Page 1 of 1
Frost melting on rain guage and creating false rain reading?
Posted: Tue 17 Jan 2012 1:35 pm
by Solorize
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?
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 1:38 pm
by mcrossley
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.
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 1:49 pm
by steve
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.
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 1:59 pm
by Solorize
Thank you both for your replies.
I think I will take your advice Steve and make a note in my log.
Cheers.
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 4:46 pm
by RayProudfoot
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.
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 4:51 pm
by steve
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.
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 5:04 pm
by RayProudfoot
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.
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 5:08 pm
by mcrossley
Code: Select all
if (average_temp_last_6hrs <= 3 && rain_count_last_3hrs < 0.02) {
//probably dew/frost
rain_day = false
} else {
rain_day = true
}
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 5:11 pm
by RayProudfoot
Brilliant!
Re: Frost melting on rain guage and creating false rain read
Posted: Tue 17 Jan 2012 6:24 pm
by steve
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.
Yes, it's a possible future enhancement. I'd have to think about the best way to do it without breaking things, though.