Vidcapper wrote:

Here's something from my VP earlier this afternoon :

Date/Time Temp Rel.Hum Dewpoint
15/03/2014 14:10 14.8 65 8.3
15/03/2014 14:11 14.8 63 7.8
15/03/2014 14:12 14.8 63 7.8
15/03/2014 14:13 14.8 56 6.1
15/03/2014 14:14 14.8 53 5.3
15/03/2014 14:15 14.8 52 5.0
15/03/2014 14:16 14.8 51 4.8
15/03/2014 14:17 14.8 51 4.8
15/03/2014 14:18 14.9 50 4.5
15/03/2014 14:19 14.9 50 4.6
15/03/2014 14:20 15.0 49 4.4
15/03/2014 14:21 15.1 48 4.1
15/03/2014 14:22 15.1 47 3.9
15/03/2014 14:23 15.2 46 3.6
15/03/2014 14:24 15.2 46 3.7
15/03/2014 14:25 15.2 45 3.4
15/03/2014 14:26 15.3 45 3.4
15/03/2014 14:27 15.3 45 3.5
15/03/2014 14:28 15.4 45 3.5
15/03/2014 14:29 15.4 46 3.9
15/03/2014 14:30 15.4 47 4.2
15/03/2014 14:31 15.5 49 4.8
15/03/2014 14:32 15.5 52 5.7
15/03/2014 14:33 15.6 57 7.1
15/03/2014 14:34 15.6 57 7.1
15/03/2014 14:35 15.6 58 7.4

And not a weather front in sight - what could have caused such a rapid
drop & rise in dewpoint?

Would I be correct in thinking that dewpoint is not measured, but calculated
from temperature and humidity?