Could be a hoar frost tonight RWood.

Gary Roberts · Unread post by **Gary Roberts** » Sat 27/05/2006 22:39

The sky has cleared, the temperature is dropping rapidly (matching the dew point), and I just drove through extremely dense fog at Buscot Corner, some patches so dense that I couldn't even see the bullbars.

We had a very light hoar frost at the same place a few weeks ago, but this is shaping up to be much more severe. If it happens I'll grab some pics first thing and post them.

RWood · Unread post by **RWood** » Sat 27/05/2006 23:02

That would be good to see.

squid · Unread post by **squid** » Sat 27/05/2006 23:15

look forward to the pics if this happends

NZ Thunderstorm Soc · Sat 27/05/2006 23:59

What is the difference betwixt hoar frost and rime, apart the fact they are spelt differently?

JohnGaul
NZTS

Gary Roberts · Unread post by **Gary Roberts** » Sun 28/05/2006 00:01

NZ Thunderstorm Soc wrote:What is the difference betwixt hoar frost and rime, apart the fact they are spelt differently?

JohnGaul
NZTS

Funny, I was wondering the exact same thing a few minutes ago! Here's what I found: http://en.wikipedia.org/wiki/Hoar_frost

NZ Thunderstorm Soc · Sun 28/05/2006 00:07

Thanks for that. Now I know the difference.

JohnGaul
NZTS

Gary Roberts · Unread post by **Gary Roberts** » Sun 28/05/2006 00:35

http://en.wikipedia.org/wiki/Image:Windbuchencom.jpg

What a beautiful picture of rime ice!

RWood · Unread post by **RWood** » Sun 28/05/2006 09:15

Gary Roberts wrote:
NZ Thunderstorm Soc wrote:What is the difference betwixt hoar frost and rime, apart the fact they are spelt differently?

JohnGaul
NZTS
Funny, I was wondering the exact same thing a few minutes ago! Here's what I found: http://en.wikipedia.org/wiki/Hoar_frost

Let me see if I understand what it's saying: for hoar frost (the one you see in Central Otago particularly), unsaturated air makes contact with surfaces that are colder than the sub-freezing dewpoint, thereby forming ice crystals on those surfaces. I guess the air itself doesn't have to be sub-freezing, though it often would be - dry enough air would still be above its dewpoint.

? Whereas with rime ice, condensation moisture already on the surfaces is simply subjected to a temperature drop that freezes it?

Gary Roberts · Unread post by **Gary Roberts** » Sun 28/05/2006 10:06

RWood wrote:
Gary Roberts wrote: Funny, I was wondering the exact same thing a few minutes ago! Here's what I found: http://en.wikipedia.org/wiki/Hoar_frost
Let me see if I understand what it's saying: for hoar frost (the one you see in Central Otago particularly), unsaturated air makes contact with surfaces that are colder than the sub-freezing dewpoint, thereby forming ice crystals on those surfaces. I guess the air itself doesn't have to be sub-freezing, though it often would be - dry enough air would still be above its dewpoint.

? Whereas with rime ice, condensation moisture already on the surfaces is simply subjected to a temperature drop that freezes it?

Yep, that's how I read it: Hoar frost is moisture freezing directly on contact with cold surfaces, whereas rime is liquid moisture condensed onto surfaces which then freezes.

Andy · Unread post by **Andy** » Sun 28/05/2006 10:32

Gary

Plenty of opportunity over the next week as a big high moves over.

Could be some dull weather inland, even better some hoar frost as there will be some fog. From recent drizzle / rain.

Gary Roberts · Unread post by **Gary Roberts** » Sun 28/05/2006 11:26

Andy wrote:Gary

Plenty of opportunity over the next week as a big high moves over.

Could be some dull weather inland, even better some hoar frost as there will be some fog. From recent drizzle / rain.

Looks like there was just enough high cloud remaining last night to prevent the temp dropping enough for a frost to form, dang it. Overnight low was +0.3Â°. Well that's my guess anyway! I'm sure RWood/TonyT/Somebody else will have a better explanation.

Andy · Unread post by **Andy** » Sun 28/05/2006 18:33

Warm here last night with a min of 7 degrees, plus 2mm of rain

Still warm today with max of 12 degrees

RWood · Unread post by **RWood** » Sun 28/05/2006 19:10

Gary Roberts wrote:
Andy wrote:Gary

Plenty of opportunity over the next week as a big high moves over.

Could be some dull weather inland, even better some hoar frost as there will be some fog. From recent drizzle / rain.
Looks like there was just enough high cloud remaining last night to prevent the temp dropping enough for a frost to form, dang it. Overnight low was +0.3Â°. Well that's my guess anyway! I'm sure RWood/TonyT/Somebody else will have a better explanation.

Was there any lower cloud? Hard to think a veil of high cloud would make all that much difference.

squid · Unread post by **squid** » Sun 28/05/2006 19:46

oh well better luck next time with winter just about upon us plenty of time to get some good frost pics.

Gary Roberts · Unread post by **Gary Roberts** » Sun 28/05/2006 19:47

RWood wrote:Was there any lower cloud? Hard to think a veil of high cloud would make all that much difference.

I don't believe there was...however there are two common inversion levels around here (on the Benmore range): one at approx 800m and another at around 1,400m. Could an inversion layer have kept the temp high?

Tonight is not shaping up any better. It's still 6Â° right now.

RWood · Unread post by **RWood** » Sun 28/05/2006 23:18

I think this is one for Tony perhaps...

Gary Roberts · Unread post by **Gary Roberts** » Tue 30/05/2006 17:02

Quite a decent frost this morning, and if it stays as clear tonight as it is right now we could be in for another one. It's been a truly magnificent day.

Andy · Unread post by **Andy** » Tue 30/05/2006 17:57

Plenty of fog in Alex this morning preventing a frost, min 1.3 degrees c.
Clearing mid morning to a sunny day.

Cloud rolled in again aready.

There was less fog up Cromwell Wanaka way.

Gary Roberts · Unread post by **Gary Roberts** » Tue 30/05/2006 19:34

Andy wrote:Plenty of fog in Alex this morning preventing a frost, min 1.3 degrees c.
Clearing mid morning to a sunny day.

Cloud rolled in again aready.

There was less fog up Cromwell Wanaka way.

Still nice and clear, temp dropping slowly, 1Â° in Omarama, 1.3Â° in Twizel.

RWood · Unread post by **RWood** » Tue 30/05/2006 20:02

Here's hoping. Andy - I bet the fog stopped at Gorge Creek (?name) on the Roxburgh sector of highway.

Andy · Unread post by **Andy** » Tue 30/05/2006 21:00

Gary Roberts wrote:
Andy wrote:Plenty of fog in Alex this morning preventing a frost, min 1.3 degrees c.
Clearing mid morning to a sunny day.

Cloud rolled in again aready.

There was less fog up Cromwell Wanaka way.
Still nice and clear, temp dropping slowly, 1Â° in Omarama, 1.3Â° in Twizel.

Yes Gary - A good frost on the way for you, one would say!

Andy · Unread post by **Andy** » Tue 30/05/2006 21:01

RWood wrote:Here's hoping. Andy - I bet the fog stopped at Gorge Creek (?name) on the Roxburgh sector of highway.

Yep thats one spot she clears

or where it starts

Gary Roberts · Unread post by **Gary Roberts** » Wed 31/05/2006 07:42

Andy wrote:A good frost on the way for you, one would say!

Low cloud rolled in late evening and kept brought the temperature right up again, so no frost at all. How about down your way Andy?

Andy · Unread post by **Andy** » Wed 31/05/2006 18:08

Gary Roberts wrote:
Andy wrote:A good frost on the way for you, one would say!
Low cloud rolled in late evening and kept brought the temperature right up again, so no frost at all. How about down your way Andy?

Yes same here.

Looks good tonight nice and clear in Alexandra.

Dull day in Wanaka all day under low cloud

Willoughby · Unread post by **Willoughby** » Wed 31/05/2006 18:39

May get the combo here tomorrow morning.. MS have us down to 0c