Get better air temperature accuracy with this new method Radiation shields are no longer the only way to protect temperature sensors against solar heating errors. A new method by METER allows the ATMOS 41 all-in-one weather station to correct temperature errors. And field tests show it fares better than its radiation-shielded counterpart. See how. Join Dr. Doug Cobos, director of environmental research at METER Group, as he shares field testing results and the science behind the ATMOS 41's breakthrough technology. In this brief 30-minute webinar, you'll find out: Why you should care about air temperature accuracy Where errors in air temperature measurement originate The first principles energy balance equation and why it matters Results of experiments comparing shielded sensor accuracy against the ATMOS 41 sampling practices How the ATMOS 41's unshielded measurement actually works

2. STOP HIDING BEHIND
A SHIELD
DR. DOUG COBOS

3. HOUSEKEEPING
ITEMS
20 minutes of presentation
10 minutes for questions
Use chat pane for Q&A
Recording/slides will be sent out following presentation

4. SPEAKER
INTRODUCTION
Stop Hiding Behind a Shield
Dr. Doug Cobos, Director of Environmental Science
METER Group, Inc. USA

5. CAN A SENSOR EXPOSED TO
SOLAR RADIATION MEASURE
AIR TEMPERATURE
ACCURATELY?

6. AIR TEMPERATURE
Ø Most commonly reported environmental
measurement
Ø Main driver of human comfort level
(operative temperature)
Ø Affects nearly all biophysical processes
Ø Key component of biological
development stages (thermal time)
Ø Accurate measurement is far more
difficult than most people realize

7. ERRORS IN AIR TEMPERATURE
MEASUREMENT
Ø Air temperature is just that – the
temperature of the atmospheric gas (air)
Ø All measurements of air temperature are
really the measurement of the temperature
of the sensor
Ø When Tsensor ≠ Tair, major errors occur

8. ENVIRONMENTAL EFFECTS ON
TAIR MEASUREMENT
§ αs= absorptivity of temperature sensor to solar
radiation (unitless ratio)
§ St = total incoming shortwave radiation (W m-2)
§ cp = specific heat of air (J mol-1 C-1)
§ k = constant describing boundary layer heat
conductance
§ u = wind speed (m s-1)
§ d = characteristic dimension of temperature
sensor (m)
Tair = Tmeasured −
αs St
cp k
u
d
⎛
⎝
⎜
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
Error term

9. HOW DO WE MAKE A GOOD AIR
TEMPERATURE
MEASUREMENT?
Ø Shade the thermometer
Ø Maximize air flow
Ø Aspirated is best, but power-hungry
Ø Louvered passive radiation shield is more common
Ø Minimize size of temperature sensor
Tair = Tmeasured −
αs St
cp k
u
d
⎛
⎝
⎜
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
Error term

10. AIR TEMPERATURE
MEASUREMENT OPTIONS
ASPIRATED SHIELDS
Apogee aspirated shield Davis aspirated shield

11. AIR TEMPERATURE
MEASUREMENT OPTIONS
PASSIVE SHIELDS

12. AIR TEMPERATURE
MEASUREMENT OPTIONS
ATMOS 41
Ø Multi-parameter, compact weather station
Ø Rain
Ø Solar radiation
Ø Wind speed and direction
Ø Air temperature
Ø Vapor pressure/RH
Ø Atmospheric pressure
Ø Lightning strikes and distance

13. AIR TEMPERATURE
MEASUREMENT OPTIONS
ATMOS 41
Ø Temperature sensor is exposed to some solar
radiation !
Ø ATMOS 41 measures solar radiation and wind
speed

14. EXPERIMENT
Ø Question: Can we use wind speed and solar radiation to correct the Tair
measurement on ATMOS 41?
14
Ø Apogee TS-100 aspirated shield chosen as Tair reference
Ø Reference Tair compared to ATMOS 41 and passive shield
Ø Air temperature data collected over a variety of
environmental conditions
Ø Optimized energy balance parameters to fit
experimental data
Ø Validated model with multiple units
Methods

15. RESULTS
MODEL OPTIMIZATION
15
(All units ∘C) ATMOS 41 uncorrected Non-aspirated ATMOS 41 corrected
Average error (bias) 0.20 0.07 -0.06
95% conf interval 0.60 0.66 0.42

16. RESULTS
MODEL VALIDATION
16
10
15
20
25
30
35
7/12 7/13 7/14 7/15 7/16 7/17
Air temperature error (C)
Time
Air temperature comparison July, 2017
ATMOS 41 #1 ATMOS 41 #2 ATMOS 41 #3 ATMOS 41 #4 ATMOS 41 #5 ATMOS 41 #6 ATMOS 41 #7 Reference

17. RESULTS
MODEL VALIDATION
17
(All units ∘C) ATMOS 41 #1 ATMOS 41 #2 ATMOS 41 #3 ATMOS 41 #4 ATMOS 41 #5 ATMOS 41 #6 ATMOS 41 #7
Bias -> 0.13 0.17 0.00 -0.03 -0.05 0.13 0.08
95% confidence interval -> 0.52 0.61 0.46 0.62 0.60 0.49 0.57
-3
-2
-1
0
1
2
3
7/12 7/13 7/14 7/15 7/16 7/17
Air temperature error (C)
Time
ATMOS 41 Air Temperature Error July, 2017
ATMOS 41 #1 ATMOS 41 #2 ATMOS 41 #3 ATMOS 41 #4 ATMOS 41 #5 ATMOS 41 #6 ATMOS 41 #7

18. CONCLUSIONS
Ø First principles energy balance air temperature correction
is effective
Ø Low bias
Ø Tight accuracy specification
Ø Correction results in air temperature measurements at
least as accurate as passive radiation shields
18