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Observing Fire Weather


Take time to review forecasts and make good fireline observations and monitor automated weather stations to ensure effective forecasts and briefings. Each Single Resource (crew, squad, and individual) is responsible for insuring that they “keep informed of fire weather conditions and forecasts” so that they may “base all actions on current and expected behavior of the fire.” The process includes obtaining and reviewing latest forecasts, taking observations to validate them through the assignment, reporting what is learned to those who need the information, and requesting forecast updates.

Fire Weather Apps

Fire Weather Calculator  URL: http://firecenter.umt.edu/weather-app/default.php

Storm by Weather Underground (iOS and Android)  URL: https://www.wunderground.com/micro/storm/  

Dark Sky  URL: https://darksky.net/app/

Sunrise – sunrise and sunset calculator URL: http://www.adairsystems.com/sunrise/

Fireline Observations

Location and Timing of Fireline Weather Observations

Four times during a 24-hour day stand out as valuable for assessing forecasts and evaluating thresholds associated with fire behavior transitions.

  • An early morning observation that represents time and conditions when the minimum temperature and maximum humidity occur.
  • A late afternoon observation that represents the time and conditions when the maximum temperature and minimum humidity occur.
  • At the times when active fire behavior seems to increase and diminish during the burn period.
  • Other times, for example hourly throughout the afternoon or when changes occur, may be called for by fireline supervisors or dictated by changing conditions to ensure situational awareness.

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.

  • Decide whether a ridgetop, midslope, or drainage bottom location is most representative.
  • If on a slope, the aspect and slope steepness is an important consideration.
  • Consider what is a representative fuelbed for the fire.
  • Attempt to find a safe site upwind or on the flank of the fire. Generally, well ventilated areas in the shade are desirable locations for the observation.
  • Minimize the fire’s influence on your observation. Avoid taking observations in the black. Avoid observations affected by gusty indraft breezes and radiant heat from the fireline

Note the type of instruments used

It’s a good idea to remark whether the observations were made with an electronic weather sensor or traditional sling psychrometer. Electronic temperature and humidity sensors should regularly be calibrated against weather instruments of reliable accuracy. Check that the batteries are fresh

Communicate and Document the Weather Observation

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 insure situational awareness. 

  • Follow instructions for periodic radio reports to fireline supervisors and/or incident communications unit.
  • Report measurements with trends, such as temperature 75 – up 5 degrees from last hour.

Provide written documentation of weather observations to fireline supervisor, situation unit, incident meteorologist, or the local Weather Forecast Office. Retain a copy for your records. 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. Forms are available.

Wind Observations and Estimations; Calibrating Forecast/Prediction

Get forecast from Incident Meteorologist or Fire Weather Forecaster

Because windspeed and direction is the most variable weather factor over the duration of an assignment, the observer will be concerned with adjusting and validating forecasted winds as much as measuring current windspeed. It is difficult for a meteorologist to produce localized wind forecasts, especially if the wind is influenced by terrain features. Forecasted winds will frequently need adjustment because they are representing a wind other than mid-flame, such as ridgetop or surface winds. See the definitions in section 1.2. It will be important to communicate with the meteorologist the factors that influence the wind measurements that are provided. 

Use Surface Wind Estimation Worksheet

Report observation type or height. Identify sheltering and aspect/slope position for the wind observation. And indicate whether local winds are influencing the observation. 

Consider possibility of Critical Wind

Estimate or validate 20-ft surface windspeed

If the weather forecast product provides windspeed as “free air or ridgetop” or if winds in the fire area are influenced by local winds, it may be necessary to use the form in section 1.2.3 to estimate the surface/20 ft windspeed. 

  • Identify speed and direction of any forecast critical wind.
  • Determine speed and direction of any Local Winds
  • Determine speed and direction of General Winds and whether they will influence the 20-ft wind.
  • Combine factors above into an estimate of local surface (20-ft) windspeed.

Visual Surface (20ft) Wind Estimate - Modified Beaufort Scale

Class

Windspeed

Terminology

 Example

Visible Effect

0

Less than 1 mph

Calm

Icon of visible wind effects as described in column 5 of this table

Calm, Smoke rises vertically

1

1 to 3 mph

Very Light Breeze

Icon of visible wind effects as described in column 5 of this table

Leaves of quaking aspen in constant motion; small branches  sway, tall grasses and weeds sway and bend with wind, wind vane barely moves

2

4 to 7 mph

Light Breeze

Icon of visible wind effects as described in column 5 of this table

Trees of pole size in the open sway gently, Wind felt distinctly on face; leaves rustle; loose  scraps of paper move, wind flutters small flag

3

8 to 12  mph

Gentle Breeze

Icon of visible wind effects as described in column 5 of this table

Leaves, small twigs in constant motion; Tops of trees in dense stands sway; light flags extended

4

13 to 18 mph

Moderate Breeze

Icon of visible wind effects as described in column 5 of this table

Trees of pole size in the open sway violently; whole trees in dense stands sway noticeably; dust is raised in the road.

5

19 to 24 mph

Fresh Breeze

Icon of visible wind effects as described in column 5 of this table

Branchlets are broken from trees; inconvenience is felt in walking against wind

6

25 to 31 mph

Strong Breeze

Icon of visible wind effects as described in column 5 of this table

Tree damage increases with occasional breaking of exposed tops & branches; progress impeded when walking against wind.

7

32 to 38 mph

Moderate Gale

Icon of visible wind effects as described in column 5 of this table

Severe damage to tree tops; very difficult to walk into wind; significant structural damage occurs.

8

39 to 46 mph

Fresh Gale

Icon of visible wind effects as described in column 5 of this table

Surfaced strong Santa Ana; intense stress on all exposed objects, vegetation, buildings; canopy offers virtually no protection

9

47 to 54 mph

Strong Gale

Icon of visible wind effects as described in column 5 of this table

Slight structural damage occurs; slate blown from roofs

10

55 to 63 mph

Whole Gale

Icon of visible wind effects as described in column 5 of this table

Seldom experienced on land; trees broken; structural damage occurs

11

64 to 72 mph

Storm

Icon of visible wind effects as described in column 5 of this table

Very rarely experienced on land; usually with widespread damage

12

73 mph or more

Hurricane Force

Icon of visible wind effects as described in column 5 of this table

Violence and destruction



Estimate or validate Midflame Windspeed

Eye level windspeed is usually assumed to be the same as mid-flame windspeed. However, as suggested in the Fireline Assessment Method (FLAME) reference, it may be too low for flames in shrub fuels and too high for flames in forest litter. In any case, it may be necessary to adjust forecasted 20 ft winds or observed mid-flame windspeed to make comparisons and validate forecasts. 

Observing eye-level windspeed in the field

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.
  • Note time and rapidity of transitions in diurnal winds
  • When using a Dwyer tube, mentally average the wind speed and note the peak gust during the sampling period.
  • Electronic sensors make wind averaging and gust measurement 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.

Estimate Effective Windspeed for slope influence

The influence of slope on fire spread is applied as a slope-equivalent “windspeed”.

Where slope is significant (generally 20% or more), all the fire behavior assessment tools in section 5 (FLAME, Lookup Tables, Nomograms & Nomographs, and BehavePlus) provide means for estimating “effective windspeed”.

This adjusted windspeed should be used instead of the mid-flame windspeed estimate in fire behavior predictions.

Temperature and Humidity Observation

Estimating temperature, relative humidity and dewpoint can provide insight to critical fire behavior thresholds for ignition and crown fire potential.

Sling Psychrometer Use

The following are instructions for determining wet and dry bulb temperatures using the sling psychrometer. These instructions are based on those from page 259 of the S-290 Instructors Manual. Several additional comments have been added. 

  1. If your sling has been in your pack, you may need to hang it in a tree, in the shade, to let it adjust to the outside air temperature. This may be a good time to take the wind observation.
  2. Stand in a shaded, open area away from objects that might be struck during whirling. If in open country, use your body shade to shade the psychrometer. If possible, take your weather observations over a fuel bed that is representative of the fuels that the fire is burning in. Stay away from heat sinks.
  3. Face the wind to avoid influence of body heat on the thermometers.
  4. Saturate the wick of the wet bulb with clean, mineral free water (distilled if available) at air temp.
  5. Ventilate the thermometers by whirling at full arm’s length. Your arm should be parallel to the ground. Whirl for 1 minute.
  6. Note the wet bulb temperature. Whirl for 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. If the wet bulb is not read at the lowest point, the calculated relative humidity will be too high. Calculate dew point each time. If it is changing significantly it may suggest a bad observation.
  7. Read the dry bulb immediately after the lowest wet bulb reading is obtained.
  8. Determine the relative humidity from the tables.

Important Tips: Sometimes beginners do not take accurate psychrometer readings because of the following common mistakes: 

  • Changing psychrometers from one observation to the next. Try and use same one throughout.
  • not ventilating the psychrometer long enough to reach equilibrium;
  • not getting the wick wet enough, or letting it dry out;
  • holding it too close to the body or taking too long to read the thermometers;
  • touching the bulb ends with the hands while reading;
  • Not facing into the breeze.

Estimating RH & Dew Point from Psychrometric Tables

Psychrometric tables included in the belt weather kit or provided here allow you to estimate Relative Humidity and Dew Point from Dry Bulb and Wet Bulb Temperatures obtained in the field. 

  1. Find the correct table based on elevation at your observing location
  2. Use your DB Temp and WB Temp to find the intersecting cell on the page
  3. Read the resulting RH (below) and Dew Point (above) in that cell.

Example Table

Each Table is labeled with an Elevation Range, including an adjustment for Alaska use.

Dry Bulb Temperature is located on the left axis and the wet bulb temp is located at the top of each column.  Cell at their intersection includes the resulting RH and Dew Point.

Psychrometric Table, Pressure 27 in, Elev 1901-3900 ft

Adjusting RH for changes in Temperature and Elevation

Under certain circumstances, it may not be possible to estimate relative humidity for a particular elevation. It may also be necessary to make field adjustment to forecasted relative humidity for some time later in the burn period. In both cases, given that the air mass is unchanging and fairly neutral, it is possible to use current estimates of dew point and temperature and to make adjustments in both cases: 

Case 1: Estimate relative humidity for an elevation above or below the observation; assuming an average lapse rate of approximately 4° F, increase the temperature by 4° F for each 1000 ft drop in elevation or decrease it by 4° F for each 1000 ft increase in elevation. Using the new temperature and the estimated dew point, look up the new relative humidity in the appropriate psychometric table. 

Case 2: Validate a forecasted relative humidity; using the estimated dew point and the forecasted temperature, look up the new relative humidity in the appropriate psychometric table. 

Vapor Pressure Deficit (VPD)

While Relative Humidity refers to the amount of water vapor in the air as a percentage of saturation levels, Vapor Pressure Deficit represents an absolute measure of the difference between the moisture in the air and the amount it could hold when saturated.

VPD rises as the moisture deficit increases and may seem counter-intuitive for users comfortable using RH.  It follows a seasonal trend of low levels in the winter and higher levels during the warm summer months, controlled by variation in temperature and atmospheric humidity.

VPD can provide important insight about the role of ambient temperature as well as atmospheric humidity.  For example, at low temperatures a low RH will not be accompanied by significantly high VPD levels.  But that same low RH under high temperatures will produce elevated VPD. The table below here provides an example interpretation.

Vapor Pressure Deficit, in millibars

Long used in greenhouse and agricultural applications, it is a good indicator of the moisture stress experienced by green vegetation.  The new Growing Season Index (GSI) uses it as a key criterion to greenup and curing phenology.  GSI considers VPD of 9 mb optimum growing conditions.

The “crossover” concept (Alberta Forest Service 1985) refers to rising temperature (in °C) and falling RH reaching the point where they are equal (highlighted boxes in the table). Interpretations suggest the onset of potential for extreme fire behavior.

Sky Observations

Airport Webcams: http://airportwebcams.net/airport-webcams-by-time-zone/ 

Synoptic (Large Scale) forecasts and representations of current conditions include reference to the relative stability of the atmosphere in the area. In that vein, there are numerous indicators that can be reviewed and interpreted. Several are referenced in section 1.4.

However, these general atmospheric conditions are influenced by the terrain and other local factors to produce more localized effects. The weather observer can provide important information to meteorologists by reporting the visual cues and the timing of changes throughout the day.

These visual cues are generally associated with a weather observation by recording them in the remarks column so that they get a time stamp.

Usually, if a visual cue is worth noting with the weather observation, photography can be very valuable supporting documentation. If a photo is taken, use a photo log or reference the photo number with the location date, time and other identifying comments.

Here are some important examples:

Lightning and Wind

  • Lightning should be reported immediately to alert fireline supervisors to take appropriate precautions and to cue meteorologists to review their lightning detection tools.
  • Sudden wind shifts may be important indicators of breaking inversions or frontal passage.

Smoke, Dust, and Fire

  • Rising smoke column indicates neutral or instable conditions. Flattening column indicates inversion at that point.
  • Smoke column change direction as it rises indicates wind shear or local wind influence.
  • Smoke column developing a pyrocumulus cap cloud indicates strong instability and impending down drafts.
  • Haze and poor visibility are indicators of inversions. Is this localized (night-time inversion) or more general and persisting throughout the day. Note if and when the haze or poor visibility abates during the burn period as indicator of increase in fire behavior.
  • Dust clouds radiating away from thunderstorms indicate potentially dangerous downdrafts.
  • Dust devils are important indicators of surface instability.
  • Firewhirls occur due to extreme vertical motion in a wildfire’s convection column. Be aware of the potential for gusty erratic winds when firewhirls are observed. A stable atmosphere generally tends to limit violent vertical motion. As a result, cloud buildups during stable conditions tend to be wider and flatter, possibly covering much of the sky. It should be noted that strong general winds are possible during stable conditions depending upon the weather pattern. The firefighter should pay attention to the fire weather forecast and keep an eye on the sky for indicators of potential severe conditions that can dominate the fire environment.

Clouds, Fog, & Precipitation

Clouds occur when moisture in the atmosphere condenses into visible droplets or ice crystals. This usually occurs when moist air becomes cooled by lifting. The shape and texture of clouds reveals much about whether the lifting process has been gradual and gentle or rapid and potentially violent.

Paying attention to the sky can help the firefighter stay aware of the current fire environment as well as anticipation of potential changes.

  • Cloud Cover, in percent, is an important input for fuel moisture shading.
  • Building cumulus, towering cumulus, or thunderstorms are all indicators of significant instability that is probably already influencing surface winds.
  • Showers or virga may be indicators of instability.

The NWCG cloud chart depicts sky signs of interest for wildland firefighters that are valuable tools in revealing the atmosphere’s current state as well as foretelling potential changes. Clouds are an important indicator of stability.

Cloud Nomenclature

Clouds that reveal variations of instability in the atmosphere:

  • Cumulus (several varieties): Weak instability. Normally not a concern for firefighters. However, when cumulus continues growing, firefighters are advised to keep an eye on the buildups due to the potential for sudden downdrafts and gusty winds.
  • Alto Cumulus (several varieties, e.g. castellanus): Upper atmosphere instability and possible weather change. These indicate increasing moisture and instability with the potential for thunderstorms.
  • Cumulonimbus: Very unstable. Fully developed mature thunderstorms contain extreme vertical motion and the strong likelihood of gusty, erratic winds that can arise suddenly miles away from the cloud buildup. Localized wind gusts over 100 mph are possible with very strong thunderstorms along with lightning, virga, and hail. Very strong thunderstorms may also be accompanied by shelf clouds or tornados. Clearly, cumulonimbus clouds portend many hazards to the firefighter exposed on the fireline.
  • Pyrocumulus: Very unstable. Pyrocumulus clouds grow above ongoing wildfires drawing energy from the heat of combustion and condensation of moisture in the fire’s convection column. A white-capped pyrocumulus cloud is a concern for firefighters for the same reason as a thunderstorm: Strong, gusty erratic winds can arise suddenly near a pyrocumulus. Virga, light raindrops, and even some lightning is possible with well-developed pyrocumulus clouds.

Clouds that indicate a stable atmosphere:

  • Stratus (several varieties): Stable and moist. Stratus clouds can cover much of the sky and blot out sunlight or even bring rain. Stratus clouds tend to mean higher humidity and decreased fire behavior. Normally not a concern for firefighters.
  • Lenticular Cloud FormationCirrostratus (several varieties): High level stratus clouds formed of ice crystals. Cirrostratus clouds are normally not a concern for firefighters. However, if these clouds increase from the west or northwest, a front may soon be approaching with strengthening general winds. Check the fire weather forecast.
  • Altostratus (several varieties): Mid to high level stratus clouds that are a good indicator of an approaching front with strengthening general winds. Check the fire weather forecast
  • Wave cloud or Lenticular cloud: Smooth almond- shaped clouds that sometimes form over mountainous terrain in patterns similar to stacked dishes. These clouds tend to remain fixed over one peak and are a good indicator of strong general winds in the upper atmosphere that may descend to the surface. Wave clouds are sometimes seen during foehn wind events. Check the fire weather forecast.

Automated Weather Stations

A wide variety of weather observing networks are available. Many of them can be observed from a variety of websites, such as:

Networks of Interest

  • Remote Automated Weather System (RAWS) are sited to assist land management agencies with monitoring air quality, fire weather, and in support of research applications. There are a variety of standards, though the fire weather stations adhere to a different standard called for by the National Fire Danger Rating System (NFDRS).
  • Incident Remote Automatic Weather Stations (IRAWS) are intended for use on or near the fireline, and can be rapidly relocated as desired by Fire Behavior Analysts (FBAs) or Incident Meteorologists (IMETs) to provide timely weather data. Fire Managers, FBAs and IMETs use IRAWS data to predict fire behavior, prescription burning times, fire weather forecasting, and canyon and ridge top winds. Generally, like RAWS equipment, mast heights may vary.
  • Automated Surface Observing System (ASOS) and Automated Weather Observing System (AWOS) stations located generally near airports to serve aviation needs. ASOS stations also serve as a primary climatological observing network. They adhere to international standards for weather observations.
  • A wide variety of other station networks are available and may be appropriate for local and ad hoc uses.

Wind Observations from Automated Sensors

Generally, four factors govern the surface wind estimate produced by automated weather observing sensors. 

  • Local terrain influences on the general winds at any location due to exposure and differential surface heating related to slope, water and ice factors and channeling of general winds or air flow. Image here demonstrates the effect of terrain on wind measurements in different locations (Bishop, 2010).
Surface Wind Variation due to Terrain

  • Sensor standards for timing and duration of observation. For example, fire weather standard averages windspeed over 10 minutes while International standard averages windspeed over 2 minutes.
  • Surface characteristics that produce differing friction factors (trees and buildings vs airports and agricultural regions). See figure on next page.
Surface Wind Adjustment for Friction

Generally, gradient winds are reduced by friction from the earth’s surface.  The surface friction in areas surrounded by large flat smooth surfaces (airports and agricultural areas) is less than that experienced in forest openings and in cities. (Lawson & Armitage, 2008)