Probability is the chance that an event will happen and is expressed in terms of percent, or per 100. For example, if there is a 40 percent chance of a spot fire starting, that means that out of 100 glowing embers that fly off, 40 embers will likely start spot fires, and 60 will not. Percent is calculated by changing the percent into a fraction by dividing by 100, then multiplying by the total number of possible situations.

Example 1 - The probability of ignition is 80 percent. How many ignitions are likely to occur if 90 glowing firebrands land on receptive fuel?

When fire moves upslope, the fuel ahead of the flame front is closer to the flame than if the slope were flatter.  This set-up "preheats" the fuels. The effects of slope on fire spread become greater as the slope increases. 

Each side of the fire is described in terms of head, flank, and rear. The head is the fastest spreading part of a fire's perimeter. The head is usually the side toward which the wind is blowing, and will also often be the upslope side of a fire. The head of the fire is of primary interest. 

The right and left flanks describe the sides of the fire. Flanks are perpendicular to the head of the fire. 

The rear of the fire is the side of the fire opposite the head.
 

Map spread, in inches, is the size of a fire as scaled to a map. See Section 5.3 for more detailed information.

Example 1. For a rate of spread of 4 chains/hour and a projection time of 3 hours, find the map distance for the fire spread. Plot the distance using a tenths ruler for accuracy. The map scale is 1:24,000.

Step 1. Convert the map scale to feet per inch. 24,000 in × 1 foot/12 in = 2000 feet.

Step 2. Convert the spread distance to feet. 12 chains × 66 feet/chain = 792 feet.

The flame length is the distance between the flame tip and the midpoint of the flame depth at the base of the flame. Flame length is an observable, measurable indicator of fireline intensity. 


Spread distance, in chains, is the distance of forward fire spread for a specified amount of time. 

spread distance (SD) = 
rate of spread (ROS) × projection time (PT) 

The rate of spread is in chains per hour (ch/h) and is defined as the speed with which the fire is moving away from the site of origin. Wind, moisture, and slope drive the fire. The flaming zone, or fire head, moves away from the origin quickly with great intensity.

Midflame windspeed (MFWS) is defined as the velocity of the winds, in miles per hour (mi/hr), taken at the mid-height of the flames. MFWS will directly affect the direction of movement of the flaming front and is important in fire spread calculations. The midflame windspeed is determined by use of the wind adjustment table, which provides values in terms of fuel overstory exposure and fuel model. 

Relative humidity is the percent of water vapor in the air compared to what would be present if the air were saturated. Fully saturated air is fog. Relative humidity is always expressed as a percentage.

Relative humidity can be determined by measuring the dry bulb and wet bulb temperatures in the field. Using these measured values, the dew point and relative humidity can be determined with the use of tables. These psychrometric tables are valid for specific elevation ranges because relative humidity and dewpoint change with atmospheric pressure, which varies with elevation.

Dewpoint is the temperature to which air must be cooled to reach its saturation point. The cooling must be done at a constant pressure and moisture content. A rising dewpoint indicates increasing moisture.

Wet bulb temperature is the lowest temperature to which the current air can be cooled. Cooling is achieved by evaporating water into air at a constant pressure. Wet bulb temperature is measured by a psychrometer. The greater the difference is between wet and dry bulb temperatures, the drier the air is.