'Everything you ever wanted to know about weather ... · Everything you ever wanted to know about...

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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

Transcript of 'Everything you ever wanted to know about weather ... · Everything you ever wanted to know about...

Page 1: 'Everything you ever wanted to know about weather ... · Everything you ever wanted to know about ... then 25 years as a marketing director in ... "Everything you ever wanted to know

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

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Your presenter: Stephen Burt• I’m  55,  married  with  two  grown-up daughters,

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

• I’ve  kept  my  own  weather  observations  for  42 years, initially with basic instruments, almost fully computerised last 20+ years

• I’m  Chairman  of  the  largest  UK  group  of  amateur  observers, the Climatological Observers Linkwww.colweather.org.uk

• I’m  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

Observer’s  Handbook  (Cambridge University Press)

• I’m  currently  completing  a  Masters  degree  (MSc) in meteorology at the University of Reading, UK

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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

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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

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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%

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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%

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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

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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

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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

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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

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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

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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

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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

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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

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Rain shield Concentricair intakes

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RM Young aspirated screen

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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’  thermometersST

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PRTs and liquid-in-glass thermometers

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Time, UTC

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

59585756555453525150

°F

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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

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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

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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

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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

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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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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

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Metadata – simple example

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Radcliffe Observatory, Oxford, England

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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!

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Daily maximum and minimum temperatures

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Daily max and min temperatures and rainfall

mm

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Daily maximum and minimum temperatures

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More advanced analyses using Excel1. Hourly pressure means

Averages over 10 years 2001-10

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More advanced analyses using Excel2. Hourly sunshine means

Averages over 10 years 2001-10

Year (Multiple Items)

Average of Sunshine hr Column LabelsRow Labels 0000 0100 0200 0300 0400 0500 0600 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 Grand Total1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.17 0.30 0.34 0.36 0.38 0.35 0.30 0.24 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.102 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.26 0.34 0.35 0.33 0.34 0.33 0.33 0.32 0.23 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.123 0.00 0.00 0.00 0.00 0.00 0.00 0.09 0.33 0.38 0.44 0.45 0.42 0.40 0.40 0.40 0.39 0.37 0.21 0.00 0.00 0.00 0.00 0.00 0.00 0.184 0.00 0.00 0.00 0.00 0.00 0.16 0.43 0.48 0.50 0.49 0.49 0.46 0.45 0.45 0.47 0.48 0.49 0.43 0.17 0.00 0.00 0.00 0.00 0.00 0.255 0.00 0.00 0.00 0.00 0.12 0.38 0.43 0.45 0.44 0.42 0.39 0.35 0.35 0.37 0.37 0.38 0.42 0.43 0.40 0.11 0.00 0.00 0.00 0.00 0.246 0.00 0.00 0.00 0.00 0.21 0.43 0.47 0.51 0.52 0.48 0.46 0.42 0.41 0.39 0.42 0.44 0.47 0.47 0.47 0.29 0.00 0.00 0.00 0.00 0.297 0.00 0.00 0.00 0.00 0.13 0.36 0.42 0.45 0.43 0.40 0.37 0.36 0.36 0.36 0.42 0.43 0.41 0.44 0.41 0.25 0.00 0.00 0.00 0.00 0.258 0.00 0.00 0.00 0.00 0.01 0.25 0.43 0.45 0.45 0.44 0.42 0.40 0.41 0.40 0.40 0.41 0.45 0.43 0.29 0.03 0.00 0.00 0.00 0.00 0.249 0.00 0.00 0.00 0.00 0.00 0.02 0.33 0.43 0.47 0.52 0.48 0.47 0.47 0.46 0.45 0.43 0.46 0.32 0.02 0.00 0.00 0.00 0.00 0.00 0.2210 0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.28 0.38 0.42 0.40 0.40 0.41 0.40 0.40 0.38 0.25 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.1611 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.09 0.30 0.35 0.38 0.38 0.40 0.37 0.33 0.23 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.1212 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.17 0.27 0.33 0.34 0.33 0.32 0.29 0.14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.09Grand Total 0.00 0.00 0.00 0.00 0.04 0.13 0.22 0.29 0.37 0.40 0.40 0.39 0.39 0.38 0.38 0.35 0.29 0.22 0.14 0.06 0.00 0.00 0.00 0.00 0.19

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More advanced analyses using Excel2. Hourly sunshine means – conditional formatting

Averages over 10 years 2001-10

Year (Multiple Items)

Average of Sunshine hrColumn LabelsRow Labels 0000 0100 0200 0300 0400 0500 0600 0700 0800 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 ANNUALJanuary 0 0 0 0 0 0 0 0.00 0.17 0.30 0.34 0.36 0.38 0.35 0.30 0.24 0.03 0 0 0 0 0 0 0 0.10

February 0 0 0 0 0 0 0.00 0.10 0.26 0.34 0.35 0.33 0.34 0.33 0.33 0.32 0.23 0.02 0 0 0 0 0 0 0.12

March 0 0 0 0 0 0 0.09 0.33 0.38 0.44 0.45 0.42 0.40 0.40 0.40 0.39 0.37 0.21 0.00 0 0 0 0 0 0.18

April 0 0 0 0 0.00 0.16 0.43 0.48 0.50 0.49 0.49 0.46 0.45 0.45 0.47 0.48 0.49 0.43 0.17 0 0 0 0 0 0.25

May 0 0 0 0 0.12 0.38 0.43 0.45 0.44 0.42 0.39 0.35 0.35 0.37 0.37 0.38 0.42 0.43 0.40 0.11 0 0 0 0 0.24

June 0 0 0 0 0.21 0.43 0.47 0.51 0.52 0.48 0.46 0.42 0.41 0.39 0.42 0.44 0.47 0.47 0.47 0.29 0 0 0 0 0.29

July 0 0 0 0 0.13 0.36 0.42 0.45 0.43 0.40 0.37 0.36 0.36 0.36 0.42 0.43 0.41 0.44 0.41 0.25 0 0 0 0 0.25

August 0 0 0 0 0.01 0.25 0.43 0.45 0.45 0.44 0.42 0.40 0.41 0.40 0.40 0.41 0.45 0.43 0.29 0.03 0 0 0 0 0.24

September 0 0 0 0 0 0.02 0.33 0.43 0.47 0.52 0.48 0.47 0.47 0.46 0.45 0.43 0.46 0.32 0.02 0 0 0 0 0 0.22

October 0 0 0 0 0 0 0.06 0.28 0.38 0.42 0.40 0.40 0.41 0.40 0.40 0.38 0.25 0.01 0 0 0 0 0 0 0.16

November 0 0 0 0 0 0 0 0.09 0.30 0.35 0.38 0.38 0.40 0.37 0.33 0.23 0.02 0 0 0 0 0 0 0 0.12

December 0 0 0 0 0 0 0 0 0.17 0.27 0.33 0.34 0.33 0.32 0.29 0.14 0 0 0 0 0 0 0 0 0.09

ANNUAL 0 0 0 0 0.04 0.13 0.22 0.29 0.37 0.40 0.40 0.39 0.39 0.38 0.38 0.35 0.29 0.22 0.14 0.06 0 0 0 0 0.19

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More advanced analyses using Excel3. Rainfall amounts by wind direction

Total rainfall (mm) over 17 years 1994-2010

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More advanced analyses using Excel4. Rainfall intensity by wind direction

Mean rainfall intensity (mm/hr) over 17 years 1994-2010

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More advanced analyses using Excel5.  Is  it  windier  when  it’s  raining?

Mean rainfall intensity (mm/hr) over 17 years 1994-2010

Averages over 10 years 2001-10

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More advanced analyses using Excel5.  Is  it  windier  when  it’s  raining?

Averages over 10 years 2001-10

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More advanced analyses6. Wind roses

Prepared using WindRose PRO software from Enviroware

www.enviroware.com

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Topics covered• 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

• Using your data

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Additional topics• Choosing a weather station

• Buying a weather station

• Earth and grass temperatures

• Sunshine and solar radiation

• Observing hours and time standards

• Dataloggers and AWS software

• Non-instrumental weather observing

• Calibration

• Collecting and storing data

• Sharing your observations

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The  Weather  Observer’s  Handbook by Stephen Burt

Published by Cambridge University Press

www.cambridge.org/9781107662285

http://www.amazon.com/Weather-Observers-Handbook-Stephen-Burt/dp/1107662281/ref=sr_1_1?s=books&ie=UTF8&qid=1380660845&sr=1-1&keywords=weather+observers+handbook

Paperback $40 (Amazon $26), Hardback $99

ISBN: Paperback 978-1-107-66228-5, Hardback 978-1-107-02681-0456 pp., 20 chapters, 4 Appendices, Index

128 b/w illus. 2 maps 50 tables, 228 x 152 mm

“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

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