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Author Topic: Summer Freeze  (Read 4673 times)

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Offline 2-Wheeler

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Summer Freeze
« on: September 18, 2006, 03:35:29 AM »
It's still summer, but I recorded sub-freezing temps early this morning in my garden!

I recently added an additional temp sensor to the Davis Leaf-Wetness/Soil Sensor unit. I set this extra temp sensor just a few inches above the ground level to be an early frost warning sensor. Sure enough, last night the "offical" low temp on the main Temp Sensor (located 5 feet above ground) was 34 F, but the new frost sensor showed 31 F. The main sensor is also in an open field, while the garden is in a different part of the yard.

The winds were totally calm and the sky was clear and our air was very dry last night so we were no where close to dew point. But this morning there was no visible frost on anything, yet most of my tender vegtables and annual flowers wilted up and died or showed signs of frost-bite. The new temp sensor revealed the hidden cause - subfreezing temps for a short time this morning.

-David Broberg   CWOP#: CW5670 / CoCoRaHS #CO-BO-218

Offline Weather Display

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Re: Summer Freeze
« Reply #1 on: September 18, 2006, 04:15:00 AM »
dry air can get colder at night much  easier than moister air (where the condensation to dew releases heat )  :wink:
given the right condition: no wind, clear skies and dry cold air to start off with, you can get a frost alright  8O

Offline administrator

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Re: Summer Freeze
« Reply #2 on: September 18, 2006, 11:18:22 AM »
given the right condition: no wind, clear skies and dry cold air to start off with, you can get a frost alright  8O

Would it be a frost? It would certainly be freezing temperatures, but I had always assumed that frost conditions implied that there was actually frozen water particles around? In very dry air there might not be any frozen water deposited, so would it be called a frost?
Chris
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Offline jbrooks987

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Re: Summer Freeze
« Reply #3 on: September 18, 2006, 05:56:52 PM »
There are still frozen water particles around - inside the plants.

Offline Weather Display

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Re: Summer Freeze
« Reply #4 on: September 18, 2006, 07:23:37 PM »
hummmm
i would think a frost is when the grass temperature gets below freezing...?
(which at grass level can happen surprisingly easily,even with a slight breeze blowing)

Offline dendrite

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Re: Summer Freeze
« Reply #5 on: September 19, 2006, 04:06:20 AM »
White frost is caused by deposition. Frost is basically dew that FORMS at temps below 32F as the water vapor deposits on objects directly into the frozen state. Then you have frozen dew which forms as normal dew (temps above 32F), yet after the liquid dew forms the temp at the surface continues to fall until it reaches below freezing. Then the droplets of dew freeze into little balls of ice. If you want to compare the 2 processes to precip form then frost:snow as frozen dew:sleet.

Ground/air temps below 32F with no frost/frozen dew is simply just a freeze. Those are the big plant killers. Frost and dew can actually work to insulate some of the heartier plants. A freeze provides them no such luck.
« Last Edit: September 20, 2006, 12:57:27 PM by dendrite »

Offline 2-Wheeler

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Re: Summer Freeze
« Reply #6 on: September 20, 2006, 01:52:37 AM »
Thanks for the clarification dendrite, it matches my observations. As I said it was a "Summer Freeze" since there was no visible frost, and yes it was a killer.  OTOH, this morning we had a light "Summer Frost" where there were ice crystals that formed on the roof of the garden shed, but not on the ground. The ground temps recorded 32. F. No additional damage to the garden was evident. 

For those who live in areas that don't freeze, it is quite common for the frost to form on the roof-tops before it it forms on the grass. Since it is usually colder on the ground, I don't completely understand that phenomenon.

-David Broberg   CWOP#: CW5670 / CoCoRaHS #CO-BO-218

Offline Weather Display

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Re: Summer Freeze
« Reply #7 on: September 20, 2006, 02:03:38 AM »
the ground temperature will be acting as a thermal insulator, i.e heat sink,i.e the ground temperature warmer the air just above it a bit..... compared to the roof, which does not have much of a heat sink...and wtih infra red /longwave radiation heast loss to space, and quickly get below freezing temperature( i.e relatively thin in thickness)


Offline up10ad

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Re: Summer Freeze
« Reply #8 on: September 22, 2006, 07:34:35 PM »
dendrite, thanks for the explanation of the differences beteween frost and frozen dew.  I seem to learn something every visit here.  :signthanks:
Rick
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Offline 7andy

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Re: Summer Freeze
« Reply #9 on: October 05, 2006, 11:50:48 AM »
For those who live in areas that don't freeze, it is quite common for the frost to form on the roof-tops before it it forms on the grass. Since it is usually colder on the ground, I don't completely understand that phenomenon.

...same reason I guess that causes car windscreens to frost over, and nothing else - happens a lot where I live in Southern UK

 

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