Fire Weather Instrument Kit
Firefighters use fire weather kits to monitor wind direction, wind speed, and relative humidity, the 3 critical weather patterns that help them determine suppression tactics and strategies.
The core tools in a fire weather instrument kit are:
- a wind meter
- a compass to calculate wind direction and fire location
- tools to calculate relative humidity that measures dry and wet bulb temperatures (e.g., psychrometer, slide rule)
Additional kit items might include a notebook and pencil to record weather measurements and related data such as location, exposure (ridgetop, slope), vegetation density, elevation, aspect, and cloud cover. Firefighters can buy a pre-assembled weather kit or build their own.
Wind Meter
The wind meter has an operational range from 2-60 mph. Wind speed and direction are measured at eye-level. The user faces the wind and holds the instrument so the wind speed can be watched. Thsi instrument requires the operator to constantly watch the instrument to determine average speed and strength of wind gusts. The data should be recorded separately because there is no storage for this information on the instrument.
The electronic instrument specification sheet that accompanies the wind meter lists the minimum and maximum wind speeds, the units of measurement, and the accuracy of those measurements. Wind meters have several measurement units to choose from (feet/second, miles per hour, kilometers per hour). The wind meters can recall average speed and maximum wind speeds for the time the instrument is powered on.
Psychrometer
The two most common types of psychrometers are the sling psychrometer and the electronic psychrometer.
The sling psychrometer is the standard instrument used to determine relative humidity. It consists of two glass thermometers with ranges of 30-110°F. One thermometer, the dry bulb, is unmodified and measures the current air temperature. The other thermometer, the wet bulb, has a muslin wick tied around the bulb that is moistened with water. The frame holding the two thermometers is swung in a circlular motion until the wet bulb temperature stops declining. With these two measurements a person can use a slide ruler to calculate relative humidity and/or the dew point.
The electronic psychrometer displays current air temperature and relative humidity. The electronic psychrometer is a 5-ounce hand held, all-in-one, instrument that measures current air temperature, relative humidity (not adjusted for elevation or latitude) and wind speed. The limitations on the operational range of the electronic psychrometer are listed on the specification sheet that accompanies its purchase. Not all electronic psychrometers have the same operational ranges. Operational ranges must be known and carefully considered before operating this type of instrument.
Both types of psychrometers need to adjust to current air temperatures prior to taking measurements. These instruments are usually carried inside packs or pockets until used and as such are subjected to radiant heat from the body, fire, and sun. The sling psychrometer consists of glass thermometers which adjust quickly to temperature changes. However, the electronic psychrometers components are contained within a closed plastic shell that may take several minutes to adjust to the changing air temperatures. If the current air temperature (dry bulb) is 90°F but the temperature inside the case is 95°F, with a wet bulb temperature of 60°F, the relative humidity will be 13% and 8% respectively. It is important to use both instruments in the shade to derive current air temperature and allow the instruments to adjust to the temperature of the air they will be measuring.
The psychrometric slide rule and tables determine relative humidity to the nearest degree from 0-100%. The psychrometric slide rule has a dry bulb range of 60-110°F and a wet bulb range of 40-120°F. Tables generally mimic the operational range of the psychrometric slide rule for both dry bulb and wet bulb temperatures. The slide rule, however, is not adjustable to differences in air pressure that result from changes in elevation and latitude. For instance, a slide rule will report a 10% relative humidity for all elevations with a dry bulb temperature of 90°F and a wet bulb temperature of 58°F. Tables accurate to the nearest 2000 feet evelation will display a relative humidity with the same dry and wet bulb measurements given previously as 15% at 6,000 feet. A 5°F difference in relative humidity may have important implications to firefighters when temperatures are high and fuel moisture is low.
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