Formula for calculating the cloudbase from the Temp and Dewpt
Thread Starter
Joined: Mar 2002
Aviation Qualifications: ATPL
Posts: 9,188
Likes: 961
From: Seat 1A
Formula for calculating the cloudbase from the Temp and Dewpt
If anybody has the formula for calculating the cloudbase from the Temp and DP could they please post here.
Thanks!
Thanks!
Joined: Jun 2001
Posts: 4,777
Likes: 9
From: Blighty
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.
Joined: Dec 2004
Posts: 117
Likes: 0
From: .
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
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
Joined: Nov 1998
Posts: 709
Likes: 146
From: Planet Earth
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
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
Thread Starter
Joined: Mar 2002
Aviation Qualifications: ATPL
Posts: 9,188
Likes: 961
From: Seat 1A
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...".
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...".
Joined: Nov 1998
Posts: 709
Likes: 146
From: Planet Earth
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
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
Sprucegoose
Joined: Sep 2000
Posts: 3,485
Likes: 1
From: Hughes Point, where life is great! Was also resident on page 13, but now I'm lost in Cyberspace....
"Oh, OK. We can just see the hills...".
