Frost

[wd40]

Does Cumulus, in the forecast, warn against a possible frost? Not sure if it is even practical if it doesn't.

I realize ground and object temperatures is an unmeasured factor in the fine off set stations and any forecast would be a good guess.

A search found post about air frost and a fix on the forum.

Randy

[daj]

Does Cumulus, in the forecast, warn against a possible frost?

No, see the wiki for the possible combinations for the forecast

http://wiki.sandaysoft.com/a/Forecast_webtag

There are a number of pieces of logic to try and predict frost, but nothing in Cumulus itself. I think I use some logic on my site (will check)

[steve]

There's a rule of thumb that I'm struggling to remember based on the dew point at a certain time in the afternoon, but I think there are other factors to take into account.

[wd40]

From what I have read, this guy as a handle on it from a practical observation view point, but no analytical information.
http://www.organicgardening.com/feature ... 70,00.html

I will dig around a bit and see what I can find.

Thanks for the reference Steve. I will look that up.

Randy

[beteljuice]

Frost is not just temperature dependent, but also RH.

It is very unlikely that your kit is accurate or even measures 98% +

A rough rule of thumb (assuming your temp. sensor is at 'regulation' height) is if temp is <4 C and >0 C = CHANCE of (ground) frost (if 0 or less really frozen)

Air frost you really need to have accurate RH readings !

[wd40]

I did a little more digging and found this reference but could not find it on line. Not sure if this publication has any use but the title is interesting.

D. R. Cook, "Predicting Frost At Your House",
Journal of Meteorology, Vol. 15, no. 153, November 1990.
Published in Great Britain.

Here is the sight I found the information.
http://www.newton.dep.anl.gov/askasci/w ... a00039.htm

[Gina]

Interesting

[nking]

There's a rule of thumb that I'm struggling to remember based on the dew point at a certain time in the afternoon, but I think there are other factors to take into account.

There are many factors but like you I had thought that the dewpoint recorded at sunset will give an approximation of the lowest overnight temperature.

[steve]

There are many factors but like you I had thought that the dewpoint recorded at sunset will give an approximation of the lowest overnight temperature.

Yes, it's something like that. It only applies in winter and assumes no passing fronts etc.

[PaulMy]

The following extracted from Davis Dew Point Help Topics:

Data: Dew Point
Dew-point is the temperature to which air must be cooled for saturation (100% relative humidity) to occur, providing there is no change in water content. The dew-point is an important measurement used to predict the formation of dew, frost, and fog. If dew-point and temperature are close together in the late after noon when the air begins to turn colder, fog is likely during the night. Dew-point is also a good indicator of the air’s actual water vapor content, unlike relative humidity, which takes the air’s temperature into account. High dew-point indicates high vapor content; low dew-point indicates low vapor content. In addition a high dew-point indicates a better chance of rain and severe thunder storms. You can even use dew-point to predict the minimum overnight temperature. Provided no new fronts are expected overnight and the afternoon Relative Humidity ³ [actual Davis text used the > sign which seemed to have been converted to 3 in my copy and paste] 50%, the afternoon’s dew-point gives you an idea of what minimum temperature to expect overnight, since the air is not likely to get colder than the dew-point anytime during the night.

Dew point is not stored in archive memory or the database. Dew point is calculated whenever it is displayed. If you edit the temperature or humidity value, the dew point will change as well.

Paul

[wd40]

Paul , that is some good information.

I've done a pretty good bit of digging and it does seem practical for a PWS program to offer a message like, "Conditions for patchy frost may be possible by morning."
Like the Cumulus forecast, the message would be a best guess with the information available from the station.

I will see if I can put together a simple manual input program or chart for my own use then keep track of when it was right and wrong for the fall.

The last programing I did was with FORTRAN and punch cards. How I hated those old days when hours of work would get you an overnight computer run that just said syntax error.

Randy