather stations are termed INSTANTANEOUS values of meteorological variables.

  • Averages over longer time periods greater than 10 minutes are termed MEAN VALUES in meteorology.

WARNING: Errors from reporting in 1 minute time intervals

Due to the misreporting of sensor time constants by many major meteorological sensor manufacturers (or omitting response time and tau τ62.3% time constants from data sheets), the reporting of 1 min averages as mentioned in the CIMO guide is NOT RECOMMENDED. In testing, we have found that time constants of many combined temperature/humidity sensors significantly exceed 2 minutes (120 s) in 1 m/s (2.2 mph or 3.6 kph) air flow. This translates to a greater than 10 minute sensor response time at this air speed. Thus every minute, the sensor has only reached 38.8% of the change in temperature. At a brisk 4 m/s (9 mph or 14.4 kph) the time constant decreases by half to 1 min (60 s) and sensor response time reduces to 5 min which far exceeds the WMO recommended 1 minute reporting time interval.

Additionally, the WMO recommended temperature sensor time constant of 20 seconds (page 65 of The CIMO guide: 2.1.3.3 Response times of thermometers) conflicts with the 1 minute reporting time interval simply due to the fact that by definition, if temperature sensor time constant is 20 seconds, its response time is five time longer, 100 seconds (20*5), which is 67% longer than the 1 minute reporting time interval. One minute equals three time constants, which means the sensor reading has only reached 95% of the change in temperature.

It is therefore easy to see that reporting air temperatures in 1 minute time intervals does not make sense when most sensor response times are greater than 1 minute. See the following article for calculation details: How does an air temperature sensor response time vary with wind speed?

  • “One-minute averages, as far as applicable, are suggested for most variables as suitable instantaneous values. Exceptions are wind (see (b) above) and wave measurements (10 or 20 min averages). Considering the discrepancy of observations between the peak gust data obtained from wind measuring systems with different time responses, it is recommended that the filtering characteristics of a wind measuring chain should be such that the reported peak gust should represent a 3 s average. The highest 3 s average should be reported. In practice, this entails sampling the sensor output and calculating the 3 s running mean at least one to four times a second.”

Upcoming revisions to the WMO CIMO Guide

WMO CIMO Task Team on Classification, Task Team on Overall Measurement Uncertainties and Task Team on Surface Measurements are currently devoting to this issue and others to revise the CIMO guidelines and to quantify the uncertainty of measurement in all meteorological applications.

References

[1] Page 540 Measurements at automatic weather stations https://library.wmo.int/doc_num.php?explnum_id=3179