Everything you ever wanted to know about weather instruments ... but were afraid to ask

40 Pages·2013·2.69 MB·English

by  Stephen Burt

Everything you ever
wanted to know about
weather instruments
Stephen Burt FRMetS
CoCoRaHS webinar, 17 October 2013
CoCoRaHS
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
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 Link
www.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
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
E
E
T
T
– Hobby/interest MI
M
O
– Education – 8 to 80 C
R
N
C
– And many more! -
K
2
-
v
E
AWS on Mt Everest, at 8000 m
• 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
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 - 2
If 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’
Preferable characteristics Avoid
Open and well-exposed - well away from Sheltered locations
trees, hedges, buildings and other
obstructions
Ground-level, on flat ground On sloping ground or in hollows
Rooftop sites (except wind, sunshine)
Above short grass Artificial surfaces – concrete, tarmac etc
Safe and secure access Insecure or unsafe locations
• Budget instruments correctly exposed on a good site will give better
results than poorly-located expensive instruments
Precipitation
What are we attempting to measure?
• Rain, drizzle, snow, rain/snow mixed, hail – also dew, frost or fog
Credit: World Meteorological Organization, Geneva
• Highly variable in space and time
• Very sensitive to exposure –
especially wind effects
– Obstructions minimum distance
N
2 x their height away O
TI
A
Z
NI
– But – very open sites may need A
G
R
O
some shielding, especially in snowfall L
A
C
GI
O
• Many different types of gauge OL
R
O
E
T
E
– International and climatic variations M
D
L
R
O
– Differing standards worldwide W
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
S
– Consider siting of gauge – will it flood? H
a
R
o
C
o
C
• Time of reading – usually morning, S,
E
G
E
R
7-9 a.m. Local Time Y
R
N
E
H
– Essential for comparability
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 RT
U
B
N
E
– Standard rain gauges prone to wind errors - H
P
E
T
up to 80 per cent S
– Wind shields can help
– Recording gauges usually useless
in snowfall – except vibrating wire types
Nipher and Alter wind shields