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  • Everything you ever wanted to know about weather instruments

    Stephen Burt FRMetS

    CoCoRaHS webinar, 17 October 2013

    This presentation is Copyright Stephen Burt 2013. All rights reserved. Reproduction and distribution is permitted for non-commercial purposes only provided the material is reproduced in its original format. Photographic copyrights remain with the original photographer as shown. www.measuringtheweather.com

    CoCoRaHS

  • Your presenter: Stephen Burt Im 55, married with two grown-up daughters,

    and I live in central southern England, about 50 miles west of London

    Ive kept my own weather observations for 42 years, initially with basic instruments, almost fully computerised last 20+ years

    Im Chairman of the largest UK group of amateur observers, the Climatological Observers Linkwww.colweather.org.uk

    Im a fellow of the Royal Meteorological Society and a Member of the American Meteorological Society and the Irish Meteorological Society

    My early working years were with the UK Met Office, then 25 years as a marketing director in the computer industry

    In 2012 I published my third book, The Weather Observers Handbook (Cambridge University Press)

    Im currently completing a Masters degree (MSc) in meteorology at the University of Reading, UK

    http://www.colweather.org.uk/

  • Topics Basic principles

    Why measure the weather?

    Instrument siting and exposure

    Measuring precipitation

    Measuring air temperature

    Measuring humidity and dew point

    Measuring barometric pressure

    Measuring wind speed and direction

    Keeping metadata

    Making the most of your observations

  • Why measure the weather?

    A global habit

    Many different reasons Input to weather and climate forecasting models Aviation and transport needs Climatology and climate change Statutory records Hobby/interest Education 8 to 80 And many more!

    Well-kept weather records by organisations and individuals alike contribute to scientific evaluation of all types of weather and climate phenomena, on scales from seconds to millennia

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    AWS on Mt Everest, at 8000 m

  • Audience survey - 1

    Q1. Do you make instrumental weather observations yourself currently, or have done so within the last year or two?

    Yes 94% No 6% No, but made weather observations some years ago

    If yes to Q1, do you make these weather observations - as part of your job? 1% for your own interest or hobby purposes? 91% both? 8%

    If yes to Q1, how long have you made weather observations yourself? less than a year? 24% More than 1 year? 76%

  • Audience survey - 2If yes to Q1, do you make instrumental weather observations using -

    fully manual instruments (such as a thermometer, raingauge)? 51% fully automatic instruments automatic weather station? 6% a mix of both methods? 43%

    If yes to Q1, how do you keep your records? Manually (manuscript, in a logbook or similar) 23% Mostly or completely on computer (spreadsheet or similar) 44% A mix of both 33%

    Q2. Which weather elements are of most interest to you? Precipitation 56% Air temperature 10% Humidity and dew point 3% Barometric pressure 8% Wind speed and direction 23%

  • Site and exposure ... the basics

    Site the area or enclosure where the instruments are exposed

    Exposure the manner in which the sensor or sensor housing is exposed to the weather it is measuring Representative and comparable

    Budget instruments correctly exposed on a good site will give better results than poorly-located expensive instruments

    Preferable characteristics Avoid

    Open and well-exposed - well away fromtrees, hedges, buildings and other obstructions

    Sheltered locations

    Ground-level, on flat ground On sloping ground or in hollowsRooftop sites (except wind, sunshine)

    Above short grass Artificial surfaces concrete, tarmac etc

    Safe and secure access Insecure or unsafe locations

  • Precipitation

    What are we attempting to measure? Rain, drizzle, snow, rain/snow mixed, hail also dew, frost or fog

    Highly variable in space and time

    Very sensitive to exposure especially wind effects Obstructions minimum distance

    2 x their height away But very open sites may need

    some shielding, especially in snowfall

    Many different types of gauge International and climatic variations Differing standards worldwide

    Credit: World Meteorological Organization, Geneva

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    Raingauge intercomparison at Vigna di Valle, Italy

  • Measuring precipitation: daily-read gauges

    National standards vary Rim height US 3-4 feet (90-120 cm), UK/Ireland 1 foot

    (30 cm)

    Round, deep funnel to minimise turbulence and outsplash

    Calibrated measuring cylinder resolution 0.1 mm or 0.01 in

    Capacity for at least! 100 year 24 h event Minimum capacity US 500 mm / 20 in Consider siting of gauge will it flood?

    Time of reading usually morning, 7-9 a.m. Local Time Essential for comparability

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  • Measuring snowfall

    Snow depth Graduated stick held vertically Average several readings Relationship snow depth:water equivalent

    very variable, average 10-12 : 1, varies 5:1 to 20:1

    Precipitation measurements Standard rain gauges prone to wind errors -

    up to 80 per cent Wind shields can help Recording gauges usually useless

    in snowfall except vibrating wire types

    Nipher and Alter wind shieldsST

    EP

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  • Measuring precipitation: recording gauges

    Used for timing and intensity only Adjacent standard raingauge should always be used

    for reference total

    Main types Tipping-bucket Vibrating wire

    Can be remotely logged or telemetered Resolution 0.1 mm or 0.01 in 1 mm / 0.04 in too coarse miss small events

    IAN

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    AYS

    Tipping bucket raingauge

  • Air temperature

    What are we attempting to measure?

    True air temperature: need protection from Solar radiation (direct or reflected sunshine) Terrestrial radiation (from Earth and Sky) Precipitation BUT: need unobstructed ventilation !

    Sensor exposure critical for representative value Not unduly influenced by exposure or siting Exposure errors are much greater than calibration errors

    Standard housings and exposure essential for comparability Open, level site with minimum obstructions/shelter from trees or buildings Typical sensor height 1.25 2 m (4-5 ft) above short grass, away from tarmac etc

  • Measuring air temperature: thermometer screens

    Basic principles Shelter from radiation (solar and terrestrial) and precipitation Even temperature environment Respond quickly to changes in air temperature (small mass, optimum ventilation)

    Response time may be much greater than sensor response time

    Thermometer screens Traditional louvred screens usually white-painted woodwork, steel stand

    such as Stevenson Screen, Cotton Region Shelter Designed for liquid in glass thermometers

    Automatic Weather Station (AWS) screens Smaller, multi-plate plastic units, usually white highly variable quality Designed for smaller electronic sensors - faster response time

    Aspirated screens Constant airflow over sensor

    Almost any screen is better than a bare sensor

  • Thermometer screens traditional louvred

    PICTURE CREDITS: TOP ROW, L TO R: STEPHEN BURT; NOAA ARCHIVES; GRANT GOODGE

    BOTTOM ROW: STEPHEN BURT; STEPHEN BURT; TAMSIN GRAY, BRITISH ANTARCTIC SURVEY; STEFAN GILGE, DEUTSCHER WETTERDIENST

    Hohenpeienberg Observatory, Germany

    St Jamess Park, London Asheville, NC

    Rothera Base,AntarcticaCannes, France Agoium, Morocco

    Granger, Utah

  • Thermometer screens modern/AWS

    Mt Everest 8000 m Fort Collins, Colorado Berkshire, England

    Birmingham University, England Fort Collins, Colorado

    PICTURE CREDITS: TOP ROW, L TO R: Ev-2K-CNR COMMITTEE; GRANT GOODGE; STEPHEN BURT

    BOTTOM ROW: STEPHEN BURT; STEPHEN BURT; GRANT GOODGE

  • Thermometer screens aspirated

    Principle: forced advection of ambient air over sensor Improves both conduction

    and response Minimises radiation

    and wetting errors Closest to true

    air temperature (probably!)

    Used in US Climate Reference Network, USCRNwww.ncdc.noaa.gov/crn/

    BUT non-homogeneous with existing louvred screens

    Fan

    Sensor

    Rain shield Concentricair intakes

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    RM Young aspirated screen

    http://www.ncdc.noaa.gov/crn/

  • Temperature sensors Traditional liquid-in-glass

    Current temperature, dry- and wet-bulb, maximum and minimum

    Continuity stable but calibration must be checked Expensive, fragile, bulky require louvred screen Must be manually read (and reset as necessary)

    Electronic sensors Resistance temperature devices (RTDs)

    Thermistor cheaper, less accurate Platinum Resistance Thermometer (PRT)

    more accurate and repeatable, easy to calibrate

    One sensor for current and extreme temperatures Smaller fit in AWS screen, faster response Physically and electrically robust - but calibration must be checked Capable of automatic logging, and so steadily replacing glass thermometers

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