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Posted: 10/23/2013 5:37:16 PM EDT
With wind speeds of 3 to 5 MPH and temps. around 28 to 33 degrees F., is frost possible?

I guess humidity levels in the air can also affect this question.

Anyone around who could answer this question?

Link Posted: 10/23/2013 5:40:32 PM EDT
Is this a serious question?
Link Posted: 10/23/2013 5:41:16 PM EDT
My answer depends on how you voted in the intelligence thread.
Link Posted: 10/23/2013 5:41:23 PM EDT
Yes.
Link Posted: 10/23/2013 5:41:27 PM EDT
yes its possible.

You trying to protect plants?
Link Posted: 10/23/2013 5:43:37 PM EDT
Is frost possible at a temperature below that which water freezes at?  Yes, in fact it is.
Link Posted: 10/23/2013 5:46:08 PM EDT
I would assume that yes it can occur at those temps at or near ground level.  Typically if its going to be 28 and breezy it wont get that cold however if there is no inversion or you are in a cold air mass then it will just be 28 and breezy.  I grow fruit trees and in the spring predominantly (sometimes in the winter) we run what we call wind machines.  They are basically big fans on like a 30' tower that suck warm air down from higher up and push the cold air out from ground level bring temps up and keeping them above critical for killing our flower buds.  Most times this works but sometimes it just gets too cold, or there is no warm air up high (no inversion layer), and things freeze.  Hope this helps.
Link Posted: 10/23/2013 6:01:27 PM EDT
Hell, I live in La, where frost appears once every 20,000 years and I know the answer to that question.

Link Posted: 10/23/2013 6:02:05 PM EDT
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Originally Posted By beardog30:
Is this a serious question?
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Yes.
Link Posted: 10/23/2013 6:03:09 PM EDT
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Originally Posted By BustinCaps:
My answer depends on how you voted in the intelligence thread.
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Now that was just mean spirited. Bless your heart.
Link Posted: 10/23/2013 6:03:41 PM EDT
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Originally Posted By Xringlover:


Yes.
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Originally Posted By Xringlover:
Originally Posted By beardog30:
Is this a serious question?


Yes.



Yes,it is possible.
Link Posted: 10/23/2013 6:04:09 PM EDT
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Originally Posted By ElevenBravo87:
yes its possible.

You trying to protect plants?
View Quote


Yes.

Trying to keep some garden plants alive over the next few days until they can "weather the storm".
Link Posted: 10/23/2013 6:04:56 PM EDT
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Originally Posted By Xringlover:


Yes.

Trying to keep some garden plants alive over the next few days until they can "weather the storm".
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Originally Posted By Xringlover:
Originally Posted By ElevenBravo87:
yes its possible.

You trying to protect plants?


Yes.

Trying to keep some garden plants alive over the next few days until they can "weather the storm".


If you run a sprinkler they won't freeze.
Link Posted: 10/23/2013 6:05:15 PM EDT
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Originally Posted By hawk78:
Is frost possible at a temperature below that which water freezes at?  Yes, in fact it is.
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Need some details. That was why I was asking!

Link Posted: 10/23/2013 6:05:38 PM EDT
[Last Edit: 10/23/2013 6:06:13 PM EDT by sd0324]
Sprinkler is your friend here..
Link Posted: 10/23/2013 6:05:53 PM EDT
Link Posted: 10/23/2013 6:06:18 PM EDT
frost possible at 32 or below, it has nothing to do with wind.
Link Posted: 10/23/2013 6:06:56 PM EDT
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Originally Posted By Xringlover:


Yes.

Trying to keep some garden plants alive over the next few days until they can "weather the storm".
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Originally Posted By Xringlover:
Originally Posted By ElevenBravo87:
yes its possible.

You trying to protect plants?


Yes.

Trying to keep some garden plants alive over the next few days until they can "weather the storm".


Cover them or if potted bring them in
Link Posted: 10/23/2013 6:08:42 PM EDT
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Originally Posted By BustinCaps:
My answer depends on how you voted in the intelligence thread.
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Link Posted: 10/23/2013 6:10:11 PM EDT
It can actually frost with the air temp above freezing.
Link Posted: 10/23/2013 6:10:22 PM EDT
Down here further south (Think Houston but 45 miles north) have seen it many a times.


Maybe high 30's very low 40's and have frost on the windshield in the morning.

OP, have you never seen this? Really?

How old are you OP? 10? 12?

Link Posted: 10/23/2013 6:11:34 PM EDT
Yes. 25 degrees this morning, all cars with a thick layer of frost.
Link Posted: 10/23/2013 6:12:58 PM EDT
If you have a clear night with no wind, that is no clouds AND the dew point gets under 32 F, you can have frost even if the air temperature is above freezing.  This is radiational cooling.  Deep space is at 4 Kelvin, the plants are at around 290 Kelvin.  Heat transfers via black body radiation at the power of 5.67 x 10-8  Watts/meter2 K4where K is the difference between the hot item and cold sink raised to the 4th.  So a 1 square meter black surface at 290 K would radiate over 350 watts of heat on a clear night.



Now plants are not perfect black body radiators, give them an emittance of 0.35 or so...depends on species etc.

       
 
Link Posted: 10/23/2013 6:14:02 PM EDT
Normally,the 1st frost can occur if the sky is clear,and the air is still. I have seen frost  when it the thermometer showed 35F,but certain areas of yard were colder. I have had success with washing plants the night before with hose,then washing them again BEFORE the sun comes up. I have a greenhouse,and If I miss judge a frost,and don't get cuttings in in time can lose a whole years crop.
Link Posted: 10/23/2013 6:15:01 PM EDT
[Last Edit: 10/23/2013 6:31:45 PM EDT by L_JE]
Frost can less likely at 33ºF and a little bit of wind than at 35ºF and no wind.

Thermal radiation.

ETA:
Keith_J, sometimes I hate you.  But, I mean that in the nicest possible way.

The atmosphere is a participating medium with respect to thermal radiation, especially at longer, emissive wavelengths.  But, Keith's example is a good first-order approximation.  350+ W/m^2 lost to the blackness of space, compared to roughly1360 W/m^2 of incident (normal) solar radiation from the sun.  But, integrate over time, and you have a rather curious balance.
Link Posted: 10/23/2013 6:20:37 PM EDT
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Originally Posted By bondryan:
I would assume that yes it can occur at those temps at or near ground level.  Typically if its going to be 28 and breezy it wont get that cold however if there is no inversion or you are in a cold air mass then it will just be 28 and breezy.  I grow fruit trees and in the spring predominantly (sometimes in the winter) we run what we call wind machines.  They are basically big fans on like a 30' tower that suck warm air down from higher up and push the cold air out from ground level bring temps up and keeping them above critical for killing our flower buds.  Most times this works but sometimes it just gets too cold, or there is no warm air up high (no inversion layer), and things freeze.  Hope this helps.
View Quote


Thank you! It does!

:)
Link Posted: 10/23/2013 6:24:06 PM EDT
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Originally Posted By _Matt_:


Cover them or if potted bring them in
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Originally Posted By _Matt_:
Originally Posted By Xringlover:
Originally Posted By ElevenBravo87:
yes its possible.

You trying to protect plants?


Yes.

Trying to keep some garden plants alive over the next few days until they can "weather the storm".


Cover them or if potted bring them in


Will be doing that over the next few days.

Seems like at the years end, it is always a struggle to keep plants alive until they finish producing.

Good advice Matt!
:)
Link Posted: 10/23/2013 6:27:52 PM EDT
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Originally Posted By Huffskies:
It can actually frost with the air temp above freezing.
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With no wind, I have seen it happen.
Seems like when a little wind is present, it doesn`t happen....for the MOST part.
Can`t tell you with any certainty, what the specific temp. was at the time. Just remembered it was near freezing. Sometimes frost, other times none.




Link Posted: 10/23/2013 6:29:49 PM EDT
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Originally Posted By AJE:
Wind doesn't effect the temperature of water freezing, either.
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Sure it does.


But by evaporation, not windchill.
Link Posted: 10/23/2013 6:30:17 PM EDT
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Originally Posted By Codyboy:
Down here further south (Think Houston but 45 miles north) have seen it many a times.


Maybe high 30's very low 40's and have frost on the windshield in the morning.

OP, have you never seen this? Really?

How old are you OP? 10? 12?

View Quote


Late 40`s.
Temps like what you just described, no frost ever.
At least not here anyway.


Link Posted: 10/23/2013 6:32:11 PM EDT
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Originally Posted By 20iner:
Yes. 25 degrees this morning, all cars with a thick layer of frost.
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If it hits that here, I am sure it won`t matter.
Everything would be toast at that point.

Link Posted: 10/23/2013 6:32:35 PM EDT
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Originally Posted By Xringlover:


If it hits that here, I am sure it won`t matter.
Everything would be toast at that point.

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Originally Posted By Xringlover:
Originally Posted By 20iner:
Yes. 25 degrees this morning, all cars with a thick layer of frost.


If it hits that here, I am sure it won`t matter.
Everything would be toast at that point.



Not if you run a sprinkler.
Link Posted: 10/23/2013 6:34:57 PM EDT
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Originally Posted By Keith_J:
If you have a clear night with no wind, that is no clouds AND the dew point gets under 32 F, you can have frost even if the air temperature is above freezing.  This is radiational cooling.  Deep space is at 4 Kelvin, the plants are at around 290 Kelvin.  Heat transfers via black body radiation at the power of 5.67 x 10-8  Watts/meter2 K4where K is the difference between the hot item and cold sink raised to the 4th.  So a 1 square meter black surface at 290 K would radiate over 350 watts of heat on a clear night.

Now plants are not perfect black body radiators, give them an emittance of 0.35 or so...depends on species etc.
         
View Quote

Keith,

A clear night and no wind near freezing, frost.

It stinks to see those poor little plants with shriveled up leaves in the afternoon after a night of those conditions.

Link Posted: 10/23/2013 6:38:48 PM EDT
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Originally Posted By TriggerM:
Normally,the 1st frost can occur if the sky is clear,and the air is still. I have seen frost  when it the thermometer showed 35F,but certain areas of yard were colder. I have had success with washing plants the night before with hose,then washing them again BEFORE the sun comes up. I have a greenhouse,and If I miss judge a frost,and don't get cuttings in in time can lose a whole years crop.
View Quote


My problem is I depend on a weatherman to let me know when it may be time to cover the plants to protect against frost.
They miss the mark sometimes. At that point......................maybe next year.
Link Posted: 10/23/2013 6:43:28 PM EDT
That whole FPNI phenomenon is just un-fucking-canny.
Link Posted: 10/23/2013 6:43:34 PM EDT
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Originally Posted By BustinCaps:


Not if you run a sprinkler.
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Originally Posted By BustinCaps:
Originally Posted By Xringlover:
Originally Posted By 20iner:
Yes. 25 degrees this morning, all cars with a thick layer of frost.


If it hits that here, I am sure it won`t matter.
Everything would be toast at that point.



Not if you run a sprinkler.


Never have tried that BC.

I have seen that done in some movies on TV but always thought it was Hollywood.
Never really gave it any thought.

Link Posted: 10/23/2013 6:47:36 PM EDT
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Originally Posted By Xringlover:


Yes.
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Originally Posted By Xringlover:
Originally Posted By beardog30:
Is this a serious question?


Yes.

I am really at a loss for words then.
Link Posted: 10/24/2013 2:54:21 AM EDT

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Originally Posted By beardog30:





I am really at a loss for words then.
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Originally Posted By beardog30:



Originally Posted By Xringlover:


Originally Posted By beardog30:

Is this a serious question?




Yes.



I am really at a loss for words then.
Plants CAN freeze if there is no wind and low humidity even if temperatures do NOT get below freezing.  Sounds like you might need to do some reading.  I would suggest John Tyndall's Contributions to Molecular Physics in the Domain of Radiant Heat.  It has been around for 130+ years so I am surprised you have not yet picked it up.  Mention Arnica Montana and people look at you funny.  But mention Wolfsbane and everyone is a Bram Stoker expert.  Well, Tyndall's book is a bit older than Dracula





 
Link Posted: 10/25/2013 8:59:42 PM EDT
It`s 31 here tonight. Predicted temps. are 28. The previous lowest in 2 nights of frost predictions where the weatherman said it would get to 33 one night and 32 the next were the first night, 37 and low frost. The second night, 37 and more frost. Both nights were cloudy and a little wind blowing.

Clear skies and no wind tonight. Plants are covered in plastic.

Here comes the litmus test.
Link Posted: 10/25/2013 11:24:56 PM EDT
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Originally Posted By Xringlover:
It`s 31 here tonight. Predicted temps. are 28. The previous lowest in 2 nights of frost predictions where the weatherman said it would get to 33 one night and 32 the next were the first night, 37 and low frost. The second night, 37 and more frost. Both nights were cloudy and a little wind blowing.

Clear skies and no wind tonight. Plants are covered in plastic.

Here comes the litmus test.
View Quote

The atmosphere can be above freezing and frost can form. It has to do with the air pressure, humidity, and the difference in temperature between the air and the objects on the ground. Complex stuff, as KeithJ pointed out. I think the warmest I can recall frost damage to crops was 38 degrees in northern Florida and southern Georgia. It was an odd event, but it happens. The term is hoar frost.
Link Posted: 10/26/2013 2:26:46 AM EDT
Th same thing causes dew to form even when there is no fog.  Plants, roofs and anything with a high emissivity cools down to the dew point of the air.  Bam.



Now, if the air's dew point is below freezing, the humidity forms frost instead of dew drops.  This is deposition which forms highly reflective crystals of water, making the frost very white instead of glare ice.



Emissivity is the characteristic of releasing radiant energy.  It is analogous to "blackness" or the ability of an object to get warm from radiant energy.  It is the opposite of reflectivity.

       
 
Link Posted: 10/26/2013 2:53:26 AM EDT
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Originally Posted By Keith_J:
If you have a clear night with no wind, that is no clouds AND the dew point gets under 32 F, you can have frost even if the air temperature is above freezing.  This is radiational cooling.  Deep space is at 4 Kelvin, the plants are at around 290 Kelvin.  Heat transfers via black body radiation at the power of 5.67 x 10-8  Watts/meter2 K4where K is the difference between the hot item and cold sink raised to the 4th.  So a 1 square meter black surface at 290 K would radiate over 350 watts of heat on a clear night.

Now plants are not perfect black body radiators, give them an emittance of 0.35 or so...depends on species etc.
         
View Quote

Bill nye?
Link Posted: 10/26/2013 3:15:23 AM EDT
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Originally Posted By hawk78:
Is frost possible at a temperature below that which water freezes at?  Yes, in fact it is.
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Yup
Link Posted: 10/26/2013 8:04:25 AM EDT
Looking up some thermal emissivity values for various leaves, I was surprised to see they are all pretty consistent around 0.96.
http://www.icess.ucsb.edu/modis/EMIS/html/leaf.html

For thermal radiation losses as a function of sky conditions:
75 W/m^2 for a clear desert night, for a typical sky-facing surface at 80ºF
60 W/m^2 with atmospheric humidity
40 W/m^2 with 50% cloud cover
7 W/m^2 with completely overcast skies
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1502-05.pdf
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