View Full Version : Cloudbase & dewpoint
tmmorris
4th Oct 2005, 07:28
I remember from the PPL Met exam that you can work out an estimate of cloudbase from the temp/dewpoint split, at 400ft per deg Celsius. True to form, on Friday the ATIS gave 12/11 and the cloudbase was at 600ft.
But yesterday the ATIS was giving 10/9 and the cloud was few at 3500 overcast at 4200. The few at 3500 were Didcot power station gunk; but why was the cloudbase so much higher? QNH was 1031 i.e. v. high pressure.
Am I right in thinking it was because there was an inversion?
Tim
Have a look at this site
http://weather.uwyo.edu/upperair/sounding.html
and the Skew-T diagrams. For Region, select Europe; for Type of Plot select GIF: Skew-T, and you get what amounts to a METAR map of data gathered from met baloons.
The generated diagram shows the profile of temperature and dew point versus height and this shows, among other things e.g. the freezing level, the temperature lapse rate.
Most of the time the lapse rate is nowhere near the book figure.
bookworm
4th Oct 2005, 07:51
Here's the Nottingham sounding for noon yesterday:
03354 Nottingham Observations at 12Z 03 Oct 2005
-----------------------------------------------------------------------------
PRES HGHT TEMP DWPT RELH MIXR DRCT SKNT THTA THTE THTV
hPa m C C % g/kg deg knot K K K
-----------------------------------------------------------------------------
1018.0 117 12.2 8.4 78 6.84 260 3 283.9 303.0 285.1
1017.0 125 12.0 6.0 67 5.80 263 3 283.8 300.1 284.8
1007.0 208 11.2 7.2 76 6.36 298 5 283.8 301.6 284.9
1005.0 224 11.1 6.9 75 6.25 305 5 283.8 301.4 284.9
1000.0 266 10.8 6.2 73 5.98 295 4 283.9 300.8 285.0
994.0 316 10.4 5.4 71 5.69 286 3 284.0 300.1 285.0
977.0 459 9.2 4.7 74 5.51 260 2 284.2 299.8 285.1
943.0 751 6.6 3.3 79 5.16 355 5 284.5 299.2 285.4
927.0 892 5.4 2.6 82 5.00 359 5 284.6 298.9 285.5
925.0 910 5.2 2.5 83 4.98 0 5 284.6 298.8 285.5
890.0 1224 2.2 1.9 98 4.96 346 7 284.7 298.8 285.5
887.0 1251 2.0 1.8 98 4.92 345 7 284.8 298.8 285.6
873.0 1379 1.3 1.0 98 4.74 335 4 285.3 298.9 286.1
850.0 1594 0.0 -0.2 99 4.46 345 8 286.1 299.0 286.9
847.0 1622 -0.3 -0.5 99 4.38 346 8 286.1 298.7 286.9
844.0 1651 6.2 -0.8 61 4.29 347 9 293.2 306.0 294.0
836.0 1729 7.1 -5.0 42 3.16 350 10 294.9 304.6 295.5
833.0 1759 7.4 -6.6 36 2.81 348 10 295.6 304.2 296.1
801.0 2080 6.2 -14.8 21 1.52 332 9 297.6 302.5 297.9
779.0 2307 5.1 -15.1 22 1.52 320 8 298.8 303.8 299.1
773.0 2371 4.8 -15.2 22 1.53 324 8 299.2 304.1 299.4
762.0 2487 4.6 -18.4 17 1.18 331 9 300.2 304.1 300.4
749.0 2626 3.7 -19.3 17 1.11 340 9 300.7 304.4 300.9
724.0 2900 1.9 -21.1 16 0.99 320 13 301.7 305.0 301.8
The inversion, a strong one as you can see, was just above the top of the cloud as usual. I think the 400 ft/degC approximation tends to work well in unstable conditions when air parcels rise from the surface under convection, and cool to their dewpoint. Yesterday it was neutral below the SC, and it was also quite humid close to the surface, giving an anomalously high dewpoint.
tmmorris
4th Oct 2005, 09:24
Excellent - I'd heard of Skew-T diagrams but hadn't realised what they were good for!
Tim
Here they call them tepigraphs or tephigrams or something like that... They are very handy for getting the cloud TOP height and the freezing level, and the stability (the lapse rate).
The problem is that the data appears only about twice every 24 hrs, and the 0000Z value isn't going to be very representative of what to expect at 1100Z during the day :O But for a flight at say 1400Z it is very good data, IMHO.
dublinpilot
4th Oct 2005, 16:26
Can anyone point me towards a "Skew-T diagrams for Dummies" guide? Just wondering how to interpret the diagram, at its simplest level.
dp
Enter
Skew-T interpretation
into Google
http://www.theweatherprediction.com/thermo/
http://www.geo.mtu.edu/department/classes/ge406/cmriley/interpret.html
etc