Rain Measurement under heating conditions

[marc@geissmann.org]

Hi

I want to share some experience with you, in particular regarding the rain-measurement. I am highly interested in precise rain measurement, as such I always compare with an other rain-gauge that is located just next to my weather station (serving as a reliable reference.). Being aware of possible calibration which is pretty tricky I learnt that the biggest difference b/w my davis and the reference station is when it is cold and the rain-collector is heated. I am not using the original Davis heating-system, rather using the one offered by Swisswetter.ch. It is a heating system that is activated only it temperature falls below a certain threshold.

Unter heating conditions I experienced deviation of 20-35% against the reference station. It seems obvious that one third of the snowflakes do not find their way through the funnel as they are most probably evaporated through heating conditions.

Is there anybody having similar experience or anybody who has hints how to get more precise data in that case?

Many thanks, Marc

[Jachym]

We use heated rain gauges at our institute, we are talking about stations worth >5000USD. But the heating mechanism still does not guarantee 100% accurate results. It is extremely difficult to set the right temperature at which you want to heat. If you set it too low, it will not melt enough, if too high, it will evaporate. And the problem is that this threshold temperature depends a lot on conditions. For example yesterday it was snowing here, even though it was +2°C. The reason being, the snow flakes were falling quite rapidly and it is colder higher above and they didnt manage to melt before hitting the ground. So this threshold temperature is not consistent and this is what causes the problems.

[Lespius]

Hello,
This topic is interesting, because here in Switzerland snow falls happen often in winter.
I have the standard heater from Davis and I know it is way too hot.
Marc, I have read your post and visited the website (where I have purchased my station btw). It is in German and the google translation is really too bad.
Could you summarize the advantages of those heaters ? Is it only that they switch on and off upon external temperature, or the power can increase or decrease upon temperature variation ? Are they really better, or in the end, s Joachym says, the loss of water will always happen ?
Thanks !

[marc@geissmann.org]

Hi Lespius

The main advantage is indeed that the heater acts in a switch on/off mode depending on a threshold temperature. On the heater I bought from Markus I can specify the temperature that triggers the heater on (e.g. 2 degree). There is also a hysteresis temperature that avoids to trigger heater on/off when temperatures move around the threshold temperature. Of course, it is a great advantage that heater is off during warmer days. Together with the heating system there is also a insulating foil delivered that you put inside the rain cone that should disseminate the heat equally around the cone. So far so good.
Nonetheless my experience is that I have significant evaporation loss during snow fall, a bit less when there is intensive wet snowfall, but more when is snowfall is light and dry. I am comparing the rain sums to a reference station (in terms of rain) that is located just next to my weather station (hellmann rain cone). With regard to the Hellmann, when I smoothly melt the snow after snowfall (I take then the cone inside where temperature is around 22 degree) and compare the results I see this differences that can amount up to 80% compared to my Davis rain sums which is huge. Sometimes the difference is much lower. But even when it rains (and heater is on b/c it is around or slightly below 2 degrees) then I have some losses around 10% that I haven't when the heater is off. Having said that I am still looking for an option that brings down my evaporation losses.
As such, Jachym is right in saying that even very professional and expensive rain measurement systems suffer evaporation losses. This is - I think - also true for Meteosuisse, they deploy Lambrecht having a 3-circle-heating system. However the evaporation losses is - I guess - brought down to a minimum.
Hope that helps.
Marc

[Lespius]

Thank you Marc for this long answer !
Interesting problem. Evaporation of course, and also snow flakes that never fall down into the cone because of wind turbulence. All that make the measurement indeed well below the truth !
Back to the heating, I think that having already the Davis heater, the extra cost of the system sold by SwissWetter is not really worth it. Not sure it will make a big difference.
Have fun with all the snow these days !!!! Btw, how many cm ?? Here in the Canton de Vaud, max 11 cm on Sunday.
Alain