### Author Topic: Chandler Burning Index  (Read 2593 times)

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#### Jáchym

• Meteotemplate Developer
• Forecaster
• Posts: 8596
##### Chandler Burning Index
« on: August 04, 2015, 11:05:36 AM »
Hi,

I was just experimenting with the Chandler Burning Index, I found the following formula to calculate it:

Code: [Select]
`CBI = (((110 - 1.373*RH) - 0.54 * (10.20 - T)) * (124 * 10^(-0.0142*RH)))/60`
RH - humidity in %, T = temp in deg C

in PHP I used:
Code: [Select]
`(((110 - 1.373 * \$RH) - 0.54 * (10.20 - \$T)) * (124 * pow(10,(-0.0142 * \$RH))))/60;`
Now the problem is, that when for example I plug in 97% for humidity and 19.4 C for temperature, I get a negative number... and likewise, some numbers lead to CBI way above 100. But the index description says it should lie between 0 and 100. I even checked this by manually calculating it on a calculator and that formula really gives these values.

Is my formula incorrect? Or can the values be negative and above 100?
« Last Edit: August 04, 2015, 11:32:57 AM by Jachym »

#### saratogaWX

• Forecaster
• Posts: 7808
• Saratoga, CA, USA Weather - free PHP scripts
##### Re: Chandler Burning Index
« Reply #1 on: August 05, 2015, 12:58:23 AM »
Yep, we use
Code: [Select]
`\$cbi = (((110 - 1.373 * \$rh) - 0.54 * (10.20 - \$ctemp)) * (124 * pow(10,(-0.0142* \$rh ))))/60;` which looks like yours
Code: [Select]
`\$cbi = (((110 - 1.373 * \$RH) - 0.54 * (10.20 - \$T)) * (124 * pow(10,(-0.0142 * \$RH))))/60;`
And this kind of code summarizes the CBI scale
Code: [Select]
`<?php         \$cbi = (((110 - 1.373 * \$hum) - 0.54 * (10.20 - \$temp)) * (124 * pow(10,(-0.0142 * \$hum))))/60;         if (\$cbi > "97.5") {              \$cbitxt = "EXTREME";         } elseif (\$cbi >="90") {              \$cbitxt = "VERY HIGH";         } elseif (\$cbi >= "75") {              \$cbitxt = "HIGH";         } elseif (\$cbi >= "50") {              \$cbitxt = "MODERATE";         } else         if (\$cbi < "50") {              \$cbitxt="LOW";         }         echo "Fire Risk for Today: " . \$cbitxt?>` (from here
Ken True/Saratoga, CA, USA main site: saratoga-weather.org
Davis VP1+ FARS, Boltek-PCI/NexStorm, microSferics ToA, Blitzortung RED, GRLevel3, WD, WL, VWS, Cumulus, Meteobridge/hub

#### Jáchym

• Meteotemplate Developer
• Forecaster
• Posts: 8596
##### Re: Chandler Burning Index
« Reply #2 on: August 05, 2015, 04:08:28 AM »
Thanks Ken,

Niko already messaged me about this, the equation is correct. What I didnt know is that the index can really be even negative or above 100, from the scale it seemed as though it should lie between 0 and 100, so I just wasnt sure I got the formula right.

Thanks.

#### dan43

• Member
• Posts: 31
##### Re: Chandler Burning Index
« Reply #3 on: January 12, 2017, 12:51:34 AM »
Jachym, sorry for this tardy reply, I just discovered the Chandler Burning Index and wanted to mention mine right now is -1.6, so your formula is probably correct.  I'm not sure if it's relevant to Canada, but I'm experimenting... it's kind of fun to see what numbers are generated each day and to learn what they mean.

#### Scalphunter

• Forecaster
• Posts: 2314
##### Re: Chandler Burning Index
« Reply #4 on: January 12, 2017, 02:08:35 AM »
Yep Jac I go from negative numbers this time of year  to 100 plus  during the summer when we  are at 13 -20% humidity and hitting the high 80's  to mid 90's. yep it gets that warm here with out any night cooling down time.

John

#### d_l

• Forecaster
• Posts: 1700
• Slide Mtn - Mt Rose
##### Re: Chandler Burning Index
« Reply #5 on: December 17, 2017, 12:19:36 PM »
If you have a Meteohub, you can do the Chandler Burning Index calculations by a virtual sensor.  Instructions on how to do that are here: http://forum.meteohub.de/viewtopic.php?f=9&t=12243.  What advantage is there to doing this?  Well you can plot a graph of the CBI over time such as I do on this web page (24 hrs in this case): https://www.tceweather.com/firedanger_graph.html  This allows you to get a feel for the trend of the CBI over time and do an eyeball projection estimate of what it might be in the future.  A future estimate of the CBI trend can be useful if a fire in a nearby area is threatening your place.

I modified the regular CBI PHP script to intake a special firedanger text file containing the just the CBI, temp, and hum that the Meteohub can output.  This data is used to generate the regular Smokey the Bear's fire danger graphic display which I've linked to a mouse click on the 24 hr CBI graph. So someone can click back and forth between the graph and the Smokey graphic.

--Dave--

Wireless VP2 w/ solar, 24hr FARS, Heater, (Envoy-WLIP)*3-Meteohub, plus custom VP2 @ 26', WL 6.0.4, WU & W4U=KNVRENO37 NetcamXL

People always talk about the weather, but they never do anything about it.  Not me.  I'm gonna measure it.  https://www.tceweather.com

#### SLOweather

• Forecaster
• Posts: 3454
##### Re: Chandler Burning Index
« Reply #6 on: December 17, 2017, 03:03:09 PM »
I'll have to see if I can find my notes or references, but I believe the CBI is supposed to be clamped or limited to between 0 and 100. Or, I may be wrong.

#### d_l

• Forecaster
• Posts: 1700
• Slide Mtn - Mt Rose
##### Re: Chandler Burning Index
« Reply #7 on: January 18, 2020, 05:32:38 PM »
I'll have to see if I can find my notes or references, but I believe the CBI is supposed to be clamped or limited to between 0 and 100. Or, I may be wrong.

I think you are right about the 0 to 100 limits; however, there is an old PHP script for a Smokey Bear fire danger graphic that didn't impose those limits either.  The constriction of the numerical ranges on the higher danger levels suggests that the upper limit could stop at 100, but I couldn't find any official definitions of the CBI.  So I've revised the Meteohub CBI virtual sensor to have those limits. Completely new instructions are here: https://forum.meteohub.de/viewtopic.php?f=9&t=14614  It took me more than a few trial-and-error attempts to string together the correct AWK conditional commands needed to constrain the limits.

As far as I can determine, the Meteohub calculates a CBI value every second or so.  When it plots a value in my 24-hour graphs, I have it aggregating the average over five-minute intervals. Here in the desert, the CBI can be pretty dynamic. A few days ago, a south wind ran it up to 100 in a matter of hours. Then as the wind veered to the west and brought in a snow storm, it dropped back to zero in another few hours.
--Dave--

Wireless VP2 w/ solar, 24hr FARS, Heater, (Envoy-WLIP)*3-Meteohub, plus custom VP2 @ 26', WL 6.0.4, WU & W4U=KNVRENO37 NetcamXL

People always talk about the weather, but they never do anything about it.  Not me.  I'm gonna measure it.  https://www.tceweather.com