Many wildland firefighters carry an instrument called a sling psychrometer. It consists of two encased thermometers, and is swung above the head on a short rope -- making the firefighters appear not unlike David readying to slay Goliath. The instrument gives a quick field reading of relative humidity, one of the most important factors in predicting what a wildfire is going to do. Quick drops in relative humidity are a sure signal that the air is getting drier and that a fire is about to turn ugly. Photo: iStockphoto Wildland firefighters know weather. They study weather reports and projections. They …