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Frost and Low RH Value

PostPosted: 2014 Apr 16, 16:14
by Richard_Griffith
After a clear and cool night with air minimum down to -0.5C and a grass minimum of -3.1C the air temperature reached +16.1C this afternoon with some low Relative Humidity values - down to 23% by mid afternoon in a light ESE breeze.
1600z ob - Air Temp +15.4C and a Dew Point of -6.0°C

Regards

Re: Frost and Low RH Value

PostPosted: 2014 Apr 16, 21:53
by Stephen Burt
Here, the afternoon humidity wasn't particularly low (low 40s) until the end of the afternoon, after which it fell irregularly and steeply towards evening. The lowest RH here was 25% at 1821z, and the lowest dew point -8.3°C 28 minutes later (just 13 minutes before sunset). At 1900 (sunset 1902z) the air temperature was +9.8°C, dew point -7.3°C, RH 29%.

I began to suspect my instruments, until I checked the hourly obs from Odiham, about 15 km SE (and upwind). There at 1800z the temperature was 12.9°C and the dew point -7.9°C, for an impressively low RH of just 21%. Not a record by any means, but very low for near sunset in spring.

At first sight, it looks as if the very dry air was imported on the day's SE'ly flow from France. There was no 12z ascent from Herstmonceux, but the 12z sounding from Trappes, near Paris, shows a hard-to-believe RH of just 2% just above the surface (see sounding below, RH column highlighted). This possible trajectory fits with Richard's low values mid-afternoon, implying the passage of a notably dry 'patch' across SE and central southern England today. Any more readings to add to the picture?

Stephen

07145 Trappes Observations at 12Z 16 Apr 2014

-----------------------------------------------------------------------------
PRES HGHT TEMP DWPT RELH MIXR DRCT SKNT THTA THTE THTV
hPa m C C % g/kg deg knot K K K
-----------------------------------------------------------------------------
1007.0 168 12.8 -36.2 2 0.17 110 6 285.4 286.0 285.4
1000.0 226 12.0 -23.0 7 0.61 110 7 285.1 287.1 285.2
954.0 616 7.0 -4.0 45 2.99 95 12 283.9 292.7 284.5
925.0 868 4.6 -4.4 52 3.00 85 15 284.0 292.8 284.5
881.0 1263 0.8 -5.2 64 2.96 71 20 284.1 292.7 284.6
862.0 1439 5.4 -8.6 36 2.33 65 22 290.6 297.7 291.0
861.0 1449 5.5 -9.4 33 2.19 65 22 290.8 297.5 291.2
850.0 1554 6.2 -17.8 16 1.12 65 22 292.6 296.2 292.8
843.0 1622 6.6 -32.4 4 0.30 64 22 293.7 294.8 293.8
707.0 3045 -1.5 -19.5 24 1.16 46 18 299.9 303.8 300.1
700.0 3124 -1.9 -18.9 26 1.23 45 18 300.4 304.4 300.6
662.0 3566 -3.7 -18.7 30 1.33 38 17 303.1 307.6 303.4
623.0 4043 -5.9 -31.9 11 0.43 31 16 305.9 307.5 306.0
600.0 4336 -7.1 -16.1 49 1.82 27 16 307.9 313.9 308.2
500.0 5730 -16.9 -24.9 50 1.02 5 13 312.4 315.9 312.6
486.0 5939 -18.4 -26.3 50 0.93 0 12 313.1 316.3 313.2
471.0 6169 -20.1 -27.8 50 0.83 30 15 313.8 316.8 314.0
400.0 7370 -28.7 -35.7 51 0.46 345 20 317.6 319.3 317.7
389.0 7566 -30.2 -37.0 51 0.41 335 21 318.2 319.8 318.3
324.0 8847 -39.9 -45.9 53 0.19 353 21 321.9 322.6 321.9

Re: Frost and Low RH Value

PostPosted: 2014 Apr 17, 20:00
by Richard_Griffith
Thanks Stephen - yes indeed a very impressive low RH value from the sounding at Trappes. My lowest RH value was 19.6% at 15:36hrs.

Regards