I have some misgivings about the veracity of the cloud height as reported by WD it rarely agrees with the Meta from Auckland airport or visual evaluation, particularly under conditions of high humidity e.g. >60% Rel…
Would appreciate comments on this from others using this feature
it relies on your weather stations humdiity and then dew point reading being correct, and then it assumes the environmental adiabatic elapse rate is standard, which it might not be, and its just as a guide as to where, if any, clouds will start forming at, but unstable conditions and a whole host of other factors means the actual cloud height will be different in practice…and so just use it as a guide…also have you set your stations altitude in the units setup?
Hi Brian,
Thanks for the info, yes the altitude is set at the height above MSL ( from GPS data) of the main sensor (temp,hum,pressure) head of the weather station.
I’ll put a note of explanation on my website.
Robert
mine currently says 3500 feet
and i would say at a guess the cloud BASE height is not too far from that, of the cumulus clouds
mine currently says 3500 feet
and i would say at a guess the cloud BASE height is not too far from that, of the cumulus clouds
in fact the airport metar says cloudy with clear patches at 3500
so for me here, not all that far from you, WD is spot on!
Brian,
Do you use the “standard” lapse rate, or the more correct wet/dry lapse rates?
Mine is currently showing 2077m whis is about 6814 feet a big difference
R
its a simple formula, but for me today, is surprisingly accurate
temp:=((temperature-dewpoint)*412)+heightabovesealevel
for feet
[quote author=oldun link=topic=11892.msg90299#msg90299 date=1130295400]
Hi Brian,
Thanks for the info, yes the altitude is set at the height above MSL
as i said, its dependent on how accurate your dew point is
my dew point is 10.6oC, which ,matches exactly auckland airport, 18km from me in a straight line
my temperature is 20oC
checking your web site, your dew point is only 7.4oC, which is too low, and hence thats why the cloud base is being reported as too high
i have a davis VP, which is giving me real good dew point readings 
This calculator lines up pretty well with WD 
yes, if i put in my current numbers, and my height, 280 feet, then it matches wd (which is now saying 4100 feet )
Time 0700 wd reports cloud height 320 meters ( temp9.4C , dew point 7.0C Height ASL=21m)
Visual zero cloud, Auckland airport concurs.???
The calculator gives me 313m for cloud height, matches up with your station report of 320m.
It doesn’t mean there are clouds present, just that if there were clouds the expected base would be as reported.
Yes I have set up the station’s height in WD and yes I also calculated the cloud height as reported, my point should be that it is not much use reporting “cloud Base” on ones web site when the reality can not be even close, it is still wall to wall blue out there, I guess that short of having a LIDAR height finder in one’s yard there is nothing one can do.
Thanks for the input,
Robert
Note just because a cloud height is reported, does not mean there is clouds to be seen…
if and when the clouds form, i.e like in the afternoon from heat build up, then that will be a good indication of the height they will form at, but you have to have accurate dew point reading in the first place, which your weather station, the WM918, is not known to be good at
Thanks Brian,
Do you have any data on the inaccuracies you mention for the wm918 station I presume it is not something nice and easy like a linear offset which could be corrected, I imagine that the errors are unit random.
Do you happen to know how often and at what times the NZAA Meta at the Auckland airport is updated it seems a bit amateurish in operation!
Robert
note that the time stamp on the metar is UTC, and we are daylight saving now…
metars are generaly only updated every hour in alot of cases, and that is certainly the case at auckland airport
nothing amateurish aboput a metar report though
re your humidity reading, you could add a 1 wire humidity sensor to your setup
Thanks Brian,
I just put the jug on for the next coffee hopefully that will improve the eyesight as well as wake the brain up UTC yes… #-o
Another question you suggest a 1 wire humidity sensor why would this be More accurate, where does one feed this into the WM918, if not where does one plumb it into the system so that it intergrates into WD.
I also note that the biggest difference in predicted cloud height and that reported by the NZAA Meta is before and just after sunrise when the big differences occur in Temp.,Hum.,and Dew point. I expect that this is where the WM918 falls down on Dew point accuracy.
Robert