### Author Topic: Chandler Burning Index  (Read 866 times)

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#### Jáchym

• Meteotemplate Developer
• Forecaster
• Posts: 8551
##### 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: 6277
• 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
Free weather PHP scripts/website templates - update notifications on Twitter saratogaWXPHP

#### Jáchym

• Meteotemplate Developer
• Forecaster
• Posts: 8551
##### 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

anything