Formula for calculating the cloudbase from the Temp and Dewpt

[Capn Bloggs]

If anybody has the formula for calculating the cloudbase from the Temp and DP could they please post here.

Thanks!

[olderairhead]

Temp minus DP x 400 = cloud base in feet

[compressor stall]

Pretty good rule of thumb in Oz, but does not work in all parts of the world!

[Dan Winterland]

400' per degree of spread is a reasonable rule of thumb in temperate climates, i.e it is only really valid if the temp is approximate to ISA. Even then there can be big variences. In tropical or very cold conditions, it doesn't work.

[Delay Approved]

My memory of MET is a little hazy but I think this formula only works for Cumuliform cloud.

[Contract Con]

Gday,

I used 300'/1degree C split quite sucessfully for years in Aus., particularly South and Central Australia.
But as Stally and others have mentioned, it may not be accurate in the more extreme environments.
Still a good rule of thumb though to give you a fair indication of as to when, if ever, you will pop out of the gloop if all you have is a METAR to go by.

Cheers,

Con:ok:

[SIUYA]

Stally............

I think that's right.

From what I can remember (and I gotta admit it's not much these days), you can also work out cloud base from LCL, but it's a lot more complicated, and the rule of thumb olderairhead quoted works OK. I dug out some of my old met notes, and here's the comparison between the two methods:

Assuming the following conditions:

Surface pressure (hPa) = 1013
Surface T (STc) = 15
Surface Dewpoint T (SDc) = 12

ESTIMATED CLOUD BASE (ft AGL)
(STc - SDc) *400 = 1,200 ft AGL

Now, if you cross-reference this ‘rule of thumb’ to the calculation for Lifted Condensation Level (LCL), you come up with:

LIFTED CONDENSATION LEVEL
LCL ft AGL = 1,200 ft

Because:

PT (LCL Temp in C) = 12

And:

LCLhPa = 973 hPa

From the following formulas:

Lifted Condensation Level Temp (PT) = ((1/(1/((SDc+273.16) 56)+LOG((STc+273.16)/(SDc+273.16))/800))+56)-273.16

Lifted Condensation Level hPa = ((Surface Pressure*1000)*(((PT+273.16)/(STc+273.16))^(3.5)))/1000

LCL ft AGL = (Surface Pressure - LCL hPa)*30

At least, I think that's the way it's done :confused:

[Capn Bloggs]

Thanks guys. I'll try it out. BOM, bring on some low cloud will ya?

And yes, Con, that's generally all we have ot go by. Apart from the girl in the office:

"How's the weather look?".

"Oh, OK. We can just see the hills...". :ugh:

[SIUYA]

Bloggs.............

The LCL calculation starts to give a lower cloud base than the rule of thumb method as surface pressure reduces. It also does as dewpoint spread increases. So if you've got a low pressure and a high dewpoint spread then 'reliability' of the rule of thumb method reduces. Not by much, but it confirms what Stally says.

Anyway, head on down to Victoria RFN, because you'll have the perfect opportunity to start trying out both methods with all the low cloud that BOM's arranged there at the moment :}

[Howard Hughes]

"Oh, OK. We can just see the hills...".
Nothing wrong with that, provided you know how high and far away the hills are...:ok: