(process): A thermodynamic change of state of
a system in which there is no transfer of heat or mass (i.e. compression
results in warming, expansion in cooling).
Chart: A thermodynamic diagram with temperature as abscissa and
pressure as ordinate.
Transport of an atmospheric property solely by the mass motion of the
Mass: A homogenous mass of air, the properties of which can be
identified as having been established while the air was situated over
a particular region of the earth's surface.
Wind: An upslope wind usually applied only when the wind is blowing
up hill or mountain as the result of local surface heating, and apart
from the effects of the larger scale circulation.
Instrument designed to measure the speed of the wind.
A closed circulation of the atmosphere, of relative high pressure. (clockwise
flow in the northern hemisphere).
Pressure: The pressure exerted by the air as a result of gravity.
It is measured by the barometer, and expressed in millibars or inches
A change of wind direction in a counter-clockwise direction (the opposite
of veering); i.e. from west to south.
Obstruction, on a large scale, of the normal west to east movement of
highs and lows.
Absence of apparent motion of the air.
The statistical collective of weather conditions over a specified period
of time (i.e. usually several decades).
The visible aggregate of minute water droplets and/or ice particles
in the atmosphere above the earths's surface.
Front: The leading edge of a relatively cold air mass.
An area of low pressure within which more than one center is found.
The physical process by which a vapor becomes a liquid.
The transfer of energy within and through a conductor by means of molecular
In meteorology, a line of constant height on a constant pressure surface
In meteorology, atmospheric motions that are predominantly vertical,
i.e. usually means upward as opposed to subsidence (downward).
Net horizontal inflow of air into a layer. If at the surface, vertical
motion results. Associated with low pressure systems.
Force: The deflecting influence of the earth's rotation on winds.
Deflects winds to the right (with wind at your back) in the northern
Fire Weather Pattern: Patterns that can quickly increase fire
danger and trigger rapid fire spread.
A principal cloud type in the form of individual detached elements,
sharp non-fibrous outlines, and vertical development.
The ultimate growth of a cumulus cloud into a mushroom shape, with considerable
vertical growth, usually fibrous ice crystal tops, and probably accompanied
by lightning, thunder, hail and strong winds.
off Low: A cold low which has become displaced to south, out
of the basic westerly flow.
Having a sense of counter-clockwise rotation about the local vertical.
A closed isobaric circulation in the atmosphere, with counter-clockwise
rotation in the northern hemisphere.
A decrease in the central pressure of a cyclonic, or low pressure system.
Point: The temperature to which a parcel of air must be cooled
to reach saturation.
front: A front across which the wind shift and temperature change
are weakly defined.
Downward (subsidence) motion results. Associated with high pressure
Daily, especially pertaining to daily cycles of temperature, relative
humidity and wind.
A period of moisture deficiency, extensive in space and time.
Devil: A small but vigorous whirlwind, usually of short duration.
Forecast: A forecast of general weather conditions for days 3
A weather term implying no precipitation and no extreme conditions of
clouds, visibility or wind.
Danger: A subjective expression of an objective assessment of
environmental (fuels and weather) factors which influence whether fires
will start and how they may spread.
Weather Watch: Issued when the forecaster feels reasonably confident
that red flag conditions will develop in the next 12 to 48 hours.
Same as cloud except base of cloud is touching earth's surface.
A transition zone between two air masses of different density.
Air: That portion of the atmosphere that is not modified by local
Air Wind: The wind at the bottom layer of the atmosphere called
free air, or just above the portion that is modified by local influences
and friction approximately 1,000 feet above the earth's surface.
Layer: The layer of air from the earth's surface to the geostrophic
wind level or level of free wind.
Wind: A horizontal wind resultant of the balance of the Coriolis
acceleration and the horizontal pressure force.
Wind: A horizontal wind velocity tangent to the contour line
or isobaric surface resulting in a balance of the coriolis, pressure
and centrifugal force.
Wind: A wind directed down a slope caused by greater air density
near the slope than at the same height at a distance from the slope.
(Also called drainage or downslope wind).
A sudden brief increase in wind speed.
Index: A lower atmospheric stability and dryness index that can
be one predictor of large fire growth due to plume dominated fires.
A brief statement at the beginning of a forecast that highlights dangerous
or changing weather conditions.
Internal energy. A form of energy transferred between systems by virtue
of a difference in temperature.
Lightning: The luminosity observed from ordinary lightning too
far away for its thunder to be heard.
Low: An area of low pressure due to high temperatures (thermal
In reference to precipitation, more than a half inch in forecast period.
Pressure: An anticyclone. An area of atmospheric pressure with
closed isobars and relative high pressure at its center. Air flows clockwise
around a high.
Clouds: Cirrus type clouds composed of ice crystals, usually
above 25,000 feet.
In reference to an airmass having similar horizontal properties or elements.
A measure of the water vapor content of the air.
An instrument that records temperature and relative humidity as a function
A property of the steady state of a system such that certain disturbances
introduced into the steady state will increase in magnitude.
Line: A band of convective activity in the atmosphere, i.e. squall
line, a line of active thunderstorms.
An increase in temperature with height, i.e. a departure from the usual
decrease of temperature with increase of altitude.
A line passing through points having equal atmospheric pressure.
A line passing through points having equal or constant values of a given
gravity, with respect to either time or space.
Affecting less than 20% percent of the area.
Stream: Relatively strong winds concentrated within a narrow
stream in the atmosphere.
wind: A wind blowing down an incline. If warm, it is called a
foehn or chinook. If cold, its called a gravity or mountain wind.
Station Forecast: A forecast that takes into account an observing
station location and the microscale effects on weather parameters in
presenting exact values of weather parameters rather than ranges.
The unit of speed in the nautical system; a nautical mile per hour.
It is equal to 1.15 statute miles per hour.
Rate: The rate of change of temperature with height. The moist
lapse rate is 3.5 degrees per 1000 feet and the dry lapse rate is 5.5
degrees per 1000 feet.
Trough: A low pressure trough formed on the lee side of a mountain
range across which the wind is blowing at nearly right angles.
Precipitation, ranging from .11 to .20 inches in a forecast period.
All the various visible electrical discharges produced by thunderstorms.
It can be cloud to cloud, cloud to ground, or cloud to air.
Activity Level (LAL): An objective rating system used in the
NFDRS that indicates the amount of cloud to ground lightning observed
or forecast in a given area.
Winds: Winds, which over a small area, differ from those appropriate
to the general pressure distribution.
Range: An extended forecast for a period greater than 5 days.
Wave: A wave in the major belt of westerlies which is characterized
by large length and significant amplitude.
An area of low atmospheric pressure having closed isobars. Used interchangeably
Aloft: An upper level cyclone (low pressure system).
Trough: A long wave trough in the large scale pattern of the
Layer: A shallow layer of air with relatively high humidity and
cooler temperatures that moves from the ocean over land. It may be associated
with diurnal land/sea breeze regimes or other features that increase
on-shore pressure gradients.
A scale that ranges in size from a few kilometers to about 100 kilometers.
The study of the phenomena of the atmosphere.
A scale that covers phenomena smaller than those in the mesoscale range.
Clouds: Clouds of the altocumulus or altostratus family, anywhere
from 7,000 to 25,000 feet in elevation.
Trough: A pressure trough in the upper air of smaller scale than
a long wave trough. It usually moves rapidly, i.e. short wave.
Height: Maximum depth to which mixing will occur.
Output Statistics: A generation of point specific output from
a numerical model.
Precipitation, ranging from .21 to .50 inches in a forecast period.
and Valley Winds: Diurnal winds along the axis of a valley, blowing
uphill and valley during the day and downhill and down valley during
Fire Danger Rating System (NFDRS): A national system used by
all land management agencies to assess fuels, weather and fire potential
on a daily basis during fire season.
Stability: The state of a parcel of air, which if displaced vertically,
will experience no buoyant acceleration.
The average value of a meteorological element over a period of years,
usually 30 in the United States.
Occurrence of a meteorological element at infrequent intervals and for
Of, pertaining to, or caused by mountains.
A cloud layer that covers most, or all of the sky.
Condition existing in which an air mass is in motion aloft above another
air mass of greater density at the surface.
The tendency for the occurrence of a specific event to be more probable,
at a given time, if that same event has occurred immediately preceding
the time period.
Front: The semi-permanent, semi-continuous front separating air
masses of polar and tropical origin.
Any or all the forms of water particles, liquid or solid, that fall
from the atmosphere and reach the ground.
Fire/Burn: A natural or human ignited fire burning under a strict
set of predetermined conditions to fulfill specified land management
Center: The center of a high (anticyclone) or low (cyclone) pressure
Wind Direction: The wind direction most frequently observed during
a given period.
The chance that a prescribed event will occur.
Forecast: A forecast of the probability of occurrence of one
or more of a mutually exclusive set of weather contingencies, as distinguished
from a series of categorical statements.
(Prog) Chart: A chart depicting some meteorological parameter
at a specified future time.
(chart): A thermodynamic chart (process), same as adiabatic,
but with saturation adiabats added.
An instrument used for measuring the water vapor content of the air.
Front: A front which is stationary, or nearly so,
The process by which electromagnetic radiation is propagated through
Cooling: The cooling of the earth's surface suffers a net loss
of heat due to terrestrial cooling.
(RAOB): Balloon-borne instument for the measurement and transmission
of temperature, humidity and pressure. When tracked by radar, also provides
wind direction and velocity (Rawin).
Flag Criteria: A locally determined set of criteria that expresses
environmental and meteorological conditions that would provide for fire
starts and rapid, dangerous fire spread.
Flag Warning: A warning issued by the forecasters when red flag
criteria are met or expected to be met within 12 to 24 hours. The warning
highlights weather of particular importance to fire behavior and potentially
extreme burning conditions or many new fires. Red flag warnings should
always be coordinated with the customer.
Humidity: (Humidity) - The ratio of the actual amount of water
vapor in the air to the possible amount at that temperature.
Usually used to denote the movement of a weather system in a direction
opposite to that of the normal flow in which the system is embedded.
An elongated area of relatively high atmospheric pressure.
The condition in which the air contains all the water vapor possible
at that temperature, i.e. 100% relative humidity.
Level Pressure: The atmospheric pressure at mean sea level.
Used in reference to thunderstorm intensity. Indicates strong winds
and large hail.
Wave: (Minor Wave) - A progressive wave in the pattern of upper
air motion with dimensions of cyclonic scale, as distinguished from
a long wave.
Precipitation from a convective cloud, characterized by the suddenness
with which they start, stop, and change intensity.
Management Parameters: The weather parameters used to forecast
smoke dispersal (mixing height and transport winds).
Weather Forecast: A specialized forecast issued by the National
Weather Service for a localized area and time, at the request of the
A strong wind characterized by a sudden onset, of longer duration than
gusts, and a rather sudden decrease in speed.
Factor: This is determined by temperature differences between
two atmospheric layers.
A property of the steady state of a system such that a disturbance introduced
into the steady state will not increase in magnitude.
In meteorology, usually refers to cyclonic storms with considerable
cloud and precipitation areas.
Descriptive of clouds of extensive horizontal development.
A descending motion of air in the atmosphere, of particular importance
due to the heating and drying of the air as it contracts.
Chart: An analyzed map showing the distribution of sea level
pressure (isobars) and location of fronts and air masses.
Pressure: The atmospheric pressure at a given location on the
A statement giving a brief general review or summary.
The degree of hotness or coldness as measured on some definite temperature
scale by means of various types of thermometers.
Belts: An area along the middle of a mountain slope that typically
experiences the least diurnal variation in temperature and humidity,
thus has the highest daily average temperature and the lowest relative
Low: (Heat Low) - An area of low atmospheric pressure due to
high temperatures and intensive heating at earth's surface, usually
stationary and have weak cyclonic circulation.
Cumulonimbus or ice top cumulus.
A local storm produced by cumulonimbus clouds accompanied by lightning
and thunder...often containing heavy rain...and sometimes strong winds
A detailed description of surface features including rivers, lakes,
A violently rotating column of air, pendant from a cumulonimbus cloud
observed as a funnel cloud.
Cumulus: The transitory stage of a cumulus into a cumulonimbus
Winds: Winds in the lower mixed layer, used for smoke dispersal
Cyclone: The general term for a cyclone that originates over
the tropical oceans. The remnants of these storms occasionally recurve
and move into the mid-latitude westerlies, bringing considerable moisture
The boundary between the troposphere and the stratosphere, usually characterized
by an abrupt change to a small lapse rate.
An elongated area of relatively low atmospheric pressure, the axis of
which is called a trough line.
A state of fluid flow in which the instantaneous velocities exhibit
irregular and apparently random fluctuations.
Air: Generally applied to levels above 850 mb (5,000 feet).
Wind: A wind directed up a slope during the hot part of the day.
Wind: A wind which ascends a mountain valley during the day.
Index: This is the product of the mixing height and transport
wind speed, and is an indicator of dispersion potential.
Wind: A change in wind direction in a clockwise manner, i.e.
south to southwest to west, the opposite of backing.
Water or ice particles falling out of a cloud but evaporating before
reaching the ground.
The greatest distance it is possible to see permanent objects with the
Front: A front that moves in such a way that warmer air replaces
A disturbance propagated by virtue of periodic motions in the atmosphere.
The state of the atmosphere, usually short term, with respect to its
effects upon life, property and human activities.
Rain: Precipitation of .10 inches or more over most of the area
The dominant west to east motion of the atmosphere across the mid latitudes.
Bulb Temperature: The temperature an air parcel would have if
cooled to saturation. It lies between the dry bulb temperature and the
dew point temperature.
A small scale rotating column of air (dust devil).
Air in horizontal motion relative to the surface of the earth.
Direction: The direction from which the wind is blowing.
Flow: The flow of air along a latitude circle.
Weather Forecast: A portion of the general fire weather forecast
issued on a regular basis during the normal fire season specifically
to fit the requirements of fire management needs. These zones or areas
are a combination of administrative and climatological areas, usually
nearly the size of an individual forest or district.