With our recent fires, I have heard a great deal about a weather condition called "crossover", when the humidity is lower than the temp. Can you explain what this means, and what it does during fire season?
Friday, July 31, 2009 | 09:44 AM PT
Question submitted by Lisa Nagy
Lisa! This is the first time I have ever been truly stumped by a question!! Congratulations.
I went to the BC Wildfire Management Branch, Ministry of Forests and Range, and got some help from a couple of great people there.
Here's what they had to say: "The formal definition of Crossover as listed by CFFC (Canadian Forest
Fire Centre) is "the point at which the numerical value of the ambient relative humidity is less than, or equal to, the numerical value of the ambient air temperature". This may be used as a rule of thumb indicator of extreme burning conditions. It is a rule of thumb indicator primarily used by fire suppression staff but there are some limitations to it being used as a definitve law."
So there you go! And thanks for the challenging question.. I now know more about fire weather forecasting too!