**Taking Good Weather Observations**

*John F Saltenberger*

*Predictive Services*

*Northwest Geographic Area Coordination Center*

*Portland, OR*

**[Preface:]{.underline}**

The purpose of taking weather observations during a wildfire or
prescribed fire project is to record environmental conditions prevalent
at the site. Depending on the size of the project and the complexity of
the terrain, several separate weather observation sites may be necessary
to adequately convey weather conditions representative of the entire
burn site.

Accurate weather observations are important in wildland fire management
for several reasons:

1^st:^ Meteorologists build their spot weather forecasts based on the
observations reported from the fireline. The forecaster uses the
observation to build a mental model of weather pattern in the vicinity
of burn site. The more accurate and representative the weather
observations are, the more effectively a meteorologist can forecast
weather conditions at the site. This results in better fire behavior
forecasts. If fireline weather observations are incomplete or
unrepresentative, the forecaster will have a much more difficult time
accounting for localized effects.

2^nd:^ Meteorologists seek to verify the accuracy of their forecasts by
comparing weather conditions observed at the site versus what was
forecast. When differences are noted, the meteorologist can potentially
learn from the errors and correct for the next forecast. The belt
weather kit observation constitutes the best record of weather
conditions near the fireline.

3^rd:^ Belt weather kits observations become part of the official fire
documentation record. If there is an investigation or litigation
following some accident on the fire, belt weather kit observations
comprise a key component for re-constructing environmental conditions
surrounding the accident. These can prove critical during litigation.

**[1. Siting]{.underline}**

***Key Point:***

*Regardless of whether the fire is a prescribed fire project or a
wildfire, the weather observer should strive to pick observation sites
that most accurately reflect environmental conditions around the fire's
location.*

***Tips:***

Consider whether to take a weather observation on a ridgetop, midslope
or drainage bottom location. Is the fire site exposed or sheltered? What
is the aspect and slope? What kind of surface texture? The choice of
observation sites should reflect the type of terrain the fire front is
most actively burning in. Selecting an unrepresentative site will result
in weather observations that don't truly reflect the environmental
conditions of that incident, division or burn site.

+----------------------------------------------+-----------------------+
| ![](media/image1.jpeg){width="3.70625in"     | **Representative      |
| height="2.775in"}                            | fuels**               |
|                                              |                       |
|                                              | Once safe             |
|                                              | representative sites  |
|                                              | have been selected,   |
|                                              | the weather observer  |
|                                              | should try to take    |
|                                              | the observations in   |
|                                              | the same fuel type    |
|                                              | the fire is most      |
|                                              | active in. If the     |
|                                              | fire is spreading in  |
|                                              | timber litter, try to |
|                                              | take the observations |
|                                              | in similar unburned   |
|                                              | timber. The same is   |
|                                              | true for brush or     |
|                                              | grass fuel types.     |
+----------------------------------------------+-----------------------+

+---------------------------------+------------------------------------+
| **Minimizing fire influences**  | ![](media/image2                   |
|                                 | .jpeg){width="3.222916666666667in" |
| The weather observer should     | height="3.8916666666666666in"}     |
| avoid taking observations in    |                                    |
| the black since burned areas    |                                    |
| tend to be unrepresentatively   |                                    |
| warm and dry. Attempt to find a |                                    |
| safe site *upwind* of the       |                                    |
| burned area if at all possible. |                                    |
| If the observer has no choice   |                                    |
| but to take an observation in   |                                    |
| the black, make sure to note it |                                    |
| in the remarks section of the   |                                    |
| log.                            |                                    |
|                                 |                                    |
| The weather observer should     |                                    |
| also take care to minimize      |                                    |
| localized weather effects       |                                    |
| generated by the *fire itself*. |                                    |
| Take weather observations away  |                                    |
| from gusty indraft breezes and  |                                    |
| radiant heat in the vicinity of |                                    |
| the flaming front. These are    |                                    |
| very localized conditions that  |                                    |
| don't reflect the area's        |                                    |
| general conditions. Generally,  |                                    |
| well ventilated areas in the    |                                    |
| shade are desirable spots for   |                                    |
| belt weather kits to be most    |                                    |
| effective.                      |                                    |
+---------------------------------+------------------------------------+

**[2. Observation process]{.underline}**

***Key Point:***

*To be effective, belt weather kits must be properly maintained and
operated. Inspect kits for defects prior to each fire assignment. Old,
dirty or broken parts should be replaced. Electronic sensors must be
calibrated routinely during fire assignments. Proper procedure must be
followed to extract the most accurate information from a weather
observation.*

***Tips:***

Here are some considerations to keep in mind:

-   Check sling psychrometers for separated mercury columns. These can
    sometimes be corrected by exposing the thermometer to very cool
    temperatures such as an ice bath.

-   Replace dirty wicks on wet bulb thermometers with a clean wick.

-   Test the ball in Dwyer wind meters for free movement. Static
    electrical charge buildup, moisture, or dirt can lodge the ball in
    the tube.

-   Electronic temperature and humidity sensors should regularly be
    calibrated against weather instruments of reliable accuracy. Check
    that the batteries are fresh.

-   Well maintained sling psychrometers are still the most accurate,
    durable and reliable indicators of temperature and moisture. They
    function as well in extreme conditions as in moderate conditions.

Regardless of whether the kit is a traditional sling psychrometer or
electronic sensor, get into the habit of letting it acclimatize for
several minutes in a shaded, breezy area before beginning an
observation. This allows the sensors to gradually adjust to
environmental temperature and humidity. Avoid exposing the kit to body
heat, sweat, ash, dirt, and direct sunlight. The observer should be
careful to store the kit in locations away from temperature extremes
such as vehicle dashboards or in line gear pockets.

+-----------------------+----------------------------------------------+
| **Avoid direct        | ![](me                                       |
| sunlight during       | dia/image3.jpeg){width="3.948611111111111in" |
| temperature and       | height="3.6479166666666667in"}               |
| humidity              |                                              |
| observation**         |                                              |
|                       |                                              |
| The wet bulb/dry bulb |                                              |
| observation *must* be |                                              |
| taken in shade. Even  |                                              |
| if the site is grassy |                                              |
| (i.e. no shadows from |                                              |
| an overstory) the     |                                              |
| instrument must be    |                                              |
| protected from direct |                                              |
| sunlight. If          |                                              |
| necessary, the        |                                              |
| weather observer can  |                                              |
| use his or her own    |                                              |
| body shadow to avoid  |                                              |
| exposing the kit to   |                                              |
| sunlight.             |                                              |
+-----------------------+----------------------------------------------+

When using a traditional sling psychrometer, be sure the wick is
thoroughly wetted before beginning to swing. Distilled water is
preferred. Bottled water is the next best alternative. New wicks may
tend to repel water droplets from the wet bulb so the observer may need
to gently rub the wick against the side of the water bottle to eliminate
air pockets. A soaked wick is the goal.

Continue swinging the sling psychrometer [until the wet bulb temperature
is reached]{.underline}. This may take several minutes. Continue to
check the wet bulb temperature every thirty seconds until it reaches its
lowest value. In very dry conditions it may be necessary to re-wet and
continue swinging until the lowest value is attained. When the wet bulb
temperature ceases descending, the correct value has been reached. Read
and record the dry bulb immediately after the lowest wet bulb reading is
obtained.

When wet bulb and dry bulb temperature have been determined, compute the
dew point and relative humidity using the paper tables included with the
kit. Select the appropriate table for the altitude range corresponding
to the observation site. Plastic slide rules for computing dew point and
humidity should only be used if a printed table is not available. Use of
the printed tables will provide greater accuracy.

A time saving strategy is to conduct the wind observation while the
temperature sensor is acclimatizing to ambient conditions. Wind
observations need not be taken in shade but they should still reflect
the ambient conditions affecting the fire.

+--------------------------------+-------------------------------------+
| ![](media/image4.png           | **Points to remember about eye      |
| ){width="2.8229166666666665in" | level**                             |
| height="3.3645833333333335in"} |                                     |
|                                | **wind observations.**              |
|                                |                                     |
|                                | -   The observer should take care   |
|                                |     to face directly into the wind  |
|                                |     and closely observe the wind    |
|                                |     speed indicator fluctuations.   |
|                                |     Exposure to sunlight is not a   |
|                                |     concern during the wind         |
|                                |     observation.                    |
|                                |                                     |
|                                | -   An eye level wind speed         |
|                                |     measurement requires at least   |
|                                |     one full minute of sampling and |
|                                |     preferably more.                |
|                                |                                     |
|                                | -   When using a Dwyer tube,        |
|                                |     mentally average the wind speed |
|                                |     and note the peak gust during   |
|                                |     the sampling period.            |
|                                |                                     |
|                                | -   Electronic sensors make wind    |
|                                |     averaging easy. They are more   |
|                                |     accurate and are preferred for  |
|                                |     eye level wind speed            |
|                                |     observations.                   |
|                                |                                     |
|                                | -   Remember: The wind direction is |
|                                |     defined as the direction the    |
|                                |     wind is [*coming                |
|                                |     from*.]{.underline}             |
+--------------------------------+-------------------------------------+

**[3. Observation logging and remarks]{.underline}**

***Key Point:***

*Careful record keeping is as important to the weather observation
process as every other step. It's a good idea for the observer to double
check recorded values for obvious errors before logging and submitting.*

***Tips:***

+-----------------------------------------------+----------------------+
| ![](m                                         | **Effective          |
| edia/image5.jpeg){width="4.002777777777778in" | logging**            |
| height="3.910416666666667in"}                 |                      |
|                                               | Carefully note the   |
|                                               | date, time and       |
|                                               | location of each     |
|                                               | weather observation. |
|                                               | Include aspect,      |
|                                               | elevation, exposure  |
|                                               | and fuel type        |
|                                               | whenever possible.   |
|                                               | If the observer is   |
|                                               | changing locations,  |
|                                               | this becomes even    |
|                                               | more important.      |
|                                               |                      |
|                                               | Don't confuse the    |
|                                               | wet bulb, dry bulb   |
|                                               | and dewpoint         |
|                                               | temperatures. Double |
|                                               | check to insure the  |
|                                               | proper values are    |
|                                               | noted in their       |
|                                               | respective columns   |
|                                               | of the observation   |
|                                               | form. *Use clear     |
|                                               | penmanship.* This is |
|                                               | a frequent source of |
|                                               | error.               |
+-----------------------------------------------+----------------------+

Compare the latest weather observation against those taken during
previous hours. *Stay alert for any obvious discrepancies*. Weather
conditions naturally fluctuate during the course of a day but large
changes may indicate either a significant weather event or an instrument
malfunction. To reduce potential errors, its a good idea to cross check
weather observations with nearby observers.

+-----------------------------------+-----------------------------------+
| **Best use of the remarks         | ![](med                           |
| column**                          | ia/image6.jpeg){width="2.58125in" |
|                                   | height="3.925in"}                 |
| The remarks column can be used to |                                   |
| log significant features such as: |                                   |
|                                   |                                   |
| -   Alto-cumulus clouds, cumulus  |                                   |
|     buildups or thunderstorms.    |                                   |
|                                   |                                   |
| -   Lightning                     |                                   |
|                                   |                                   |
| -   Smoke column developing a     |                                   |
|     pyro-cumulus cap cloud.       |                                   |
|                                   |                                   |
| -   Lenticular clouds             |                                   |
|                                   |                                   |
| -   Sudden wind shifts            |                                   |
|                                   |                                   |
| -   Changing cloud cover          |                                   |
|                                   |                                   |
| -   Dust clouds radiating away    |                                   |
|     from downdraft impacts.       |                                   |
|                                   |                                   |
| -   Inversions                    |                                   |
|                                   |                                   |
| -   Dust devils                   |                                   |
|                                   |                                   |
| -   Showers or virga              |                                   |
|                                   |                                   |
| -   Upper level winds             |                                   |
|     significantly different than  |                                   |
|     surface winds                 |                                   |
|                                   |                                   |
| -   Other significant features    |                                   |
|     since last observation        |                                   |
+-----------------------------------+-----------------------------------+

Lastly, it's a good idea to remark whether the observations were made
with an electronic weather sensor or traditional sling psychrometer.

**[4. Transmission of observations]{.underline}**

***Key Point:***

*The most accurate weather observation is of little use unless it is
properly received by those who need its information. The weather
observer should make sure that the chain of communication is functioning
rapidly and efficiently at both ends.*

***Tips:***

+----------------------------------------+-----------------------------+
| ![](media/im                           | Work out a plan to transmit |
| age7.jpeg){width="3.372916666666667in" | weather observations to the |
| height="2.986111111111111in"}          | Incident Communications     |
|                                        | Unit or local fire dispatch |
|                                        | office on a regular         |
|                                        | schedule as necessary.      |
|                                        | Double check that each      |
|                                        | observation has been        |
|                                        | received clearly by asking  |
|                                        | to have it repeated back.   |
|                                        | This technique is effective |
|                                        | at reducing errors.         |
|                                        |                             |
|                                        | Remember that weather       |
|                                        | observations may need to be |
|                                        | recorded and transmitted    |
|                                        | more than once per hour.    |
|                                        | Fast breaking weather       |
|                                        | changes can endanger        |
|                                        | firefighter safety so be    |
|                                        | ready to act rapidly when   |
|                                        | the situation requires.     |
+----------------------------------------+-----------------------------+

It's a good idea make sure the observation is being properly relayed to
the Incident Meteorologist, weather office, or other affected users.
This is particularly true during critical burning conditions or
significant weather changes. *Don't assume that weather observations are
automatically being received by the proper users.* The weather observer
may need to take the initiative to verify that the information is being
passed up the line.