Frost melting on rain guage and creating false rain reading?

Solorize · **Posted:** Tue Jan 17, 2012 1:35 pm

I have done a quick search on the forum but can not seem to find anything on the following:

I am not 100% sure as I was at work when my rain guage recorded rain fall today. But I am
pretty sure that we have had no rainfall today so far.

When I got us this morning there was a lot of frost about and it got me thinking that
if the rain guage has a lot of frost on it and then later in the day the temp got up
high enough for it to melt, it could then trickle down into the rain gauge and be
recorded as rainfall.

Therefore I wonder if other people have had this happen? and if so is there something
I could do / modify, to prevent this from happening in the future?

mcrossley · **Posted:** Tue Jan 17, 2012 1:38 pm

I just live with it - normally I only get a single tip (like today). Some argue it's all 'precipitation' anyway, same as a heavy dew.

steve · **Posted:** Tue Jan 17, 2012 1:49 pm

The Met Office Observer's Handbook says that it should count towards the rain total, and if it's known for certain that it didn't actually rain, a note should be made in the log for that day to say where the precipitation came from.

Solorize · **Posted:** Tue Jan 17, 2012 1:59 pm

Thank you both for your replies.

I think I will take your advice Steve and make a note in my log.

Cheers.

RayProudfoot · **Posted:** Tue Jan 17, 2012 4:46 pm

steve wrote:

The Met Office Observer's Handbook says that it should count towards the rain total, and if it's known for certain that it didn't actually rain, a note should be made in the log for that day to say where the precipitation came from.

I set the RainDayThreshold to 0.02 (inches) so melted frost (usually 0.01") doesn't count as a rain day but is added to the rainfall total. I've had two incidents of that in the last week.

I think that's the reason Steve added that option in Cumulus.ini.

steve · **Posted:** Tue Jan 17, 2012 4:51 pm

Yes, the correct way to handle it (according to the UKMO book) is to include it in the total but not count it as a 'rain day'. But there's no good way to handle it automatically; a single tip due to real rain should count as a rain day.

RayProudfoot · **Posted:** Tue Jan 17, 2012 5:04 pm

steve wrote:

Yes, the correct way to handle it (according to the UKMO book) is to include it in the total but not count it as a 'rain day'. But there's no good way to handle it automatically; a single tip due to real rain should count as a rain day.

I've interrogated dayfile.txt and I have 105 entries where the rainfall is 0.01". Impossible to say whether it's proper rain or melt. You could include an additional indicator for each record to indicate if it was melt. Down to the user then to check it when apprpriate. Only for true nerds probably. :lol:

mcrossley · **Posted:** Tue Jan 17, 2012 5:08 pm

Code:

if (average_temp_last_6hrs <= 3 && rain_count_last_3hrs < 0.02) {
  //probably dew/frost
  rain_day = false
} else {
  rain_day = true
}

RayProudfoot · **Posted:** Tue Jan 17, 2012 5:11 pm

Brilliant!

steve · **Posted:** Tue Jan 17, 2012 6:24 pm

RayProudfoot wrote:

You could include an additional indicator for each record to indicate if it was melt. Down to the user then to check it when apprpriate. Only for true nerds probably. :lol:

Yes, it's a possible future enhancement. I'd have to think about the best way to do it without breaking things, though.

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Frost melting on rain guage and creating false rain reading?

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