Learning Weather - Part Two - Frost

stephenprudence

Learning Weather - Part Two - Frost

Post by stephenprudence » Tue Nov 15, 2011 11:17 pm

In the second of the weather guides, I'm going to discuss frost. I want to make it as relevant to plants and gardening as possible.

First of all frost is a common blight in winter, and in general frost is an intechangeable word, but it can be sometimes confusing when you realise that there are different types of frosts, and a white frost doesn't necessarily mean the temperature has gone below 0C.

It often surprises people who wake up to a white frost that their temperature reading is above 0C, and for this reason, it is worth nothing first of all that there are multiple meanings to the word 'frost'.

The most important terms in Britain are;

- Air frost
- Ground frost
- Hoar frost.

Air Frost

Air frost is fairly straight forward it is the point at which the air at 2 metres above the ground (the official height for measuring temperature, globally), falls to 0C (32F). When the temperature reaches this figure, the water vapour in the air will turn to ice and so the term air frost is given.

Ground Frost

Ground frost is the point at which the ground surface; whether that be concrete, grass or a car roof, hits 0C. Quite often you will see hoar frost on the surfaces of various objects when the ground temperature hits 0C, therefore if this occurs you know that the ground temperature is below 0C.

Hoar Frost

Hoar frost (also known as white frost) is just an indicator that the ground temperature is below 0C, in hoar frost, ice crystals are arranged in a particular way on a surface, however even if the surface is below 0C, hoar frost doesn't always result as explained below.

So why is there sometimes no frost below 0C on the ground?

This is to do with dewpoints and also potentially cloud cover. Hoar frost is the calling card of radiation frost, that is when the ground loses its heat to a clear sky, with no thermal blanket the dew which forms on a surface often arranges itself into crystals, especially with a lowish dewpoint. However occasionally in situation where there is relatively high dewpoint, or under cloudy conditions, you will not get frost where the surface or ground temperature is below 0C, insteas you will get frozen dew. In these situations the crystals cannot arrange themselves into patterns, and therefore dew just develops as standard ice.

I hope this gives some insight into the main kinds of frosts we may receive in Britain.. again any questions let me know.

Conifers
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Re: Learning Weather - Part Two - Frost

Post by Conifers » Wed Nov 16, 2011 1:22 am

Two queries . . .

1. Why was 2 metres chosen? Seems they should have chosen ground level, as that's where ice affects people (slip-up accidents) and plants (freeze damage on foliage).

2. Often the weathermen on the radio claim that the temperature is above freezing, but it clearly isn't, with hoar frost (or snow) on tree branches 2m above ground refusing to melt. What's going on there?

stephenprudence

Re: Learning Weather - Part Two - Frost

Post by stephenprudence » Wed Nov 16, 2011 6:04 am

1. 2 metres is chosen with regard to measuring air frost - this is because official temperatures are taken at this level. Taking it at this height assures you are high enough not be directly affected by the ground and low enough not to be be affected by mixing at a higher altitude. As air frost is a measure of the freezing point of air, it is not to blame for accidents (unless contributing to freezing fog etc.) slips and other accidents are usually caused by ground frost (the ground temperature being below 0C which allows ice and frost to form on footpaths and other surfaces. So although linked, air and ground frost are not the same technically, although as ground loses heat it is the catalyst for air frost development

2. Most likely the weather forecasters are quoting temperatures for urban areas and cities which can be significantly warmer than outlying areas. These areas may stay below freezing all day whilst major conurbations rise above freezing at both air and ground. However it is possible to rise slightly above air frost levels and maintain a ground frost so it may give the impression it is below freezing when the ground is frozen still - this happens after deep frosts and on potentially overcast days if it was clear the night before - frost sometimes survives in the shade.

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Re: Learning Weather - Part Two - Frost

Post by redsquirrel » Wed Nov 16, 2011 7:58 am

really appreciate what you are doing Steve,thanks mate icon_thumright
mars ROVER broken down. headgasket faillure

Andy P

Re: Learning Weather - Part Two - Frost

Post by Andy P » Wed Nov 16, 2011 9:19 pm

Thanks for posting this Stephen. Very informative. It does however throw up a question.
A few months ago i would have assumed that my Washie palm,for example, would be perfectly safe from any frost damage aslong as the temperature stayed above 0 centigrade. After reading your post ( and other stuff i`ve been reading on the subject lately on the net) I`m now led to believe that frost damage could occur even at temps of say 2 or 3 degrees. At what temperature would you advise to switch on rope lights?
I`m inclined to think that cloud cover makes quite a difference. E.G. If i`m expecting 2 degrees and have thick cloud cover then leave the rope lights switched off - If i`m expecting 2 degrees with clear skies then switch them on for the night,just to be safe??? I know that`s probably simplifying the matter a lot, but without very accurate temperature predictions specific to my garden ( and with my very limited knowledge on meteorology) it`s the best i can deduce.

stephenprudence

Re: Learning Weather - Part Two - Frost

Post by stephenprudence » Wed Nov 16, 2011 9:28 pm

Hoar frost (what you have described) is a common problem for gardeners, because it causes damage on the surface cells of the plants, but it is unlikely to kill the plant if the air temperature is above 0C. It would cause aesthetic damage most likely on a Washingtonia. If you don't want this it really depends, you have to keep an eye out on humidity.. the more relatively humid it is at a low temperature, the more chance of frost at 0C. It really does depend on the dewpoints, and if you have a temperature sensor which gives you temperatures and humidity, then you can work it out.

If the air is dry, then there is less worry for hoar frost and it is unlikely your plants will be damaged in that situation if the temperature is above 0C.

They say dry is the best way in winter, and in most cases on cold (but not freezing) nights, it probably is.

In general though if the skies are clear and the temperature is falling rapidly or hovering around 2-3C, yes switch the lights on, because it may not stay at 2-3C

Darlo Mark

Re: Learning Weather - Part Two - Frost

Post by Darlo Mark » Sun Nov 20, 2011 2:08 pm

cheers for this Stephen!

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Re: Learning Weather - Part Two - Frost

Post by Yorkshire Kris » Sun Nov 20, 2011 2:12 pm

I guess another point is that when the minimum temps reach 2-3 degress then your tender aloes etc might be killed as where the plants are (in the ground could be below 0)!!

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Re: Learning Weather - Part Two - Frost

Post by redsquirrel » Wed Nov 23, 2011 7:37 am

the % in humidity,is that dryness?
my lowest point last night was temp,1.4c,dew point of 2.0 and humidity 99%.
is that why i didnt get frost?
mars ROVER broken down. headgasket faillure

stephenprudence

Re: Learning Weather - Part Two - Frost

Post by stephenprudence » Wed Nov 23, 2011 9:49 am

Humidity is just a measure of how much water vapour is in the air, I suppose you could use the term 'the dryness', in that the lowest % is the driest, and the highest % is the most moist.

Your dewpoint would have been 1.4C too, as dewpoints cannot be higher than the temperature, however weather stations often round number up, so it probably updated at 1.5C and rounded the dewpoint up to 2C.

Frost does not typically occur when the dewpoint is the same as the temperature (at temperatures above 0.5C) however you most likely had dew turning to ice on the coldest surfaces.

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Re: Learning Weather - Part Two - Frost

Post by redsquirrel » Wed Nov 23, 2011 6:08 pm

i expected to find ice on the cars.there was none.
mars ROVER broken down. headgasket faillure

Troppoz

Re: Learning Weather - Part Two - Frost

Post by Troppoz » Thu Nov 24, 2011 12:34 pm

Interesting reading Stephen icon_thumleft

Im just glad I dont have to deal with the stuff...

jungle jas

Re: Learning Weather - Part Two - Frost

Post by jungle jas » Fri May 04, 2012 5:51 pm

Just been told min temp tonight will be 3c and we can expect a radiation frost! What is a radiation frost. icon_scratch Thanks Jas.

Conifers
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Re: Learning Weather - Part Two - Frost

Post by Conifers » Fri May 04, 2012 6:21 pm

jungle jas wrote: What is a radiation frost. icon_scratch
A ground frost.

jungle jas

Re: Learning Weather - Part Two - Frost

Post by jungle jas » Fri May 04, 2012 9:31 pm

Conifers wrote:
jungle jas wrote: What is a radiation frost. icon_scratch
A ground frost.
THANKS. icon_thumright

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