Temperature v. Frost

[Kristen]

Newbie naive question: what caused my ground frost to be "significant" last night?

I have a El Cheapo weather station. The outside sensors are mounted on top of a pergola, about 8' up

We had a pretty heavy frost last night - still very evident (11am) in areas still shaded from the morning sun, current air temperature 5C.

Here's what the Weather Station says (Graph is from Cumulus which seems to be much better than the Easy Weather software that came with the weather station)

Temperature I understand , Dew Point only briefly touched 0C, air could be colder at the ground level than at 8' (though how much colder I wonder?)

Is it the Effective Temperature? and what makes that so much lower?

Wind speed was pretty constant around 1 m/s with gusts to 2 / 2.5; Humidity slightly rising during the night from 83% at 23:00 to 90% at 07:00

[stephenprudence]

I assume by ground frost you mean the white frost (known as hoar frost?), sometimes you can have ground frost without white frost, if humidity is low enough.

But basically a combination of high humidity, radiation loss and light breezes.

Frost usually occurs under radiation anyway, but if it's humidity and cold, layers of frost will add on to the top of each other during the night, therefore by morning it looks very frosty. The wind also acts as an icing agent by moving around molecules in water, which then allows to form ice - if there is no breeze evidence, molecules would simply sit still and you would have super cooled water rather than ice.

Here is another link to a frost which I wrote a month or so ago.

http://www.hardytropicals.co.uk/forum/v ... 19&t=14456

[Kristen]

I assume by ground frost you mean the white frost (known as hoar frost?)

Yes, thanks, that's the one . Dunno about other lay-folk, but I tend to refer to Hoar Frost as the white you get on trees, typically after freezing fog or the like.

Here is another link to a frost which I wrote a month or so ago.

http://www.hardytropicals.co.uk/forum/v ... 19&t=14456

Off for a read, thanks.

[stephenprudence]

freezing fog often causes rime frost, although hoar frost can often form before the onset of freezing fog, then sit there whilst layers of time are added to it.. fascinating stuff, but best kept well away from our plants I reckon.

[Kristen]

Very useful linked-post, thanks.

I understand the principles now but I'm still a bit unsure for my particular graph.

The ground is radiating heat, and gets to the point where hoar frost forms - have I got that right? i.e. if hoar frost forms on the grass then the temperature [of the grass] is 0C or lower?

How does the temperature at 8' stay significantly higher than that? I realise that cold-air-sinks, but my thinking is that there must be some cooling of adjacent higher air layers? and 8' is not all that far from the ground

The thing that particularly puzzles me, and maybe you could clarify, is that the Dew Point only touched 0C for a brief period last night, and given the amount of hoar frost on the grass my thinking is that I ought to have some readings from my weather station that indicate the amount of hoar frost that I actually had on the ground.

But maybe this is just the difference between a reading at 8' and at ground level? if so I think I will need to get some monitoring at ground level, 'coz it strikes me as a big difference in only 8' and I'm going to get caught out by my weather-station readings at 8'!!

I did see a website that had readings at ground-level and a-couple-of-feet-below-ground. I'd quite like to have those figures here too I have a number of "1-wire" sensors from another project, and could maybe redeploy them for various heights in the conservatory, and outside.

[stephenprudence]

Ground loses heat on a clear night, and then the air temperature is affected by that. The ground will always been colder than the air on a clear night. When the ground is warmer than the air in winter you often see mist hugging the ground. However if you were to climb to 1000 feet you would find the temperature will most likely be higher than near ground level on a night like last night.

It looks like the difference between the temperature and dewpoint caused frost for you last night, despite the air temperature being above 0C. It is the same principle with snow, snow will not form unless the dewpoint is below the temperature (around 0C). Seems for you the dew must have been building all night and then the dewpoint hit 0C, a rapid morph to frost occurred. Now the dewpoint is lower today in many cases, the frost has remained in shady spots.

[Kristen]

Ah ... I think I've got it!

So the Dewpoint was low enough for dew to form on the grass (and humidity was 90%-ish ...)

Then Dewpoint (or maybe the temperature?) hit 0C at ground level and that froze the dew that was on the grass. Given that my recording is at 8', the Dewpoint (or Temperature) will have hit 0C earlier, and for longer, at ground level

If I've got that right that's the essential "bit" that I was missing; Thanks

[stephenprudence]

Yes you have it; for example here's a typical frosty night in the UK


6pm
Air temperature: 3C
Dewpoint 1C
Ground temperature 1C
(dew on ground)

9pm
Air temperature: 1C
Dewpoint: -1C
Ground temperature: -2C
(Frost on ground)

12am
Air temperature: -0C
Dewpoint -1C
Ground temperature: -3C
(Frost on ground)

3am
Air temperature: -1C
Dewpoint: -2C
Ground temperature: -4C
(Moderate frost on ground)

6am
Air temperature: -2C
Dewpoint: -2C
Ground temperature: -5C
(Heavy frost on ground)

9am
Air temperature: -1C
Dewpoint: -1C
Ground temperature: -4C
(heavy frost on ground)

12pm
Air temperature: +4C
Dewpoint: +2C
Ground temperature: 0C
(Only patches of frost in shade)

[Kristen]

Brill! Thanks very much for your help and patience.

I need some extra recording-sensors now ...

[Kristen]

Temperature now falling like a stone