Fire Weather Terms and Definitions

This page courtesy of WFO Medford, OR


Cumulus Buildups

  • Clouds which develop vertically due to unstable air. Characterized by their cauliflower-like or tower-like appearance of moderately large size (Stage 2 or better).

Deepening

  • A decrease in the central pressure of a surface low pressure system. The storm is intensifying.

Dry Thunderstorms

  • Generally a high-based thunderstorm when lightning is observed, but little if any precipitation reaches the ground. Most of the rain produced by the thunderstorm evaporates into relatively dry air beneath the storm cell. May also be referred to as "dry lightning".

Filling

  • The opposite of deepening. A general increase in the central pressure of a low pressure system.

Forecast periods

  • Today - sunrise to sunset
  • This afternoon - noon until 600pm
  • This evening - 600pm until sunset
  • Tonight - sunset to sunrise
  • Tomorrow - sunrise to sunset the following day
Fronts
  • The boundary area between two different air masses, usually where temperature, humidity, wind, and pressure change most rapidly with time and distance. In a cold front, colder air replaces existing warmer air. Normally, cold fronts produce more violent weather than warm fronts, especially with regards to winds.

High Pressure Ridge

  • A large area of clockwise circulating air generally characterized by broadscale subsidence or sinking air. The subsiding air is responsible for warm, dry conditions and a general lack of cloudiness.
Haines Index
  • A Lower Atmospheric Stability Index used to forecast the potential for large fire growth and/or erratic fire behavior. The Haines Index focusses on dry, unstable air, whereas most conventional atmospheric stability indices key on moist, unstable air.

Humidity Recovery

  • The change in relative humidity over a given period of time; generally between late evening and sunrise. The moisture change in the fine fuels during this period is directly related to the amount of humidity recovery.

Inversions

  • A condition in which temperature increases with height through a layer of the atmosphere. Vertical motion is restricted in this very stable air mass. Inversions are common during late night and early morning hours - especially in mountainous terrain - during the summer on clear nights. This type of inversion usually dissipates with daytime heating. Inversions aloft caused by large scale subsidence may persist for several days.

Low Pressure

  • A large area of rising air through a relatively deep layer of the atmosphere. As the air rises, it cools and condenses water vapor into clouds and precipitation.

Mixing Heights

  • Mixing height is a forecast of the altitude in which the atmosphere will be well mixed. The forecast will reflect the diurnal change between the highest mixing height (generally occurring in the afternoon) and the lowest mixing height (generally occurring during the early morning hours). Mixing height information will be given in either above Mean Sea Level (MSL) heights or Above Ground Level (AGL) heights. AGL height is the distance upward in the atmosphere above a given point. MSL height refers to a height in the atmosphere in relation to sea level, which would be uniform over an entire area. MSL is useful in complex terrain because on days with good mixing conditions, smoke will tend to rise to a uniform elevation in the atmosphere, regardless of the fire's elevation. To determine an AGL height from an MSL height forecast, subtract the elevation location of the fire from the MSL height.

Lightning Activity Level (LAL)

  • Lightning Activity Levels (LALs) are numbered 1 through 6.
    • LAL 1 - No Thunderstorms
    • LAL 2 - Few building cumulus with isolated thunderstorms
    • LAL 3 - Much building cumulus with scattered thunderstorms. Light to moderate rain
    • LAL 4 - Thunderstorms common. Moderate to heavy rain reaching the ground
    • LAL 5 - Numerous thunderstorms. Moderate to heavy rain reaching the ground
    • LAL 6 - Dry lightning (same as LAL 3 but without the rain).

Pressure Gradients

  • The change in value of atmospheric pressure per unit distance. The greater the change in pressure per unit distance, the stronger the pressure gradient, and the stronger the wind.

Probability of Precipitation (PoP)

  • Slight Chance - 20%
  • Chance - 30-50%
  • Likely - 60-70% Chance
  • No Remark - 80% and higher

 Red Flag  Warning

  • Highlight statement used in the fire weather forecast to alert land management agencies of the onset, or possible onset, of critical weather and fuel moisture conditions that could lead to rapid or dramatic increase in wildfire activity. This could be due to strong winds, dry lightning, dry cold fronts, etc.

Residual Moisture

  • Atmospheric moisture which lingers over an area after the main weather system has departed.

Shower Terminology

  • Isolated or Widely Scattered - Less than 24% areal coverage
  • Scattered - 25-54% areal coverage
  • Numerous - 55-74% areal coverage
  • Widespread - 75% or greater areal coverage
  • No Qualifying Remark - 75% or greater areal coverage
Sky Cover Terminology
  • Clear - Zero to 1/10th opaque cloud cover.
  • Mostly Sunny - The prevailing condition is sunny but some clouds may be present either over a portion of the area or for a short time over the entire area. 1/10th to 2/10ths cloud cover.
  • Fair - Less than 4/10ths of the sky is covered by opaque clouds. No precipitation. No extremes in weather, visibility, temperature, or wind.
  • Partly Cloudy/Partly Sunny - 3/10ths to 6/10ths of the sky will be covered by opaque clouds.
  • Mostly Cloudy/ - Cloudiness will be subject to some variability in amount or location. 7/10th to 8/10ths of the sky will be covered by opaque clouds.
  • Cloudy - The sky is essentially covered (9/10ths to 10/10ths) with clouds throughout the forecast period.

Smoke Dispersal

  • Describes the ability of the atmosphere to ventilate smoke. Depends on the stability and winds in the lower layers of the atmosphere (i.e., a combination of mixing heights and transport winds).

Smoke Dispersal Terms

  • Very Poor - High smoke pollution potential. Usually occurs in a very stable air (strong inversion) and light winds. Normally occurs late at night and early in the morning hours, but could occur during the daytime when a shallow pool of cold air intrudes into the area creating strong low level inversions. Burning is not advised under this category.
  • Poor - Moderate to High smoke potential. Burning not advised under this category. Most likely time of occurrence is from evening through the early morning.
  • Fair - Marginal smoke pollution potential. Dependent on trend of weather and local conditions. Generally acceptable for small burns of dry fuels.
  • Good - Moderate to Low smoke pollution potential. No inversion and gentle winds expected. Most likely to occur in the late morning and afternoon when surface heating usually breaks through the low level inversions.
  • Very Good - Low smoke pollution potential. Transport winds or mixing height lower than that for Excellent. Transport winds stronger than that for Good. Most likely to occur in the late morning and afternoon.
  • Excellent - Low smoke pollution potential. Unstable airmass and/or brisk winds. Best time to conduct burning operations if fire can be controlled. Most likely to occur in the late morning and afternoon or when a strong weather system affects the area, eliminating all low level inversions and generating moderate winds.

Split Flow

  • A flow pattern high in the atmosphere characterized by diverging winds. Storms moving along in this type of flow pattern usually weaken.

Stable Conditions

  • A temperature/height relationship that tends to suppress vertical motion. This condition will minimize convective activity, including vertical development of cumulus clouds, which might otherwise lead to shower activity. An inversion is a very stable condition which may trap smoke or fog near the earth's surface. Stable conditions are not favorable for turbulent surface winds or erratic fire behavior

Subsidence

  • Sinking air usually found around high pressure systems. Strong subsidence leads to very warm, dry air aloft, often appearing at high elevations first. It may arrive at day or night. Poor humidity recovery at higher elevations is usually a sign of strong subsidence.

Temperature Recovery

  • The change in temperature over a given period of time. Generally, the period between late evening and sunrise. Windy or cloudy conditions will tend to produce slow temperature recovery, while clear, calm weather can cause rapid recovery.

Thermal Trough or Heat Low

  • An area of low pressure caused by very warm, dry air. Heat lows or thermal troughs often build north along the coasts of California and Oregon in the summer. Thermal troughs can cause "east winds" in the Washington and Oregon Cascades. If however, a relatively strong disturbance in the upper atmosphere moves across the NW, it will force the thermal trough east of the Cascades. In most cases, a moderate to strong push of marine air will follow, along with strong, gusty, west winds along the east slopes of the Cascades. Gusty winds and thunderstorms can be associated with the passage of a thermal trough.

Trends

  • This term refers to the changes in temperature, relative humidity, wind speed, and fuel moisture between one day and the next. The trends delineated in the daily narrative forecasts are area wide averages for the extreme part of the day. These should not be confused with NFDRS trends for a particular zone at a particular time of the day (1300 LST).

Unstable Conditions

  • A temperature/height relationship in the atmosphere which favors vertical motion and is usually associated with cumulus clouds and possible shower or thundershower activity. Unstable conditions are favorable for turbulent surface winds and erratic fire behavior. Smoke generally disperses well in an unstable atmosphere.

Upper Level Ridges/Troughs

  • Often referred to as a high or low aloft. They occur in the upper levels of the atmosphere and may Or may not be reflected at the surface.

Ventilation Terms   (Based on Mixing Height and Transport Winds)

  • Excellent - 150,000 Knot Feet and Greater
  • Very Good - 100,000 to 150,000 Knot Feet
  • Good - 60,000 to 100,000 Knot Feet
  • Fair - 40,000 to 60,000 Knot Feet
  • Poor -  Less than 40,000 Knot Feet

Wetting Rain

  • An appreciable amount of continuous rainfall over a broad area. Usually greater than .10 inches.

Winds

  • Eye Level - Wind speed and direction measure at eye level.
  • Surface Wind (20 ft) - Air movement measured at 20 feet above the average vegetative cover. Averaged over a 10 minute period. Unless otherwise noted, this is the wind referred to in the general weather forecast.
  • Mid-Flame - The wind that acts directly on the flaming fire front at a level one- half the flame height.
  • Free Air - The wind speed and direction at a level in the atmosphere free from the effects of friction and terrain.
  • Drainage - Normal nighttime airflow directed downslope or down valley, caused by cooling of the air near the earth's surface. Air sinking toward lower elevations is usually quite gentle (light) in nature.
  • Transport - The mean wind speed and direction of all measured winds within the mixed layer.

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