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Unit 8: Keeping Current with The Weather

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Forecasting   ::   AFFIRMS   ::   Warnings   ::   Management   ::   Services   ::   Mobile Units   ::   Forecasts   ::  Monitoring  ::   Exercises

Monitoring the Weather

There is much that a firefighter can do to monitor the weather if he has a belt weather kit and has learned a few basic skills. It's important that he know where, when, and how to take observations, either visually or with instruments.

Taking Weather Observations at Fires

WHEN : The two best times are during periods when the weather has stabilized for the extreme, i.e., the warmest and driest period and the coldest and wettest period.

HOW OFTEN : The frequency might be once every hour, but during periods when you expect changes in the weather, they might be more frequent.

WHERE : The best locations are near the fire in areas representative of fireline conditions, but not inside the burn.

NUMBER OF LOCATIONS : On large fires with varied terrain and dangerous topography and/or fuel conditions, take observations at each problem area. Also select locations with good visibility for weather watches to monitor changing weather conditions.

Visually Monitoring Weather Elements

Weather elements that can be monitored visually are:

  1. Thunderstorm buildups.
  2. Approaching cold fronts.
  3. Inversion layers.
  4. Air stability and instability.
  5. Wind shifts.
  6. Cloud cover.

When we have strong concerns that one of these weather processes could change the weather for the worse and create serious control and safety problems, it is usually wise to set up a security weather watch. In a security weather watch, one or more observers are posted at strategic locations in the proximity of the fire to detect and warn fire personnel of pending critical weather changes that might significantly affect the fire.

Security Weather Watch : One or more observers are posted at strategic locations in the proximity of the fire to detect and warn fire personnel of pending critical weather changes that might significantly affect the fire.

Security Weather Watch
Security weather watch

The graphic above illustrates a security weather watch. Oftentimes, regular fire lookouts can be used for this purpose, or you may need to locate a person on a good vantage point to observe changing weather processes. Several factors are important here. The weather watch must be able to detect wind shifts or increased winds before they affect the fire and provide adequate warning time to fire personnel. This can mean placing the observer between the fire and the wind source at a considerable distance from the fire. In the case of approaching cold fronts, watchers have been located up to 20 miles to the west of a fire. Then, of course, the watcher must be able to communicate rapidly with fire personnel.

Weather Observations Using the Belt Weather Kit

Finally, we wish to discuss the belt weather kit, which is a standard item on many fires. This small, compact unit, which can be strapped to your belt, contains all the weather instruments and miscellaneous items needed to take a basic weather observation. The two primary instruments to accomplish the job are the windspeed meter and the sling psychrometer. Other items are the psychrometric tables for determining relative humidity, a small bottle of distilled water, a pencil, a recording pad, and a compass.

On the next pages are instructions on how to use the instruments, observations to be taken, and standards of accuracy for taking the observations. If you have a belt weather kit available to you, we suggest you remove the items from the kit and inspect them carefully. Read through the instructions; then go outdoors and take various observations using the kit. If you have any problems or questions, find someone who is familiar with the belt weather kit.

Determine Windspeed (Eye Level) Using the Windspeed Meter

  1. Face the wind and hold the meter at arm's length about head high. The scale side should be facing you.
  2. Hold the meter about midway from either end being careful not to block the two holes at the bottom or small inhole in top stem.
  3. Observe action of the ball in relation to left scale. If it is bouncing between 2 and 9, read from the left scale. If ball is rising up near 10 on left scale, cover the top stem with your finger and read from the right scale.
  4. To obtain a reading, observe the height attained by the ball in relation to the appropriate scale value. As the height of the ball usually varies even during a short time, a certain amount of subjective averaging is often necessary to obtain a value. Observe for a period of 1 minute.
  5. The standards for accuracy are +/- 2 mph.

Determine Wind Direction Using Compass

  1. Be sure the proper declination for your area is set on the compass.
  2. Take a reading to the nearest of the eight cardinal points (N, NE, E, SE, S, SW, W, NW).
  3. The direction should also be noted as downcanyon, upslope, erratic, etc.

Determine Wet and Dry Bulb Temperatures Using the Sling Psychrometer

  1. Stand in a shaded , open area away from objects that might be stuck during whirling.
  2. Face the wind to avoid influence of body heat on the thermometers.
  3. Saturate the wick of the wet bulb with clean, mineral-free water.
  4. Ventilate the thermometers by whirling at full arm's length. Your arm should be parallel to the ground. Whirl for 1 minute.
  5. Note the wet bulb temperature. Whirl another 40 or 50 times and read again. If the wet bulb is lower than the first reading, continue to whirl and read until it will go no lower. Read and record the lowest point.
  6. Read the dry bulb immediately after the lowest wet bulb reading is obtained.
  7. The standards for accuracy are +/- 1 degree for either temperature.

Determine Relative Humidity Using Tables or a Computer

  1. See unit 4 for instructions on use of psychrometric tables. Check the elevations for these tables.
  2. For wet bulb depression tables, use the difference between the wet bulb and dry bulb temperatures, and enter this at the top of the table. Read relative humidity as with standard psychrometric tables.
  3. For computer, see instructions with computer.

Provide Weather Remarks

  1. Report cloud cover or buildups, thunderstorms, dust devils, inversion layers, behavior of smoke, etc.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 10). Unit 8: Keeping Current with The Weather. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License