Instruments Weather Glossary
following is a
glossary of weather terms, both common and uncommon, probably way more
information than most of us would ever need - and is quite
free to distribute, copy, or link
to this page.
HUMIDITY In a
system of moist air, the ratio of
the mass of water vapor to the total volume of the system; usually
grams per cubic meter (g/m³ ).
with which an instrument measures a
variable in terms of an accepted standard value or true value; usually
in terms of inaccuracy but expressed as accuracy; often expressed as a
percentage of full-scale range.
density of a parcel of air expressed
in units of mass per volume.
equation in night visual range
theory, relating the illuminance of a point source of light to distance
transmissivity of the atmosphere.
A type of
wind shield used around the mouth
of a precipitation gage to reduce the effect of wind on catch.
measurements or devices in which the output
varies continuously (e.g., voltage or rotation signals); compare to
term for instruments designed to
measure the speed or force of the wind.
barometer which measures atmospheric
pressure using one or a series of aneroid capsules.
Observing System. A network of instrumented
weather stations deployed primarily by the U.S. National Weather
Service to make
weather observations without observer involvement.
Observing System. A Federal Aviation
Administration specification describing an automatic weather station
making aviation weather observations without operator involvement.
BAR A unit of
equal to 100 kilopascals.The millibar (1/1000
bar) is commonly used in aviation and meteorology. The pascal
is the S.I. unit for pressure.
for measuring the pressure of the
atmosphere; the two principle types are aneroid and mercurial.
atmospheric pressure at a
given point due to the gravitational force on the column of air above
BAUD A unit
describing the rate of character transmission and
equal to approximately one bit per second.
Decimal. A code format in which decimal digits (0-9)
are expressed as four digit binary numbers.
A thermometer, the sensitive
element of which consists of two metal strips which have different
of expansion and are brazed together. The distortions of the system in
to temperature variations are used as a measure of temperature.
curved, flexible tube of elliptic
cross section which is deformed, according to type, by variations of
atmospheric pressure or temperature and so provides a measurement of
ascribed to the lowest layer of clouds or obscuring
phenomena when it is reported as broken, overcast, or obscuration and
classified as thin or partial. The ceiling is termed unlimited when
conditions are not satisfied.
used to measure cloud base height.
scale having the freezing point of pure water at
0° and the boiling point at 100° under standard sea
pressure. Also called
older name for the Celsius temperature scale. The use of this name was
officially abandoned by international agreement in 1948.
For a given
cloud or cloud layer, the lowest
level in the atmosphere at which the air contains a perceptible
of the cloud base above the local
cardinal points of the compass: N, NNE,
protective coating applied to circuits
usually by spray deposition.
blowing perpendicular to the course of a
moving object. Often used when referring to winds affecting ballistics.
Anemometer which measures wind speed by the
speed of rotation of 3 or 4 hemispherical or conical cups, each fixed
end of a horizontal arm projecting from a vertical axis.
parameter used to describe the response of
a wind vane to a sudden change in wind direction. It is defined as the
the actual damping to the critical damping, where critical damping is
value of damping which gives the fastest transient response without
DEAD BAND The
through which the input may be varied
without initiating a response; usually expressed as a percentage of
passage of air necessary over a wind
vane to cause the vane to respond to 5O% of a step function change in
difference between temperature
measurements taken at two significant levels above the ground.
10 and 40 meters are commonly used.
The temperature to which a
given parcel of air must be cooled at constant pressure and constant
vapor content in order for saturation to occur. Any further cooling
results in the formation of dew or frost.
measurements or devices in which the output
varies in discrete steps (e.g., on-off pulse signals); compare to
passage of air necessary over a wind
speed sensor to cause the sensor to respond to 63% of a step function
actual temperature of the
air, used for comparison with wet bulb temperature.
erasable programmable read-only memory.
programmable read-only memory.
difference between the indication by an instrument
and the true value of the measured signal.
Aviation Administration. A branch of the U.S.
Department of transportation responsible for regulating aviation
within the U.S.
temperature scale having the freezing point
of pure water at 32° and the boiling point at 212°
standard sea level
instructions which are stored in
consisting of numerous minute water drop- lets and
based at the surface; droplets are small enough to be suspended in the
Temperature of solidification of a liquid
under given conditions.
direction opposite to the course of a moving object. Often used when
to winds affecting ballistics.
FILM ANEMOMETER Anemometer which measures wind speed by measuring the
cooling of a metal film heated by an electric current.
the air; may be expressed as absolute humidity, specific humidity,
humidity, or mixing ratio.
Instrument resulting from the combination of a thermograph and a
furnishing, on the same chart, simultaneous time recording of ambient
temperature and humidity
difference noted in
a sensor's output as a response to first an increasing, and then a
input signal of the same value. If Yi is the value of the output with
increasing input of value X, and Yd is the value of the output with
input of value X, then the hysteresis can be defined as the maximum
value of the difference (Yi - Yd) for any value of X. This is
expressed as a percentage of the sensor's full scale range and usually
included as a component of total sensor error.
a sensor (or sensors), the associated transducer(s), and the data
ISOBAR A line
SPRING A spring which is designed to achieve a fixed spring constant
wide temperature range. Usually, this involves the use of an alloy with
nickel content such as Ni-Span C. It is common for these springs to be
relieved at elevated temperature after forming.
SCALE An absolute temperature scale based upon the
triple point of pure water defined as 273.16° K. The size of
degree is the
same as on the Celsius scale, and the zero point is absolute zero.
KNOT A unit
mile per hour or approximately 1.15 statute miles per hour. The
is closely related to the geographical mile which is defined as the
one minute of arc on the earth's equator. By international agreement,
nautical mile is now defined as 1852 meters.
LAW A basic equation in daytime visual range theory, relating the
luminance of a distant black object, the apparent luminance of the
background-sky above the horizon, and the extinction coefficient of the
energy per unit
area, equal to 1 gramcalorie/cm² and commonly employed in
from the wind.
A technique used to detect atmospheric constituents or related
as atmospheric extinction coefficient. Light is produced in a modulated
and the resulting backscattered or reflected light is analyzed to
property of the atmosphere.
an instrument's actual response curve from the "best fit" straight
line response. It applies only to instruments with more or less linear
response, and is usually stated as a percentage of full scale range.
THERMOMETER Thermometer in which the difference in the rates of
temperature of a liquid and its receptacle is used as a measure of the
temperature. The liquid used may be ethyl alcohol, toluene, petroleum,
The meteorological visual range, which can be
estimated from the average extinction coefficient using the Koschmieder
where Lv is
meters and (sigma)e is the extinction coefficient in meters^-1.
LEVEL WIND SHEAR A local variation in the wind direction or speed. This
condition can present danger to aircraft, especially at landing, when a
shift from headwind to tailwind can cause a rapid loss of airspeed and
THERMOMETER Thermometer used for measuring the highest temperature
during a given interval of time; for example, a day.
An aneroid barograph designed to record atmospheric pressure changes of
to 100 Pascals.The Pascal (Newton/meter² ) is the S.I. unit
THERMOMETER Thermometer used for measuring the lowest temperature
during a given interval of time; for example, a day
RATIO In a system of moist air, the dimensionless ratio of the mass of
vapor to the mass of dry air.
MILE see KNOT.
An instrument used to measure the scattering coefficient of an air
caused by suspended particles. The measurement can be used to determine
visual range through the medium.
reflecting type telescope
with a 45° mirror, so that the primary image is observed
hole in the
side of the tube.
Weather Service, administered by the U.S. Department
of Commerce, and responsible for the collection of weather data, the
production of weather forecasts, and the issuance of weather warnings
limiting values of ceiling, visibility and wind, or runway visual
established as safety minima for aircraft landings and take-offs.
Safety and Health Administration, a regulatory
office of the U.S. Department of Labor.
output which has a separate
communication path (or wire) for each bit of a digital character This
transmission makes each bit available simultaneously, and thus results
molding compound formed by the reaction
of phenol and formaldehyde. It can be heavily reinforced or "filled"
with glass fibers or other materials. Phenolics are known for their
strength, excellent wear characteristics, and dimensional stability
over a wide
that measures luminous intensity,
luminous flux, light distribution or color.
variable resistor having three terminals
and a movable wiper. Precision potentiometers can be used to create a
resistance proportional to angular or linear displacement.
all forms of water particles, liquid
or solid, that fall from the atmosphere and reach the surface.
aneroid barometer with a scale
graduated in altitude instead of pressure units.
(BAROMETRIC TENDENCY) The change in
barometric pressure within a specified period of time (typically 3
read-only memory. Read-only memory which can be
programmed by the user using a special hardware programmer.
set of rules
or conventions used to standardize
data transfer between devices.
consisting of a wet-bulb and a
dry-bulb thermometer and used to measure the water vapor content of the
which measures diffuse and direct
balloon-borne instrument for the simultaneous
measurement and transmission of meteorological data. It includes
for the measurement of pressure, temperature, and humidity, a
switching mechanism, and a radio transmitter.
for measuring the depth of water
from precipitation that is assumed to be distributed over a horizontal,
impervious surface and not subject to evaporation.
memory.The memory of a computer which can be read and
written into at any location without passing through preceding
between the lower and upper measuring limits of an
instrument e.g., a thermometer with a range of -35° to
(compare to span).
upper air observation consisting of
an evaluation of the wind speed and direction, temperature,
humidity aloft by means of a balloon-borne radiosonde tracked by radar
degree of agreement between a sensor's
output values in response to the same input value when this same input
condition is presented multiple times. The repeatability is usually
as a percentage of full scale range (i.e. 1% repeatability).
change in the parameter being
measured that causes a detectable change in the output of the
the quantity being measured as
indicated or otherwise provided by a measuring instrument.
required for an instrument to
register a designated percentage (frequently 90%) of a step change in
variable being measured.
RMS Root Mean
Square.This notation is used frequently with error
analysis. In that context, it is the square root of the arithmetic mean
squares of the deviations of the individual calibration points from the
theoretical or ideal response.
Any type of memory which cannot be readily
rewritten. A memory that cannot be altered in the normal use of a
usually used to store information permanently, such as firmware
RS 232 A
of the Electronic Industries
Association defining a standard serial data interface. A standard
between a computer input/output port and a peripheral device.
RS 422 A
similar to RS 232 which makes use of
differential transmission to provide high speed data transmission over
significantly longer distances.
RS 485 A
similar to RS 232 which permits data
interchange on multi-drop networks of up to 32 nodes using a single
pair cable. In order for this protocol to be used, each device on a
must have some level of intelligence in order to establish orderly data
transfer over a single path.
RANGE(RVR) The range
over which the pilot of an aircraft on the center line of a runway can
runway surface markings or the runway lights.
indicating marks and figures in relation to which
the position of an index is observed (e.g., a scale plate on a
A measure of the attenuation
due to scattering of light as it traverses a medium containing
of the output of an instrument to
the input (i.e. -gain).
part of a
measuring instrument which responds directly to
changes in the environment.
data output in which the
characters are sent one bit at a time over a single communication path.
Psychrometer to which a small chain or
rotary handle is attached so that the observer can rotate the
rapidly to properly ventilate the thermometer bulbs.
noticed primarily in wet snow
conditions when snow clings to the sides of a precipitation gage and
accumulates until the gage orifice is capped with accumulated snow.
can be minimized by using large collectors, and wind screens around the
and instructions which direct a
SOLID STATE A device
which is able
current without the use of moving parts or vacuum tubes.
of the differences between the upper and lower
limits of an instrument's range.
HUMIDITY In a
system of moist air, the
dimensionless ratio of the mass of water vapor to the total mass of the
A type of
wind vane having a split or
V-shaped tail. The apex orients itself into the direction of the wind.
PRESSURE VENT A
vent used with pressure
sensors to reduce the effect of wind on the pressure inlet. It is
mounted remotely and connected to the sensor using airtight tubing.
A unit of
distance equal to 5280 feet. It is
sometimes referred to as a land mile.
A power supply which
achieves its output regulation by means of one or more active power
devices which are alternately placed in the "off" or "on"
states. It is more efficient than linear supplies which vary the
power devices to achieve output regulation.
device containing a rotor and a stator and
capable of converting an angular position into an electrical signal, or
electrical signal into an angular position. When several synchros are
connected, all of the rotors will align themselves in the same angular
position. This is useful, since one synchro whose angular position is
change, can drive another synchro to indicate the angular change.
A factor relating the
response characteristics of a device with changes in the ambient
used to give a graphic record of
the time variations of temperature.
used in the measurement of
used to switch electrical current at a
selectable set-point temperature.
input value to a sensor which will
cause the sensor to respond. Commonly used with mechanical wind sensors
describe the wind speed necessary to cause the anemometer and wind vane
required for an instrument to register
63.2% of an instantaneous change in the measured parameter.
Anemometer in which the
sensor rotation is transmitted to a mechanical counter which directly
integrates the air movement past the instrument.
common support used to fixture maximum
and minimum thermometers. It is designed to hold the thermometers at
angles for measuring, and it also simplifies resetting of the
measure of luminous flux remaining in a
light beam after it has passed through a specified distance of the
instrument which measures the
transmissivity of the atmosphere between two points for the
Temperature to which absolutely
dry air would have to be brought in order for it to have the density as
air considered at the same pressure.
distance at which it is possible
with the unaided eye to recognize a prominent dark object against the
sky. At night, it is defined as the greatest distance at which a
intense, unfocused light source can be seen on the horizon.
General term for an instrument used to
make direct measurements of visual range or measurements of the
characteristics of the atmosphere which determine the visual range
VISUAL RANGE The maximum
horizontally, at which a given object or light source is just visible
particular conditions of transmittance and background luminance.
The temperature of the wet
bulb thermometer at equilibrium with a constant flow of ambient air at
of 2.5 to 10.0 meters per second.
thermometer with a
muslin-covered bulb which is moistened; used to measure wet-bulb
direction from which the wind is
blowing. Usually measured in degrees azimuth.
WIND GUST The
momentary wind velocity within a given
interval of time.
distance or length of flow of the air
past a point during a given interval of time.
WIND ROSE A
diagram indicating the relative
frequencies of different wind directions for a given station and period
The rate of
wind movement in units of
WIND VANE An
used to indicate wind direction.
WIND VECTOR A
of the wind (often using Cartesian
coordinates; i.e. X and Y wind vectors). The term can also apply to the
resultant wind vector which is sometimes drawn as an arrow with length
to wind speed.
term which includes both wind speed
and wind direction.
the side from which the wind blows.