Weather is a key element in fire behavior. Monitoring temperature, relative humidity, and wind allow you to anticipate how your fire is going to behave.
Belt Weather Kit
The Belt Weather Kit contains a variety of items to take weather observations.
The Sling Psychrometer
The dry bulb measures ambient air temperature. The wet bulb measures the lowest temperature that can be reached under current ambient conditions by the evaporation of water. Note the wick on the wet bulb thermometer.
Clean water, preferably distilled, is used to saturate the wet bulb thermometer.
Anemometer or Wind Gauge
The anemometer or wind gauge, the white ball in the center of the gauge indicates wind speed. Note the two small holes on the back and bottom of wind gauge. This is where wind enters the gauge. These two holes measure winds 10 miles per hour or less. Do not block them with your hand when taking weather observations. Note the hole on the top of wind gauge. Covering this hole measures wind speeds greater than 10 miles per hour.
Compass for measuring wind direction. Wind direction is the direction the wind is coming from.
The calculation tables allow you to obtain the relative humidity and dew point where the dry and wet bulb temperatures converge. Color-coded tables that are easier to read are located in the Interagency Wildland Fire Module Field Guide, 2012 edition. The dry bulb readings are on the left side of the chart. The wet bulb readings are across the top of the chart. Note the elevation on the charts. The chart with your current elevation must be used to avoid faulty readings.
For example, a dry bulb temperature of 88 degrees, and a wet bulb temperature of 62 degrees converge here on the table. This gives us outputs of 22% relative humidity, and a 45% dew point.
Fire Weather Observer’s Record Book
Weather observations will be recorded here. It is a good idea to fill out the header before taking weather observations. Recorded weather observations include time, dry bulb, wet bulb, relative humidity, wind speed, and direction.
How to Spin Weather?
Stand in an open shaded area, away from objects that might be struck while spinning sling. If in open country, use your body to shade the sling. If possible, take your weather observations over a full bed that is representative of the fuels that the fire is burning in. If your sling has been in your pack, you may need to hang it in a tree, or in the shade, to let it adjust to the outside air temperature.
Fill out the header of your Fire Weathers Observer’s Record while sling adjusts to outside air temperature. Use the wind gauge and compass to measure wind speed and direction. Saturate the wick of the wet bulb with clean, mineral free water. If you have a new wick on your wet bulb thermometer, you may need to let the wicks oak to become fully saturated.
When using the sling, face the wind to avoid the influence of your body heat on the thermometers. Ventilate the thermometers by spinning at full arm’s length. Your arm should be parallel to the ground. Spin for one minute. Note the wet bulb temperature. Spin for another 40 or 50 times and read again. If the wet bulb is lower than the first reading, continue to spin and read until it will go no lower. Read and record the lowest point.
If the wet bulb is not read at the lowest point, the calculated relative humidity will be too high. Read the dry bulb immediately after the lowest wet bulb reading is obtained and record. Cross-reference dry and wet bulb temperatures to determine the relative humidity from the tables, and record. You have now completed Fire Weather Observations.
The most accurate weather observation is of little use unless it is properly communicated in a timely fashion to those who need it. Make sure that current observations are reported verbally over the radio to ensure situational awareness.
- Not ventilating the sling long enough to reach equilibrium.
- Not getting the wick wet enough, or letting it dry out.
- Holding the sling too close to the body or taking too long to read the thermometers.
- Touching the bulb ends with the hands while reading.
- Not facing the breeze.
- Not being at the same aspect and elevation as fire or project.
- Minimize fire influences. Taking observations too close to the fire or in black will cause too high of temperatures and too low of relative humidity.
The weather has a significant influence on fire behavior. Utilizing weather observations and monitoring fire weather conditions will allow you base all actions on current and expected fire behavior. The Belt Weather Kit remains the standard and most accurate set of weather observation tools on wildland and prescribed fires.