What is the highest temp at which frost can form?

[Cocoa Beach Jason]

I am about to set an alarm on my weather station to alert me when the temp reaches a certain low. My goal is to be ready to cover palms when the theeat of frost is real. I know frost often forms above 32. What is the highest temperature at which you have seen frost form? Thank you in advance for your input.

[Zeeth]

The NWS uses 36˚ as their guideline, but it can form at warmer temps. 

[_Keith]

Up to around 38 seems right to me.

[Monòver]

I have seen frost in my garden around 36°.

[Mr. Coconut Palm]

I think somewhere between 36F and 38F depending on the conditions and the surface it's forming on (some surfaces cool off faster and more so than others).  I have had heavy frost before with temps around 35F or 36F for the low.

[Tropicdoc]

Seen frost at 38 here especially on rooftops but I guess we don't really care about that frost 

[Mr. Coconut Palm]

I don't really have any Zone 11 palms, but all my in ground Zone 10 palms seem to not be phased too much by frost.  My young coconut palms in pots that I used to have, were only slightly frostbitten with frosty temps in the mid thirties, but once they were in the ground, they got wiped out by two very chilly (but no freezes at my place) prolonged winters in which they were exposed to 2 weeks straight twice each of those winters to low temps in the 30's and 40's and highs only in the 40's and low to mid 50's with some drizzle and light rain.  Coconut palms can take some frost and even some light freezes easier than they can take 2 straight weeks of chilly damp weather.  This happened when our normal lows should have been over that period of time between 50F and 54F and our normal highs should have been between 65F and 69F.  Anyway, most Zone 10 palms that I know of can handle some frost even a couple of times in one winter without too much injury, but Zone 11 palms I am sure is a whole different story.  I have a 6ft. tall Archontophoenix alexandrae on the north side of my house that experienced 9 mornings of frost (5 of which were heavy frosts) over the span of two winters when it was even smaller than its 6ft. height now, with all its leaves totally exposed to the frost without any injury whatsoever.  The temps during these frosts were between 33.6F and 38F.

[nachocarl]

36F almost always guarantees frost around here in open spaces. 

 

[Hammer]

When it hits 37°f here I can pretty much bet on some frost. Especially if the air is still.

Anything predicted below 40°f I start to sweat and take whatever precautions I can.

[Ed in Houston]

 

There is some patchy frost on some roof tops in my neighborhood this morning with a low of 38.

Ed in Houston

[Walt]

95% of all cold weather in my area that causes foliage damage is from radiational freezes, and that is when dew forms on the foliage and then freezes. Most folks call it frost but it's really black ice, as technically, frost can only form (change of state from vapor directly to a solid) below 32F/0C. But that is all beside the point for all intents and purposes. After living in south central Florida for more than 18 years I can attest to ice on my exposed (out in the open) palms and tropical plants foliage when the ambient air temperature between 4-6 feet above the ground is in the mid to high 30s.

According the below website, and under the right conditions, frost can actually form on super cooled surfaces when the ambient air temperature above said surfaces is as high as 42 degrees F:

http://www.weather.gov/arx/why_frost

 

[Tropicdoc]

On January 9, 2016 at 5:55:16 PM, Mr. Coconut Palm said:

I don't really have any Zone 11 palms, but all my in ground Zone 10 palms seem to not be phased too much by frost.  My young coconut palms in pots that I used to have, were only slightly frostbitten with frosty temps in the mid thirties, but once they were in the ground, they got wiped out by two very chilly (but no freezes at my place) prolonged winters in which they were exposed to 2 weeks straight twice each of those winters to low temps in the 30's and 40's and highs only in the 40's and low to mid 50's with some drizzle and light rain.  Coconut palms can take some frost and even some light freezes easier than they can take 2 straight weeks of chilly damp weather.  This happened when our normal lows should have been over that period of time between 50F and 54F and our normal highs should have been between 65F and 69F.  Anyway, most Zone 10 palms that I know of can handle some frost even a couple of times in one winter without too much injury, but Zone 11 palms I am sure is a whole different story.  I have a 6ft. tall Archontophoenix alexandrae on the north side of my house that experienced 9 mornings of frost (5 of which were heavy frosts) over the span of two winters when it was even smaller than its 6ft. height now, with all its leaves totally exposed to the frost without any injury whatsoever.  The temps during these frosts were between 33.6F and 38F.

That experience with frost is very different from others on this forum especially with king palms. I planted my kings under canopy.

[Mr. Coconut Palm]

2 minutes ago, Tropicdoc said:

That experience with frost is very different from others on this forum especially with king palms. I planted my kings under canopy.

That particular King palm is from Southern California.  I ordered it online about 7 or 8 years ago.  It was just about a foot tall when I got it.  I grew it in a pot in Bryan for a few years, keeping it in my greenhouse over the winter, but it has been in the ground at my place in Corpus Christi for about 2.5 years now.  I think it would be at least 8 to 10ft. tall in overall height now, if we hadn't had the worst drought in Texas' history about a year and a half ago, and if I could have kept it more watered over that time.  I have come to find out that they like a lot of water, and since our way above normal rainfall over this past year, it is really happy and growing twice as fast now.  The ones I have seen here in Corpus Christi are all exposed and look like they are doing fine.  Mine and the others are grown in full sun here.

[George Sparkman]

I see it around 38/39f under the right (wrong) conditions.

[SubTropicRay]

On a cold surface like a car or house rooftop, 37-40F in my experience. 

[displaced_floridian]

I have had frost on car tops with temps as high as 41* on calm clear nights.

[Tropicdoc]

23 hours ago, Mr. Coconut Palm said:

That particular King palm is from Southern California.  I ordered it online about 7 or 8 years ago.  It was just about a foot tall when I got it.  I grew it in a pot in Bryan for a few years, keeping it in my greenhouse over the winter, but it has been in the ground at my place in Corpus Christi for about 2.5 years now.  I think it would be at least 8 to 10ft. tall in overall height now, if we hadn't had the worst drought in Texas' history about a year and a half ago, and if I could have kept it more watered over that time.  I have come to find out that they like a lot of water, and since our way above normal rainfall over this past year, it is really happy and growing twice as fast now.  The ones I have seen here in Corpus Christi are all exposed and look like they are doing fine.  Mine and the others are grown in full sun here.

Good growing! I hope my kings are as hardy as yours. I'm still glad they are under canopy though I may rode an "Inge" king this spring supposedly from a hardy mom.

[Mr. Coconut Palm]

I hope your kings do well too.  I think the difference is you live in a 9A climate, and I live in a 10A climate.  I moved here from Bryan, TX 3.5 years ago to get away from the borderline 8B/9A climate there that I and my tropical plants couldn't handle.  It's nice being able to grow things in my yard now that I could only overwinter in a greenhouse up there.

[Funkthulhu]

Technically, at 100Kbars solid water can form at over 600Kelvin...

[Hammer]

6 hours ago, Funkthulhu said:

Technically, at 100Kbars solid water can form at over 600Kelvin...

At that temp I am thinking palms might show damage pretty quickly.

[steve99]

I  would have thought frost can only occur on the ground or whatever when the temp is at or below freezing?      

Edited January 15, 2016 by steve99
grammer

[Pando]

9 hours ago, steve99 said:

I  would have thought frost can only occur on the ground or whatever when the temp is at or below freezing?      

It's true that frost can only form when the surface is at or below freezing.

I think they are talking about the surrounding air temp, which can be a few degrees above the temperature of the surface where frost can form. This is due to heat energy radiating off the surface, and due to evaporation of water from the surface, making it a few degrees colder than the surrounding air. It's the same effect of feeling cold when your skin is wet - the evaporating water takes energy from the skin, making it cooler.

Edited January 15, 2016 by Pando

[Walt]

On cold radiational cooling nights I sometimes walk my property to check how the air temperature (about 4-5 feet above the ground) varies. I have a pocket digital thermometer that reads in 1/10th degrees, both Centigrade and Fahrenheit. The thermometer is fast to respond to temperature changes. It may read 35 degrees F air temperature, then when I touch the thermometer to a flat surface exposed to the sky, it may read  up to 10 degrees lower. 

Years ago I observed the difference between air temperature and exposed flat surface areas facing up to the sky. At that time I had an Oregon Scientific base station digital thermometer with three remote sensors. I placed one sensor flat on the ground facing skyward. I then placed another sensor flat on the ground facing skyward, but then placed a lawn chair over top of it. The lawn chair had a webbed seat, so it had openings in it. Still, the temperature of the sensor was a few degrees warmer than the totally exposed sensor only two feet away.

[Mr. Coconut Palm]

Walt,

So the moral of your story is canopy (i.e. the lawn chair) truly does make a difference, in both the formation of frost and absolute lowest temps on any given morning.

John

[Mr. Coconut Palm]

Walt,

I have a question for you.  You have stated several times that higher elevation where you live REALLY makes a difference, just like it does in Southern California.  Does that apply to a ditch in front of one's house?  I have a ditch that is about 13ft. to 14 ft. wide and about 24" to 26" deep that is adjacent to the street.  The side of the ditch that is closest to my house starts about 30 ft. from my house.  I want to plant two coconut palms about 8 ft. to 10 ft. away from the ditch on the backside of a Sea grape.  Do you think this is enough elevation to make a difference on a chilly clear night, or would it actually draw colder air from closer to the house towards my palms?  I know it wouldn't make the SIGNIFICANT difference like what you experience in your area, but do you think it could make a positive difference by say 2F to 4F on the upside for my yard and my palms?

John

[Walt]

1 hour ago, Mr. Coconut Palm said:

Walt,

So the moral of your story is canopy (i.e. the lawn chair) truly does make a difference, in both the formation of frost and absolute lowest temps on any given morning.

John

John,

Almost any canopy will reflect long-wave radiation (rising ground heat) back down towards the ground, keeping the air between the ground and canopy several degrees (maybe more) warmer. I walk around with either my pocket thermometer or thermal couple and as soon as I go from out in the open to under canopy the temperature goes up. Canopy definitely decreased the chance and/or severity of frost. I've found that almost all of my tender species of palms (leaves) can take five degrees lower temperature if they aren't exposed to frost. It's the frozen dew/frost that does more of the damage than the low temperature. This at least applies to my area where the duration of low temperature is short and doesn't go lower than 25 degrees.

 

[Walt]

1 hour ago, Mr. Coconut Palm said:

Walt,

I have a question for you.  You have stated several times that higher elevation where you live REALLY makes a difference, just like it does in Southern California.  Does that apply to a ditch in front of one's house?  I have a ditch that is about 13ft. to 14 ft. wide and about 24" to 26" deep that is adjacent to the street.  The side of the ditch that is closest to my house starts about 30 ft. from my house.  I want to plant two coconut palms about 8 ft. to 10 ft. away from the ditch on the backside of a Sea grape.  Do you think this is enough elevation to make a difference on a chilly clear night, or would it actually draw colder air from closer to the house towards my palms?  I know it wouldn't make the SIGNIFICANT difference like what you experience in your area, but do you think it could make a positive difference by say 2F to 4F on the upside for my yard and my palms?

John

John, IMO, the ditch would only make a marginal difference, if any.  When I'm talking about elevation, I'm estimating about one degree rise for every 10 feet in elevation. I based this on topographical maps I have for my area and measurements I've taken. But I've also found that the air layer stratification changes from one night to another. I don't fully understand it, other than to say the laws of physics dictate that the air will rise in temperature (from, say sea level) up to maybe 200 feet, then it starts to drop in temperature.

What puzzles me is that the University of Florida has 10 meter high weather towers spread across the state in rural areas (to help the agricultural folks). These towers have temperature sensors at 2 feet, 6 feet, and 30 feet. One night about five years ago I was checking various towers to see what the temperatures were. The Belle Glade tower showed a 14 degree temperature rise from 2 feet to 30 feet! But this was maybe around 9 p.m. Later that night that difference dropped way back to maybe only a couple of degrees.

http://fawn.ifas.ufl.edu/

[Mr. Coconut Palm]

Hey Walt,

Thanks for the info.  Like you said, the ditch benefit would probably be very marginal at best, but maybe at least would draw the colder air and frost down to the bottom of the ditch before it would effect my palms

John