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Humidity spikes

Discussion specific to Davis weather stations
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TXRweather
Posts: 9
Joined: Sun 31 Jul 2016 3:19 pm
Weather Station: Davis Vantage PRO 2
Operating System: PI 3B+ running CMX3.1.0b3191
Location: Lebanon TN.

Humidity spikes

Post by TXRweather »

This may be normal, I am not sure. I have noticed recently my humidity readings have been changing by almost 5% within a minute. Not sure if it is the RH sensor, or it is something to do with the sampling rate in CumulusMX. I have had my Davis VP2 connected to my Mac since 2016. I have an after-market usb to serial data logger, which has never had any issues with bad readings. The temp/RH sensor is housed in a fan aspirated solar shield. I did some major cleaning, and parts replacement on the unit about 2 years or so ago. New solar batteries, new anemometer sending unit, new wind direct sensor and might have changed out the temp/RH sensor, that I can't remember.

This week, I finally took the plunge and changed over from the Mac to a Raspberry PI 3B+. That was fairly easy and it was only offline for about an hour. I do live in the woods and am wondering if the relative humidity actually changes that much in 60 seconds with the winds blowing cooled air past the Davis unit. I am running Cumulus MX 3.19.0 b3191, which I believe is the latest version for Pi. I attached a 7 hour screen cap of the humidity chart for reference.
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HansR
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Weather Station: GW1100 (WS80/WH40)
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Re: Humidity spikes

Post by HansR »

I don't see an abnormal chart. When you think of an airflow as large chuncks of air which do not mix passing by the sensor you can imagine the humidity not being the same in every chunk. In turbulent airflows the effect will be less (the blocks of consistent air being smaller, mixing of air is faster). In laminair airflows - low windspeeds - the humidity will be much more consistent.

From your chart I don't see an issue.

What you see is the sum of variation in humidity and the sensor. Added to that may be noise. No doubt you will find studies on the subject.
Hans

https://meteo-wagenborgen.nl
https://meteo-wagenborgen.nl/CMX4
CMX build 3183 ● Ecowitt GW1100/WS80/WH40 ● RPi 3B+ ● Raspberry OS 5.15.32 ● Mono 6.8.0.105
CMX build 40xx ● Ecowitt GW1100/WS80/WH40 ● RPi 4B ● Raspberry OS 5.15.32 ● .NET 6.0.6
freddie
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Re: Humidity spikes

Post by freddie »

I agree with all Hans says here. In addition, in light wind situations in the summer half of the year you may see large fluctuations during the daytime - due to a process called convection in which air near the ground is warmed by the ground, becomes buoyant and rises. There is compensatory downward mixing of air from higher up, which is warmed and dried by descending. This can give large short-period humidity fluctuations.
Freddie
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HansR
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Re: Humidity spikes

Post by HansR »

Ah yes, good point. I assume that effect be more present in graslands (wetlands) than on rocky soils.
Hans

https://meteo-wagenborgen.nl
https://meteo-wagenborgen.nl/CMX4
CMX build 3183 ● Ecowitt GW1100/WS80/WH40 ● RPi 3B+ ● Raspberry OS 5.15.32 ● Mono 6.8.0.105
CMX build 40xx ● Ecowitt GW1100/WS80/WH40 ● RPi 4B ● Raspberry OS 5.15.32 ● .NET 6.0.6
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