Author Topic: formula for calculating if a frost occurred?  (Read 3241 times)

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Offline JHellis

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formula for calculating if a frost occurred?
« on: March 05, 2013, 08:54:52 AM »
Hi

I do not seem to be able to find a formula for calculating from Temperature (and anything else) if a frost has occurred.

I am using a Vantage VUE to record data.

I store daily records in an Excel file; I would like to be able to get an automatic record of frost days. I am now retired  =D> so don't get up so early in the morning, thus I miss some lighter frosts that have dissipated by the time I arise.  :-(

John
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Offline SLOweather

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Re: formula for calculating if a frost occurred?
« Reply #1 on: March 05, 2013, 10:39:28 AM »
I doubt that frost can be calculated just from temperature. It also depends a lot on still air and lack of cloud cover to allow for radiational cooling.

The simplest thing to do might be to add a ground level temperature probe, exposed to the sky, and see how those readings correlate with observed frost. A few years ago I did some informal experiments when an IR temperature gun and was very surprised at the temperature difference between the air and frosted surfaces.

Offline JHellis

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Re: formula for calculating if a frost occurred?
« Reply #2 on: March 05, 2013, 11:27:23 AM »
Thanks SLOwether

I was actually wondering if something like-
if temp <0 and Dew Point<0 then FROST

though I very much doubt it would be that simple.

I am thinking that some time this year I will add a 1 Wire system. Perhaps something might be possible using your ground level Temp idea.
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Offline JACK10562

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Re: formula for calculating if a frost occurred?
« Reply #3 on: March 05, 2013, 11:32:24 AM »
There might be too many variables to incorporate into a formula. I have seen some pretty heavy frost coating on my east/west-facing (unheated) porch roof, and temps recorded never dipped below 35˚f.  Other times overnight temps went well below freezing and no frost at all was evident.

Sky cover, wind/direction or lack thereof, and relative humidity/dew point all  must play a factor if there is going to be frost.
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Offline JHellis

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Re: formula for calculating if a frost occurred?
« Reply #4 on: March 05, 2013, 02:35:28 PM »
Thanks Jack10562

Unfortunately you have confirmed my thoughts, and there was I thinking I could enjoy my morning tea in bed, sometimes I hate weather recording  :lol:
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Offline JHellis

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Re: formula for calculating if a frost occurred?
« Reply #5 on: October 25, 2015, 12:00:19 PM »
Calculation-wise, the Frost Point (FP) is roughly 9/10's of the Dew Point (DP) value, ie:  FP ≈ 0.9×DP

SOURCE: Walter J. Saucier, PRINCIPLES of METEOROLOGICAL ANALYSES, 1955/1983/2003, Dover Publications, "Table 1.05b - Dewpoint-Frostpoint Conversion", page 11.

Thank you Old Tele Man, my snug winter mornings look assured.

John
Davis Vantage Vue
Belfryboy Data Logger