small fire

What were the chances of this small fire in Spring Gulch on July 7th growing into a major blaze?

We know that hot weather carries hazards all by itself, but they are complicated by the accompanying problems of drought and wildfire. Smokey Bear tells us that we can prevent wildfire, and around here, most of us do. Responsible outdoorsmen (outdoorspeople?) know not to leave fires unattended and know how to put them completely out. They watch for dragging chains and don’t throw cigarette butts out the car window. They don’t use exploding targets or incendiary ammunition, drive or park in tall grass, or use fireworks dangerously. If they catch a flaming fish, they throw it back in the water immediately. (Ok, Smokey didn’t say that one, I made it up.)

But once a fire does start, either by someone who didn’t listen to Smokey or by a natural cause like lightning or, less likely, hot lava, it’s important to know how likely it is that the fire will grow. We know that dry brush burns easily, but other factors can lead to fast-burning fires too. We can measure these factors and alert firefighters to be ready for extreme fire behavior and take quick action should one start.

Two measurable factors that influence wildfire growth are the humidity and the instability of the air. Humidity, of course, is the amount of water vapor in the air. Meteorologists look at the difference between the air temperature and the dewpoint temperature, that other line on your weather station’s graph. The bigger the difference, the drier the air, and the more likely a wildfire will grow.

The other factor, instability, is a little more challenging to measure. When the air is unstable, if something gives it a nudge upward, it will keep on going up, faster and faster. In addition, as surrounding air moves in to replace the rising air, fire-fanning wind will result. If the heat of a small fire gave it that nudge, the upward motion of the air and the resulting wind would let the small fire grow quickly into a big fire. To know how unstable the air is, you have to compare the temperature at the surface to the temperature aloft, probably with data from a weather balloon.

Back in the 1980s, a researcher named Donald Haines realized how important these two factors were for firefighters to know. He weighted them equally and combined them into one number called the Haines Index. He rated the dryness of the air on a scale of one to three, rated the instability of the air on a scale of one to three, and then he added the two numbers together. So the Haines Index goes from two to six. When the Haines Index is six, as it has been at times all over SE Idaho recently, firefighters know to expect extreme fire behavior should one start. The National Weather Service issues Fire Weather Watches and Red Flag Warnings. It doesn’t mean a fire will start, but if it does, watch out!