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Everything you ever wanted to know about weather instruments
• 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
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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
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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
Hohenpeißenberg Observatory, Germany
St James’s 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
– 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’ thermometersST
Stevenson screen vsAspirated screen1 minute averages5 October 2013
Stevenson screenAspirated screen
Response time - sampling and logging intervals
• Sampling interval – 5-15 sec ideal for air temperatures
• Logging interval – WMO recommend 60 sec running average
• Many consumer AWSs provide only ‘spot’ values
60 minutes 5
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Humidity and dew point
What are we attempting to measure?
• The amount of water vapour in the air– Warmer air can ‘hold’ more water vapour:
at 30 °C, 7 x as much as at 0 °C› Expressed in relative terms –
‘Relative Humidity’› Or, as an absolute measure –
humidity mixing ratio, g/kg› Or, as the ‘dew point temperature’› The dew point temperature is
the temperature at which the amount of water vapour in the air just equals the maximum amount that the air can holdat that temperature
› The larger the difference between dew point and the current temperature, the lower the humidity
“17 °C, 50% RH”
17 °C, SVP ~ 20 hPa
50% RH VP ~ 10 hPa
VP 10 hPa dew point 7°C
Measuring humidity and dew point
• Dry- and wet-bulb thermometers – ‘traditional’ method– The wet bulb is cooled by evaporation– Liquid-in-glass thermometers or electronic sensors– RH, dew point (etc) calculated from tables, by logger or by PC/smartphone apps– Disadvantages: water supply needed!
• Capacitative sensors – used by most AWSs
– Polymer film between electrodes, impedance varies directly with RH– Advantages: small, low power, remote logging, inexpensive, works below 0°C, no
water supply required– Disadvantages: slow response near saturation, calibration drift
• Usually exposed in thermometer screen alongside temperature sensors– Accuracy at best typically 3% RH or 0.5 degC / 1 degF dew point
Barometric pressure
What are we attempting to measure?
• ‘Air pressure’ – force per unit area exertedby a vertical column of air
• ‘Pressure trend’ useful for short-term local forecasting
• Easiest of all elements to measure– You can do this one indoors!– But locate away from sources of heat,
or draughts, or A/C
• Most AWSs now use a small solid-state sensor– To compare readings, they must be reduced to a common or standard level,
usually Mean Sea Level, or MSL– Beware of calibration drift – check 2-3 times per year
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Household aneroid barometer
Wind speed and direction
What are we attempting to measure?
• The most variable element of all!
• Challenging for instruments, logging equipment and site/exposure
• Wind is a vector quantity – it has both direction
and speed
• Wind direction is where the wind is coming from– Measured by electrical wind vane– Referenced to True North, not Magnetic North
• Mean wind speeds refer to 10 minute means, gust speeds refer to 3 second means– Measured by cup anemometer, rotor anemometer or sonic anemometer
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Cup anemometer
Wind speed and direction
• Greater level of difficulty and complexity to obtain representative readings
• Ideal exposure is at 10 m (33 ft) above ground in open site– No obstacles within 10 x their height in any
direction– Rooftop sites are not ideal, but are often all
that is available– Planning or zoning restrictions may apply
• Most vulnerable to weather exposure
• Instruments will need occasional maintenance – ensure safe access SW
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Vallée de la Sionne, Canton Valais – 2696 m
Metadata
• Literally ‘data about data’
• Description of – Site (latitude, longitude, altitude) and surroundings– Instruments in use and changes over time– Where the records are kept, and what format they are in– Units in use and any changes
• Essential for any future users of the records– You may not be around to tell them!
• Document in simple structured text file, and keep updated annually
• Add photographs – and take new ones every so often
Metadata – simple example
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Radcliffe Observatory, Oxford, England
Making the most of your observations
A means to an end
• Keep your observations in spreadsheets– ‘Export’ function from AWS, ‘Import’ into spreadsheet– Better analysis and presentation facilities than AWS software– Develop a format and structure that work, then stick to it– Hourly, daily and/or monthly files– Don’t forget metadata!
• Files quickly build into useful datasets
• Try simple plots of (say) daily maximum and minimum, then try more advanced plots and analyses– Augment records with longer-period nearby sites if available
› Be careful of inhomogeneities
– Practice builds experience – go ahead, try it!
Daily maximum and minimum temperatures
Daily max and min temperatures and rainfall
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Daily maximum and minimum temperatures
More advanced analyses using Excel1. Hourly pressure means
Averages over 10 years 2001-10
More advanced analyses using Excel2. Hourly sunshine means
“I would highly recommend this comprehensive weather-observing guide to hobbyists, professionals, teachers, and college instructors. The author has done an outstanding job making the book accessible to anyone interested in observing the weather, even if they do not have a technical background. At the same time, there is plenty of useful information for those of us who have been professionally involved in observing the weather for quite some time.”
Bulletin of the American Meteorological Society, May 2013