MOVING ON FROM MERCURY:maintaining homogeneity in meteorological records

• Stephen BurtDepartment of Meteorology, University of Reading, UK

•WMO TECO Conference, Madrid, 30 September 2016

AGENDA

• The mercury toxicity issue

• Mercury in meteorology

• Synoptic and climatological observational requirements for barometers and thermometers

• Sensor substitution options and benefits/drawbacks

• Maintaining homogeneity during the transition

• Summary

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THE MERCURY ISSUE

• Neurological toxin – both vapour and compounds

• UNEP Minamata Convention on Mercury• European ban from 2014, Global ban from 2020•Guidance in WMO CIMO Commission for Instruments flyer (online)• ‘Act now to ensure network data quality’

• In meteorology, traditional working constituent of• Thermometers

• Sealed: Hg ~ 1 cm3 or ~ 14 g per thermometer

• Barometers• Vented: Hg ~ 200 cm3 or ~ 3 kg per barometer

• Vapour pressure Hg at 20 °C 0.002 hPa•Negligible inhalation risk at room temperature 3

SENSOR COMPARISON ‐ PRESSUREFeature M ercury barom eter

Cost Expensive –precision m anufacture item

Useful lifetim e > 100 years

Precision 0.1 hPa

Accuracy (at station level pressure)

0.2 hPa

Reading accuracy Typical 0.2 hPa

Stability/drift < 0.1 hPa per decade

Supplem ental readings needed

Hg tem peratureAir tem perature

Notes Very fragile –requires great care in handling and transport 4

SENSOR COMPARISON ‐ PRESSUREFeature M ercury barom eter Pressure sensor

Cost Expensive –precision m anufacture item

M ass-producedRequires logger

Useful lifetim e > 100 years > 10 years

Precision 0.1 hPa < 0.1 hPa

Accuracy (at station level pressure)

0.2 hPa < 0.1 hPa

Reading accuracy Typical 0.2 hPa Logger-dependent, < 0.1 hPa

Stability/drift < 0.1 hPa per decade < 0.1 hPa per annum

Supplem ental readings needed

Hg tem peratureAir tem perature

Air tem perature

Notes Very fragile –requires great care in handling and transport

Sm all, robust, light, low-power (radiosonde, sm artphone)

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SENSOR COMPARISON ‐ TEMPERATUREFeature M ercury therm om eter

Cost Expensive –precision m anufacture item

Useful lifetim e 10-50years

Precision 0.1 degC

Accuracy 0.2 degC

Reading accuracy 0.1-0.2 degC

Stability/drift < 0.1 degC per decade

Notes Requires physical access to read therm om eterVery fragile –requires care in handling and transport

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SENSOR COMPARISON ‐ TEMPERATUREFeature M ercury therm om eter Electricalsensor (PRT)

Cost Expensive –precision m anufacture item

M ass-producedRequires logger

Useful lifetim e 10-50years > 10 years

Precision 0.1 degC < 0.1 degC

Accuracy 0.2 degC < 0.1 degC

Reading accuracy 0.1-0.2 degC Logger-dependent, < 0.1 degC

Stability/drift < 0.1 degC per decade Requires regular calibration checks

Notes Requires physical access to read therm om eterVery fragile –requires care in handling and transport

Sm aller –physical access not required ( sm aller screens)Not restricted to> -40°C 7

REPLACING MERCURY THERMOMETERS …

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?AWS + logger

Stevenson screen, manual instruments

Different exposure- tim e constants

and response tim e- ventilation, therm al inertia- calibration- HOMOGENEITY RISK

© Stephen Burt

© Stephen Burt

… AND AUTOMATING SITES

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Kew Gardens, west London

© Stephen Burt

SUGGESTED GUIDELINESfor mercury‐replacement climatological equipment

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ESSENTIAL• Conformance WMO CIMO spec• Homogeneity with existing record• 5/6 day week capability• Engage observer philosophy ‐‘display and record’ unit design

• Robust and weatherproof unit (including display)

• Field‐swappable units and sensors• Field‐calibratable units and sensors• Low power operation• Solar cell/battery power supply• Lifetime minimum 10 years

SUGGESTED GUIDELINESfor mercury‐replacement climatological equipment

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ESSENTIAL• Conformance with WMO spec• Homogeneity with existing record• 5/6 day week capability• Engage observer philosophy ‐‘display and record’ unit design

• Robust and weatherproof unit (including display)

• Field‐swappable units and sensors• Field‐calibratable units and sensors• Low power operation• Solar cell/battery power supply• Lifetime minimum 10 years

DESIRABLE• Emulate existing obs routine as far as possible

• Minimise observer retraining• Cost comparable to four mercury thermometers

• Retrofit existing screens (minimise exposure change, reduce costs)

• Not locally reconfigurable (for NMS rollouts)

• Capable of sensor expansion• Optional sub‐daily logging capability

System software

andadministrator configuration

setup

SYSTEM DESIGN

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2 x PRT

500 m mflyingleads

12 v supply

Display unit and user interface

17:06:00 UTCDry 22.8 °CWet 17.7 °C o Metadata

o Time settingo Unitso Logging 

yes/no, interval

o Terminal hours and extremes period(s)

o Calibration adjust/offsets

o etc

Optionalexpansion

Soil/earth tempsSurface temps

TBRsunshine, etc

Sensorinterface

USER INTERFACE

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Date/time/battery state

Current time and temperature(s)

Extremes since last reset, with times

Derived values e.g. RH, dew point

Extremes day‐1 ‐ date

Extremes day‐2 ‐ date

Extremes day‐3 ‐ date

etc etc

Buttonpush

Auto reset at pre‐configured terminal hour (e.g. 0900 UTC)

Autostore all extremes for display, to day‐7

Extremes cannot be accidentally reset

Reverts to sleep mode after 2 min to conserve power

ACHIEVING AIR TEMPERATURE HOMOGENEITY

• Retain existing screen where practicable to do so

• Retrofit replacement sensors and logger into existing screen

• Train observer(s) in operation new equipment

• Replicate existing observational routine as far as possible•Manual entries of temperature records into observation register

• Check calibration, homogeneity and consistency in overlap period•Overlap period ideally 10% of existing record, but minimum 6 months• Carefully analyse results, test for homogeneity

• Remove mercury thermometers only after successful overlap14

PROTOTYPE TRIALS

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University of Reading Atmospheric Observatory51.441°N, 0.938°W, altitude 66 m above MSL

The University of Reading has offered the facilities of its atmospheric observatory to the UK Met Office for evaluation trials of ‘mercury replacement’ systems in the interests of meteorological science; there is no commercial interest or favour in hosting prototype trial units.

© Stephen Burt

PROTOTYPE TRIAL UNITFairmount Weather Systems Intellisense

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Copies of overlap comparison data available on request©

Stephen Burt

SUMMARY

• Legislation is forcing withdrawal of mercury‐based instruments – the deadline is October 2017

• Barometric pressure sensors already offer accurate, stable, robust and low‐cost alternatives to mercury barometers

• Resistance‐based temperature sensors also offer viable alternatives to mercury thermometers, but with a greater homogeneity risk to existing temperature records

• Recommendations –• Retain existing thermometer screen and retro‐fit sensor/logger combination• Emulate existing observational routine and actively engage observer(s)• Carefully overlap new and existing methods and compare

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THANK YOU

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s.d.burt@ reading.ac.uk