Dew point error?

[SUBYDAZZ]

Back in April this year I managed somehow to get a dew point of -13.4oC (appears in the all-time and yearly records). Since the minimum temperature was about 10oC that day is this even possible and how can I find out whether it was some kind of glitch or genuine?

Is there anywhere I can view dew points for a selected day or something other than the graphs which are limited to the last 48 hours?

[steve]

Back in April this year I managed somehow to get a dew point of -13.4oC (appears in the all-time and yearly records). Since the minimum temperature was about 10oC that day is this even possible and how can I find out whether it was some kind of glitch or genuine?

That's definitely a glitch - you probably got an abnormally/erroneously low RH reading.

Is there anywhere I can view dew points for a selected day or something other than the graphs which are limited to the last 48 hours?

In 'Select-A-Graph'?

[SUBYDAZZ]

OK so I have viewed that day and it doesn't even show that value. So I'll delete it and reset the new minimum dew point. Any idea how I could go about finding the proper minimum dew point from the history I have or is it just better to reset it?

When I fetch the log and dayfile data the two values I get are the existing erroneous one and another of -11.1 oC in the middle of January (Summer here).

[steve]

You can look at alltimelog.txt, see what the previous record was, and reinstate that. Or you can edit dayfile.txt and/or your monthly log file(s) to remove the bad values - set them to the same as the ambient temperature, for example - and then use the editor to fetch the values again. The problem with the latter approach is that you might just uncover another bad value...

[SUBYDAZZ]

Any way to be able to determine whether the values for dew point / humidity might be erroneous or not? I guess other than looking at the graph and seeing an obvious spike? I assume very low humidity will give a very low dew point.

[bnwrx]

Along the lines of this thread about dewpoint....When my station reports below 10% humidy, it just shows 2 dashes on the console(ie:--)Cumulus then reports an erroneous reading for dew point very close to the actual temp. reading. Yesterday our temp was 82F with single digit humidities and Cumulus reported dewpoints in the low 80sF while other stations in my area had dewpoints in the single digits. Is this an anomoly with Cumulus? I tried setting Cumulus to calculate the dewpoint, but no change. When the humidity goes above 10%, all is well again with the dewpoints. Odd!!!

[steve]

When my station reports below 10% humidy, it just shows 2 dashes on the console(ie:--)Cumulus then reports an erroneous reading for dew point very close to the actual temp. reading

It should transmit a value of 255 for humidity when that happens; Cumulus checks for this and ignores it, so it should just continue using the last good value. If it sends some other out-of-range figure, that should be ignored too. But in either case, it's difficult to know what to do. Should it assume 10%? Some other value? In some circumstances, it sets the dew point to the ambient temperature when it can't make sense of the humidity reading, this is presumably what you're seeing. What was it displaying for humidity at the time?

I'll have a look at the code and see if I can get it to behave a bit better.

[bnwrx]

Steve, looking at the data log, it appears that Cumulus was logging 0% as the humidity level but I can't remeber what the display showed. I'll try to notice it the next time it happens. It did this for several hours and when the station reported humidity at 10%, all seemed OK with Cumulus. My station only reports down to 10% and this year it is VERY dry here. Unusually so.
Thanks for any help.....