Frost?

Hi All, Hope your all well.

This may seem a silly question ( is knowing me ).

At what temperature are frosts formed?
Also what is a hard frost compaired to, well normal old frost really.

Thank You.

PS

Temperature's in the S.E of England tonight are -1 to -3. Brrrrr

 

i am certain that Walnut, PeterS, Strongy or someone will know. I have difficulty with ground and air frosts. :D

 

John, You know more than me my friend.

 

Not a meteoroligist by any means VP, but ground frost can form at any temp below about 4C I think.
The temps on the ground on clear nights are much lower that they are at the normal recording height, which is I think about four feet or so.

Air frost is when the actual air temperature drops to, or below 0C at that height.
Lots of different factors come into it, wind speed, humidity etc.

I think the term "hard frost" is a generalisation for a very cold night.

Thats how I've always seen it, but I'm sure there are more technical ways of looking at it.

 

Try this link to a previous discussion V Phttp://www.gardenerscorner.co.uk/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=003425#000005 we had a debate about it before, Pete is right on a clear night the ground surface loses temperature rapidly and a "grass frost" will occur if the temperature continues to fall the frost will then start to freeze the ground beneath the grass the lower the temperature the deeper into the ground the frost penetrates,cars are quick to show frost because the steel loses heat very quickly.An air frost when the temperature above the ground falls to freezing accompanied by a wind is very damaging to plants, it causes dessication,moisture is taken away from the plants quicker than it can take it up from the roots due to the plant being dormant or semi dormant,boxwood and conifers are highly susceptible to this hence the browning of the leaves.

 

I remember that thread Walnut! It was very useful. It set me thinking - I hadn't realised how much heat is lost at low temperatures by radiation.

Hi Veggie - by definition, frost is formed when water vapour freezes. For this to happen the temperature at that point must be 0C or less - it can never be higher. But as Pete pointed out official temperatures are measures at four foot or so above the ground (I couldn't Google the exact height). It may be 4C at that height, but as cold air is heavier than warm air the cold air sinks, and at ground level it may be 0C or less. Thats why it is called ground frost, because it can occur even when the air is warmer.

Windy nights are usually warmer, as the lower cold layer is mixed up with the upper warm layer. But as Walnut said, high and freezing winds can be very damaging to plants. Also you usually get frosts on clear nights rather than cloudy nights. On a clear night heat is radiated out into space and lost, leading to a fall in the temperature of the ground. But on cloudy nights this heat is reflected back by the clouds.

You have to be careful where you measure temperatures. I have a couple of max/mix thermometers in my garden. One at four inches, and the other at six feet. The upper one usually reads about two degrees more than the lower one over night.

 

Thank you all for your helpful infomation once again.