Reply To: Suck or blow

Forums Telescopes Suck or blow Reply To: Suck or blow

Martin Lewis

Hi Grant,
The reason breezy nights are less dewey is that the warming effect of the ambient air prevents surfaces facing the much colder night sky from being so depressed relative to the air temperature. On still nights surfaces with high emmisivity like plastics and painted surfaces can be several degrees below ambient temperature and well below the prevailing dewpoint. If there is a breeze around it has a moderating effect.

I’ve got measurements of a 2.4C depression for a plastic surface on a still night dropping to 1.5C on a breezy night. Radiative sky temperature in UK is typically 30C lower than ambient so there is a strong cooling effect for anything facing it – like opening the freezer door!

Aluminium foil, with its very low emmisivity was 0.9C and 0.2C respectivley on those two different nights making shiny aluminium a great anti-dew blanket!