Page 3 of 3

Re: Fire Weather, a new approach

Posted: Sat 21 Sep 2019 10:56 am
by freddie
Interesting stuff. I would have thought it would be easier to use the station pressure of the PWS (not the sea level pressure). That should be sufficient to calculate saturated vapour pressure without involving altitude.

Re: Fire Weather, a new approach

Posted: Sat 21 Sep 2019 3:41 pm
by HansR
Ehmm.. OK, how would you calculate the saturated water vapour pressure from the station pressure (that is: the atmospheric pressure at station level)?

Re: Fire Weather, a new approach

Posted: Sat 21 Sep 2019 4:17 pm
by freddie
As you said in your blog, you already have the station relative humidity. You also have the station temperature.
So, with station-level pressure, do you not have everything you need - and have no need to know altitude?

Re: Fire Weather, a new approach

Posted: Sat 21 Sep 2019 5:12 pm
by HansR
I am not sure about what you are saying, but even if I measure RH and temperature, I still do not know my saturation pressure of water at that temperature so I have to calculate that. Atmospheric pressure is not in that equation. The relation - approximation of the P(sat) of water - is not described by atmospheric pressure, only by temperature. And that approximation is at sealevel so I need to correct for the altitude. These are not simple relationships. I may be wrong, but than I need some good convincing ;)

Re: Fire Weather, a new approach

Posted: Sat 21 Sep 2019 6:18 pm
by freddie
Yes, I understand that the relationship is complex. Okay, I'll rephrase what I am asking: if there is no dependence on pressure, then why does altitude matter?

Re: Fire Weather, a new approach

Posted: Mon 23 Sep 2019 8:12 am
by HansR
freddie wrote:
Sat 21 Sep 2019 6:18 pm
[...] I'll rephrase what I am asking: if there is no dependence on pressure, then why does altitude matter?
Sorry I thought I had answered, but apparently forgot to press submit :(

[edit:] Wrong note, so I better remove it.

[edit:] And to continue on this: the solution probably lies in the Psychrometric constant.

to be cont.

Re: Fire Weather, a new approach

Posted: Thu 26 Sep 2019 10:14 am
by HansR
I guess you are right @freddie, I made an error in thinking.
Nevertheless, I had to change and add some other things. So here is the blog.

Re: Fire Weather, a new approach

Posted: Thu 26 Sep 2019 4:26 pm
by freddie
HansR wrote:
Thu 26 Sep 2019 10:14 am
I guess you are right @freddie, I made an error in thinking.
Vapour pressure is a difficult concept to get your head around.
You have opened up a can of worms with your blog. You have made the assumption that water is solid when its temperature is below zero. This is not actually true, as water can exist in liquid form at temperatures as low as -40 Celsius. This fact is very apparent in the atmosphere - especially when it comes to precipitation growth within clouds. If you Google "Bergeron Findeisen" you can read about this yourself. If I was you, I would use a linear "sliding scale" with your August-Roche-Magnus coefficients, such that at 0 they are the values for SVP over water, and at -15 they are the values for SVP over ice. That should capture the most likely values for SVP during those transition temperatures.

Re: Fire Weather, a new approach

Posted: Thu 26 Sep 2019 6:08 pm
by HansR
freddie wrote:
Thu 26 Sep 2019 4:26 pm
You have opened up a can of worms with your blog. You have made the assumption that water is solid when its temperature is below zero. [...]
No I have not, that is too easy a conclusion. I am well aware of the existence of undercooled water, but it is not purely the theoretical subject I try to solve. I am working on the subject of meteorology and Wild vegetations (forests, grasslands, bogs etc...) and their fires. And undercooled water does not exist too much in and around forests. It may exist on high altitudes, large surfaces, but not around in vegetation situations. On the contrary I would say. If you walk a forest on a winters day, you will see white ice crystal covered leaves on the ground and grass. Almost the physical view of the frozen wet bulb. And yes, vegetation has its own effect on humidity. But that's in a way the idea behind what I am building.

So in general, as long as the wet bulb thermometer will not freeze I am good with the assumption of the above zero ARM coefficients. Below freezing I use the other set. I do not see why I should use a sliding value of coefficient on temp. Never seen that in literature anywhere and I think a linear sliding scale does not match the exponential character of the equations.

And above that, I would like to stick to what is accepted science in meteorology. More or less. There is a load of equations (I found around 20 I think), all dealing with the estimation of the Saturated Vapour Pressure of water (I include one paper below). That is very nice and good for the scientists who earn their money. But look at the variations. Within the domain of where I write my software for, for a certain goal with a the temperature range of let's say -40 up to +50 degrees, all equations suffice. So I chose to use a more or less standardized equation. the ARM equation, which is apparently adopted by the WMO. That makes me happy, decision taken. I will not slide away in the confusion of choice. I make my choice and I publish what I do. I don't think my blog opens a can of worms, meteorology is just physical chemistry on a large scale.

Attachment:
190926 Saturation vapor pressure formulations.pdf

Re: Fire Weather, a new approach

Posted: Sun 06 Oct 2019 9:40 am
by HansR
A discussion on the pwsFWI will follow up here (if there is follow up ;)) so that other thread can be on the software.
Several discussions on fire weather have already been made in the other thread. The last one initiated this move.

Re: Fire Weather, a new approach

Posted: Thu 17 Oct 2019 9:31 am
by HansR
Now pwsFWI has reached a maturity stage it is interesting to look at how it behaves. And that gives some surprising results half October. Especially in San Sebastian where despite a lot of rainfall, strong winds and high temperatures drive the warning level to Moderate, which must be regarded as pretty high (without reaching real danger levels), given the time of year. I keep getting surprised by the result of my own invention!

And actually, I think the relation with reality is correct given the map of a more global Spanish fireweather system. pwsFWI is known (has a built in) delay on weather changes because wood does not dry that fast. If the weather situation in Basque country continues like this I presume the warning level will go to yellow. And that I guess is interesting and not very common over there with 1300 mm rain per year!

Knipsel.JPG
Knipsel2.JPG

Re: Fire Weather, a new approach

Posted: Fri 18 Oct 2019 7:55 am
by kocher
Good morning Hans and good morning everyone

I am very happy with your creation pwsFWI.
Especially effective is the calculation of the FWI with smooth changes after episodes of rain.

http://kocher.es/fwi-hans.php

http://kocher.es/NOAAsql/NOAA.php

Indeed there has been a curious situation during the last 5 days in San Sebastián (Basque Country, Spain), due to an episode of high temperatures and strong South wind (wind of African origin), but this is quite usual in San Sebastián, even in the month of December.
It usually corresponds to isolated episodes, without being able to consider it a trend.

The fires in our area are really rare although, cyclically, we can see some year with repeated fires, especially in the summer months coinciding again with times of heat and strong South wind.

I am very grateful for your interest in this very important issue and for your intelligent way of dealing with it.

Saludos desde San Sebastian

Javier

Re: Fire Weather, a new approach

Posted: Fri 18 Oct 2019 8:14 am
by HansR
Thanks for your reply and good info.

And yes, I check all sites daily for anomalies and if I don't understand I get curious :)
Curiosity is the basis for understanding this complex issue. I have seen many fires in my years in the South Of France (only 400 km of San Sebastian) and I became aware of how local meteorological conditions influence the susceptibility for fire more than anything else (and of course the availability of wood/fuel). That's where this program originated and that's why I sometimes ask clarification on the relation between what the program tells me and the actual situation around the station.
kocher wrote:
Fri 18 Oct 2019 7:55 am
I am very grateful for your interest in this very important issue and for your intelligent way of dealing with it.

Saludos desde San Sebastian

Javier
Thnx for the compliment!

Groeten vanuit Wagenborgen :)
Hans