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FIRE WEATHER TERMINOLOGY
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 till 6 p.m.
This evening.....................6 p.m. till sunset
Tonight.............................sunset till sunrise
Tomorrow.........................sunrise to sunset of 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.
MARINE PUSH...A replacement of the current air mass with air from off
the ocean. Temperatures are
much cooler and relative humidities much higher. The air mass is generally
much more stable in this situation.
MIXING HEIGHTS...The height to which a parcel of air, or a column of
smoke, will rise, mix or disperse. A column of smoke will remain trapped
below this height.
LALs (L)ightning (A)ctivity (L)evels 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.
POPs (P)robability (o)f (P)recipitation.
Slight Chance ...................10-20% chance
Chance .............................30-40% chance
Good chance ....................50% chance
Likely ................................ 60-70% chance
No remark .........................80% or greater
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 few..... Less than 15% areal coverage
Widely scattered..... 15-24% areal coverage
Scattered..... 25-54% areal coverage
Numerous..... 55% or greater areal coverage
Widespread...... 55% 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/Considerable Cloudiness...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.
Variable Cloudiness/Occasionally Cloudy...An irregular condition in which
cloud cover increases or decreases several times during the forecast period.
Variable High Clouds...Same as above but clouds above 18,000 feet.
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.
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 OR UPPER LEVEL 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.
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 downvalley,
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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