Frost?

hi,

can anyone tell me when frost forms??
Is it simply when he ambient temp gets to 0 deg C (or 32F)

or has it something to do with dewpoint as well???

cheers

dew point does come into it
also, the temperature at ground level is colder than normal station temperature (i.e “shoulder height”), and that is what grass miniumum measures (i.e the ground radiated heat, and this then cools the air directly above the ground, and so the calmer the wind, the more cooling of this air layer (i.e no mixing)
BUT
when the temperature reaches the dew point, dew forms on the grass , which is cold,(i.e colder temperatures can not hold as much water, and so the relative humidity increases - once at 100%, the moisture condenses out).
the formation of the dew (and note that the temperature at ground level is colder and so it reaches the station height dew point earlier) releases latent heat, which then counteracts the cooling.
Frost is when the dew freezes…
(and the ice crystals keep increases in size as more moisture from the air condenses)
Now, to get a frost, you need to have no wind and clear night skies (cloud traps the escaping long range radiation (infra red) radiated heat)…
AND, enough time…i.e long winter nights…OR the temperature starting off cold enough. Most often the frost is most sever actualy just affer sun rise, as it takes a while for the heat from the sun to start to counteract teh heat still being lost to spave…

now, the higher the starting dew point, the sooner the dew will form, and the whiter the frost, BUT this dw formation releases heat, and slows down the rate of cooling,…so its a catch 22

:slight_smile:

Brian,

thanks for the info.

although i’m not any more clearer.

I am using the log files & producing my own reports(using ancient Clipper compiler).

So if I simply say if dewpoint & min temp <=0 then frost has occured., do you think that will be mostly correct. or I guess I could say also no wind & also check how long the temps have been 0.

what do you reckon.

If you were to build a module into Wd that said here had most probably been a frost, would you use the above references.

cheers

humm
well, from my experience, you can get a light ground frost with air temperature minimums of only 3.5 to 4 oC (with no wind)

and then you can have a cold wind blowing and minium air temperatures of say 2oC but no frost (becuase the ground air layer keeps getting disturbed)
the best thing is to have a grass minumum temperature

Can dewpoint ever be below 0/32?

yup
if the air is dry enough (i.e it has originated from cold latitudes, where it is below freezing to start with…i.e the dew point of a airmass is slow to increase or decrease as it leaves its place of origin…i.e as air moves from the polar region, where it might start with a high humidity reading (but relatively low dew point, i,e the dew point and temperature are nearly the same) then as it warms up, as it moves towwards the equator, the relative humidity drops (i.e the dew point stays the same low value, but the temperaure increases), and so the cloud base gets higher and higher and rain gets less and less until eventualy you get cloud less skies

the opposite happens to an air mass leaving the tropics and heading to the poles.

another thing that can create dew points below freezing is the dry side of a mountain rainge (fohn wind), where all the moisture in the air has been dropped as rain on the ranges…