Cloudbase calculation

[goldrush]

Steve
This is not a complaint, but an observation you may wish to know.

My station is at 553 feet amsl.
Tonight Cumulus calculated cloudbase as 532 feet, whereas Weather Display from the same data and station calculated it as 1062 feet which looks about right.
Does Cumuls calculate cloudbase as from station altitude which would put it too at roughly correct?
I realise this calculation is very innacuarate and unreliable.
Cumulus 1.8.4 build 584
WH1081
Vista crap

[steve]

Yes, Cumulus gives the height AGL, which I was lead to believe was the normal way of presenting it?

Steve

[goldrush]

Hope this does not get duplicated, having problems submitting this morning

Confusingly it looks like cloudbase can be shown as altitude or height with height agl "normal"?.

Some formulae add the station height to the calculations, others do not.

http://mst.nerc.ac.uk/file_format_ceilo ... _base.html

talks of...
"Cloud base altitude of first cloud layer (m). Altitude implies above mean sea level. Accuracy: +/- 5 m. "

The calculator at
http://www.csgnetwork.com/estcloudbasecalc.html

requires input of station height and gives the result as

Estimated Cloud Base Altitude

Incedentally, it gives the same results as Cumulus this morning without station height, same as Weather display with it!
Further this morning Cumulus gives about the right cloudbase. I suspect yesterday it may have been due to the dewpoint/temperature/humidity combination which should have given "low flying cloud" which Cumulus predicted, but which did not actually occur.


Good aint it?

You amy be interested that apparantly the Australian Weather Institute has carried out an investigation into the relationship between dew point and snow/ice prediction with the following results

dewpoint 0,1 - 0,8°C = normally sleet
dewpoint >0,8°C = rain
dewpoint <0,0°C= snow

[steve]

Confusingly it looks like cloudbase can be shown as altitude or height with height agl "normal"?.

I think I'll leave it as it is, then

You amy be interested that apparantly the Australian Weather Institute has carried out an investigation into the relationship between dew point and snow/ice prediction with the following results

dewpoint 0,1 - 0,8°C = normally sleet
dewpoint >0,8°C = rain
dewpoint <0,0°C= snow

Will Hand, a UKMO employee on the uk.sci.weather newsgroup has a more sophisticated rule of thumb which uses dewpoint and temperature:

Add together screen temperature and dewpoint to get a temperature T

If T = 7 chance of rain turning to snow = 10%
6 20%
5 30%
4 40%
3 50%
2 60%
1 70%
0 80%
<0 90%

Thus an air temperature of 5C with a DP of -1C gives T=4 which is 40%
chance.

Steve

[goldrush]

..........I think I'll leave it as it is, then

Good ploy.... "if in dowt... do nowt" works every time

[goldrush]

Just for info, I have discovered that WD can be set to display cloudbase either as height agl, or altitude amsl