Discussion: frost

Hey Blair, on a totally non orienteering direction. i s there a particular temperature that a frost forms? Does it have to get to 0 degrees or lower to form or is it just when it is bloody cold overnight?

You need the ground temperature to stay below 0 for long enough for ice to form, and for there to be sufficient moisture for condensation to occur - the second is not normally an obstacle in Canberra or Wagga but can be in more arid parts (or in Seoul, as I discovered in February).

Under clear calm conditions the minimum ground temperature will normally be 3-5 degrees colder than that at the standard measuring height (1.2m), so with a screen-level temperature of, say, +2, you'd expect a ground temperature of around -2, and probably some frost formation.

Minimum temperatures (at both ground and screen level) are also extremely sensitive to local topography, so it's quite possible under the right conditions to find places which are several degrees warmer or colder than the nearest official observing site.