Windward
Instruments Weather Glossary
The following is a
glossary of weather terms, both common and uncommon, and is quite
long.
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to this page.
Weather Glossary:
ABSOLUTE HUMIDITY In a
system of moist air, the ratio of
the mass of water vapor to the total volume of the system; usually
expressed as
grams per cubic meter (g/m³ ).
ACCURACY The degree
with which an instrument measures a
variable in terms of an accepted standard value or true value; usually
measured
in terms of inaccuracy but expressed as accuracy; often expressed as a
percentage of full-scale range.
AIR DENSITY The mass
density of a parcel of air expressed
in units of mass per volume.
ALLARD'S LAW A basic
equation in night visual range
theory, relating the illuminance of a point source of light to distance
and the
transmissivity of the atmosphere.
ALTER SHIELD A type of
wind shield used around the mouth
of a precipitation gage to reduce the effect of wind on catch.
ANALOG Pertaining to
measurements or devices in which the output
varies continuously (e.g., voltage or rotation signals); compare to
digital.
ANEMOMETER A general
term for instruments designed to
measure the speed or force of the wind.
ANEROID BAROMETER A
barometer which measures atmospheric
pressure using one or a series of aneroid capsules.
ASOS Automated Surface
Observing System. A network of instrumented
weather stations deployed primarily by the U.S. National Weather
Service to make
weather observations without observer involvement.
AWOS Automated Weather
Observing System. A Federal Aviation
Administration specification describing an automatic weather station
capable of
making aviation weather observations without operator involvement.
BAR A unit of pressure
equal to 100 kilopascals.The millibar (1/1000
bar) is commonly used in aviation and meteorology. The pascal
(Newton/meter² )
is the S.I. unit for pressure.
BAROGRAPH A
continuous-recording barometer.
BAROMETER An instrument
for measuring the pressure of the
atmosphere; the two principle types are aneroid and mercurial.
BAROMETRIC PRESSURE The
atmospheric pressure at a
given point due to the gravitational force on the column of air above
it.
BAUD A unit used in
describing the rate of character transmission and
equal to approximately one bit per second.
BCD Binary Coded
Decimal. A code format in which decimal digits (0-9)
are expressed as four digit binary numbers.
BIMETALLIC THERMOMETER
A thermometer, the sensitive
element of which consists of two metal strips which have different
coefficients
of expansion and are brazed together. The distortions of the system in
response
to temperature variations are used as a measure of temperature.
BOURDON TUBE Closed,
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
the
particular parameter.
CEILING The height
ascribed to the lowest layer of clouds or obscuring
phenomena when it is reported as broken, overcast, or obscuration and
not
classified as thin or partial. The ceiling is termed unlimited when
these
conditions are not satisfied.
CEILOMETER Instrument
used to measure cloud base height.
CELSIUS A temperature
scale having the freezing point of pure water at
0° and the boiling point at 100° under standard sea level
pressure. Also called
Centigrade scale.
CENTIGRADE TEMPERATURE
SCALE The
older name for the Celsius temperature scale. The use of this name was
officially abandoned by international agreement in 1948.
CLOUD BASE For a given
cloud or cloud layer, the lowest
level in the atmosphere at which the air contains a perceptible
quantity of
cloud particles.
CLOUD HEIGHT The height
of the cloud base above the local
terrain.
COMPASS POINTS The
cardinal points of the compass: N, NNE,
NE, etc.
CONFORMAL COATING A
protective coating applied to circuits
usually by spray deposition.
CROSSWIND A wind
blowing perpendicular to the course of a
moving object. Often used when referring to winds affecting ballistics.
CUP ANEMOMETER
Anemometer which measures wind speed by the
speed of rotation of 3 or 4 hemispherical or conical cups, each fixed
to the
end of a horizontal arm projecting from a vertical axis.
DAMPING RATIO A
parameter used to describe the response of
a wind vane to a sudden change in wind direction. It is defined as the
ratio of
the actual damping to the critical damping, where critical damping is
that
value of damping which gives the fastest transient response without
overshoot.
DEAD BAND The range
through which the input may be varied
without initiating a response; usually expressed as a percentage of
full-scale
range.
DELAY DISTANCE The
passage of air necessary over a wind
vane to cause the vane to respond to 5O% of a step function change in
wind
direction.
DELTA TEMPERATURE The
difference between temperature
measurements taken at two significant levels above the ground.
Temperatures at
10 and 40 meters are commonly used.
DEWPOINT TEMPERATURE
The temperature to which a
given parcel of air must be cooled at constant pressure and constant
water
vapor content in order for saturation to occur. Any further cooling
usually
results in the formation of dew or frost.
DIGITAL Pertaining to
measurements or devices in which the output
varies in discrete steps (e.g., on-off pulse signals); compare to
analog.
DISTANCE CONSTANT The
passage of air necessary over a wind
speed sensor to cause the sensor to respond to 63% of a step function
change in
wind speed.
DRYBULB TEMPERATURE The
actual temperature of the
air, used for comparison with wet bulb temperature.
EEPROM Electrically
erasable programmable read-only memory.
EPROM Erasable
programmable read-only memory.
ERROR The algebraic
difference between the indication by an instrument
and the true value of the measured signal.
FAA The Federal
Aviation Administration. A branch of the U.S.
Department of transportation responsible for regulating aviation
operations
within the U.S.
FAHRENHEIT A
temperature scale having the freezing point
of pure water at 32° and the boiling point at 212° under
standard sea level
pressure.
FIRMWARE Programs or
instructions which are stored in
read-only memory.
FOG A hydrometeor
consisting of numerous minute water drop- lets and
based at the surface; droplets are small enough to be suspended in the
earth's
atmosphere indefinitely.
FREEZING POINT
Temperature of solidification of a liquid
under given conditions.
HEADWIND A wind blowing
in a
direction opposite to the course of a moving object. Often used when
referring
to winds affecting ballistics.
HOT
FILM ANEMOMETER Anemometer which measures wind speed by measuring the
degree of
cooling of a metal film heated by an electric current.
HUMIDITY Water vapor
content of
the air; may be expressed as absolute humidity, specific humidity,
relative
humidity, or mixing ratio.
HYGROTHERMOGRAPH
Instrument resulting from the combination of a thermograph and a
hygrograph and
furnishing, on the same chart, simultaneous time recording of ambient
temperature and humidity
HYSTERESIS The
difference noted in
a sensor's output as a response to first an increasing, and then a
decreasing,
input signal of the same value. If Yi is the value of the output with
an
increasing input of value X, and Yd is the value of the output with
decreasing
input of value X, then the hysteresis can be defined as the maximum
absolute
value of the difference (Yi - Yd) for any value of X. This is
frequently
expressed as a percentage of the sensor's full scale range and usually
is
included as a component of total sensor error.
INSTRUMENT A term used
to describe
a sensor (or sensors), the associated transducer(s), and the data
read-out or
recording device.
ISOBAR A line of equal
or constant
barometric pressure.
ISO-ELASTIC
SPRING A spring which is designed to achieve a fixed spring constant
over a
wide temperature range. Usually, this involves the use of an alloy with
high
nickel content such as Ni-Span C. It is common for these springs to be
stress
relieved at elevated temperature after forming.
KELVIN TEMPERATURE
SCALE An absolute temperature scale based upon the
triple point of pure water defined as 273.16° K. The size of the
degree is the
same as on the Celsius scale, and the zero point is absolute zero.
KNOT A unit of speed,
one nautical
mile per hour or approximately 1.15 statute miles per hour. The
nautical mile
is closely related to the geographical mile which is defined as the
length of
one minute of arc on the earth's equator. By international agreement,
the
nautical mile is now defined as 1852 meters.
KOSCHMIEDER'S
LAW A basic equation in daytime visual range theory, relating the
apparent
luminance of a distant black object, the apparent luminance of the
background-sky above the horizon, and the extinction coefficient of the
atmosphere.
LANGLEY A unit of
energy per unit
area, equal to 1 gramcalorie/cm² and commonly employed in
radiation
measurement.
LEEWARD Facing away
from the wind.
LIDAR Light detecting
and ranging.
A technique used to detect atmospheric constituents or related
parameters such
as atmospheric extinction coefficient. Light is produced in a modulated
source
and the resulting backscattered or reflected light is analyzed to
quantify some
property of the atmosphere.
LINEARITY The maximum
departure of
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.
LIQUID
THERMOMETER Thermometer in which the difference in the rates of
expansion with
temperature of a liquid and its receptacle is used as a measure of the
temperature. The liquid used may be ethyl alcohol, toluene, petroleum,
or
mercury.
LOCAL VISUAL DISTANCE
The meteorological visual range, which can be
estimated from the average extinction coefficient using the Koschmieder
equation:
Lv = 3.9/(sigma)e
where Lv is the visual
Range in
meters and (sigma)e is the extinction coefficient in meters^-1.
LOW
LEVEL WIND SHEAR A local variation in the wind direction or speed. This
condition can present danger to aircraft, especially at landing, when a
sudden
shift from headwind to tailwind can cause a rapid loss of airspeed and
lift.
MAXIMUM
THERMOMETER Thermometer used for measuring the highest temperature
attained
during a given interval of time; for example, a day.
MICROBAROGRAPH
An aneroid barograph designed to record atmospheric pressure changes of
very
small magnitude.
MILLIBAR A unit of
pressure equal
to 100 Pascals.The Pascal (Newton/meter² ) is the S.I. unit for
pressure.
MINIMUM
THERMOMETER Thermometer used for measuring the lowest temperature
attained
during a given interval of time; for example, a day
MIST Drizzle or heavy
fog.
MIXING
RATIO In a system of moist air, the dimensionless ratio of the mass of
water
vapor to the mass of dry air.
NAUTICAL
MILE see KNOT.
NEPHELOMETER
An instrument used to measure the scattering coefficient of an air
sample
caused by suspended particles. The measurement can be used to determine
the
visual range through the medium.
NEWTONIAN TELESCOPE A
reflecting type telescope
with a 45° mirror, so that the primary image is observed through a
hole in the
side of the tube.
NWS The National
Weather Service, administered by the U.S. Department
of Commerce, and responsible for the collection of weather data, the
routine
production of weather forecasts, and the issuance of weather warnings
within
the U.S.
OPERATIONAL WEATHER
LIMITS The
limiting values of ceiling, visibility and wind, or runway visual
range,
established as safety minima for aircraft landings and take-offs.
OSHA The Occupational
Safety and Health Administration, a regulatory
office of the U.S. Department of Labor.
PARALLEL OUTPUT An
output which has a separate
communication path (or wire) for each bit of a digital character This
form of
transmission makes each bit available simultaneously, and thus results
in very
fast communications.
PHENOLIC A plastic
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
high impact
strength, excellent wear characteristics, and dimensional stability
over a wide
temperature range.
PHOTOMETER Instrument
that measures luminous intensity,
luminous flux, light distribution or color.
POTENTIOMETER A
variable resistor having three terminals
and a movable wiper. Precision potentiometers can be used to create a
variable
resistance proportional to angular or linear displacement.
PRECIPITATION Any and
all forms of water particles, liquid
or solid, that fall from the atmosphere and reach the surface.
PRESSURE ALTIMETER An
aneroid barometer with a scale
graduated in altitude instead of pressure units.
PRESSURE TENDENCY
(BAROMETRIC TENDENCY) The change in
barometric pressure within a specified period of time (typically 3
hours for
meteorological observations).
PROM Programmable
read-only memory. Read-only memory which can be
programmed by the user using a special hardware programmer.
PROTOCOL A set of rules
or conventions used to standardize
data transfer between devices.
PSYCHROMETER Instrument
consisting of a wet-bulb and a
dry-bulb thermometer and used to measure the water vapor content of the
air.
PYRANOMETER instrument
which measures diffuse and direct
solar radiation.
RADIOSONDE A
balloon-borne instrument for the simultaneous
measurement and transmission of meteorological data. It includes
transducers
for the measurement of pressure, temperature, and humidity, a
modulator, a
switching mechanism, and a radio transmitter.
RAIN GAUGE Instrument
for measuring the depth of water
from precipitation that is assumed to be distributed over a horizontal,
impervious surface and not subject to evaporation.
RAM Random-access
memory.The memory of a computer which can be read and
written into at any location without passing through preceding
locations.
RANGE The interval
between the lower and upper measuring limits of an
instrument e.g., a thermometer with a range of -35° to 50° C
(compare to span).
RAWINSONDE A method of
upper air observation consisting of
an evaluation of the wind speed and direction, temperature,
pressure,and
humidity aloft by means of a balloon-borne radiosonde tracked by radar
or a
radio theodolite.
REPEATABILITY The
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
expressed
as a percentage of full scale range (i.e. 1% repeatability).
RESOLUTION The smallest
change in the parameter being
measured that causes a detectable change in the output of the
instrument.
RESPONSE The value of
the quantity being measured as
indicated or otherwise provided by a measuring instrument.
RESPONSE TIME The time
required for an instrument to
register a designated percentage (frequently 90%) of a step change in
the
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
of the
squares of the deviations of the individual calibration points from the
theoretical or ideal response.
ROM Read-Only Memory.
Any type of memory which cannot be readily
rewritten. A memory that cannot be altered in the normal use of a
computer;
usually used to store information permanently, such as firmware
programs.
RS 232 A specification
of the Electronic Industries
Association defining a standard serial data interface. A standard
interface
between a computer input/output port and a peripheral device.
RS 422 A protocol
similar to RS 232 which makes use of
differential transmission to provide high speed data transmission over
significantly longer distances.
RS 485 A protocol
similar to RS 232 which permits data
interchange on multi-drop networks of up to 32 nodes using a single
twisted
pair cable. In order for this protocol to be used, each device on a
network
must have some level of intelligence in order to establish orderly data
transfer over a single path.
RUNWAY VISUAL RANGE(RVR) The range
over which the pilot of an aircraft on the center line of a runway can
see the
runway surface markings or the runway lights.
SCALE The array of
indicating marks and figures in relation to which
the position of an index is observed (e.g., a scale plate on a
recorder).
SCATTERING COEFFICIENT
A measure of the attenuation
due to scattering of light as it traverses a medium containing
scattering
particles.
SENSITIVITY The ratio
of the output of an instrument to
the input (i.e. -gain).
SENSOR The part of a
measuring instrument which responds directly to
changes in the environment.
SERIAL OUTPUT A digital
data output in which the
characters are sent one bit at a time over a single communication path.
SLING PSYCHROMETER
Psychrometer to which a small chain or
rotary handle is attached so that the observer can rotate the
instrument
rapidly to properly ventilate the thermometer bulbs.
SNOW BRIDGING An effect
noticed primarily in wet snow
conditions when snow clings to the sides of a precipitation gage and
gradually
accumulates until the gage orifice is capped with accumulated snow.
This effect
can be minimized by using large collectors, and wind screens around the
gages.
SOFTWARE The programs
and instructions which direct a
computer.
SOLID STATE A device which is able
to control
current without the use of moving parts or vacuum tubes.
SPAN The absolute value
of the differences between the upper and lower
limits of an instrument's range.
SPECIFIC HUMIDITY In a
system of moist air, the
dimensionless ratio of the mass of water vapor to the total mass of the
system.
SPLAYED TAIL A type of
wind vane having a split or
V-shaped tail. The apex orients itself into the direction of the wind.
STATIC PRESSURE VENT A
vent used with pressure
sensors to reduce the effect of wind on the pressure inlet. It is
normally
mounted remotely and connected to the sensor using airtight tubing.
STATUTE MILE A unit of
distance equal to 5280 feet. It is
sometimes referred to as a land mile.
SWITCHING POWER SUPPLY
A power supply which
achieves its output regulation by means of one or more active power
handling
devices which are alternately placed in the "off" or "on"
states. It is more efficient than linear supplies which vary the
conduction of
power devices to achieve output regulation.
SYNCHRO A motor like
device containing a rotor and a stator and
capable of converting an angular position into an electrical signal, or
an
electrical signal into an angular position. When several synchros are
correctly
connected, all of the rotors will align themselves in the same angular
position. This is useful, since one synchro whose angular position is
forced to
change, can drive another synchro to indicate the angular change.
TEMPERATURE COEFFICIENT
A factor relating the
response characteristics of a device with changes in the ambient
temperature.
THERMOGRAPH Thermometer
used to give a graphic record of
the time variations of temperature.
THERMOMETER Instrument
used in the measurement of
temperature.
THERMOSTAT A device
used to switch electrical current at a
selectable set-point temperature.
THRESHOLD The smallest
input value to a sensor which will
cause the sensor to respond. Commonly used with mechanical wind sensors
to
describe the wind speed necessary to cause the anemometer and wind vane
to
turn.
TIME CONSTANT The time
required for an instrument to register
63.2% of an instantaneous change in the measured parameter.
TOTALIZING ANEMOMETER
Anemometer in which the
sensor rotation is transmitted to a mechanical counter which directly
integrates the air movement past the instrument.
TOWNSEND SUPPORT A
common support used to fixture maximum
and minimum thermometers. It is designed to hold the thermometers at
the proper
angles for measuring, and it also simplifies resetting of the
thermometer
markers.
TRANSMISSIVITY A
measure of luminous flux remaining in a
light beam after it has passed through a specified distance of the
atmosphere.
TRANSMISSOMETER An
instrument which measures the
transmissivity of the atmosphere between two points for the
determination of
visual range.
VIRTUAL TEMPERATURE
Temperature to which absolutely
dry air would have to be brought in order for it to have the density as
moist
air considered at the same pressure.
VISIBILITY The greatest
distance at which it is possible
with the unaided eye to recognize a prominent dark object against the
horizon
sky. At night, it is defined as the greatest distance at which a
moderately
intense, unfocused light source can be seen on the horizon.
VISIBILITY SENSOR
General term for an instrument used to
make direct measurements of visual range or measurements of the
physical
characteristics of the atmosphere which determine the visual range
VISUAL RANGE The maximum distance,
usually
horizontally, at which a given object or light source is just visible
under
particular conditions of transmittance and background luminance.
WET BULB TEMPERATURE
The temperature of the wet
bulb thermometer at equilibrium with a constant flow of ambient air at
a rate
of 2.5 to 10.0 meters per second.
WET BULB THERMOMETER A
thermometer with a
muslin-covered bulb which is moistened; used to measure wet-bulb
temperature
WIND DIRECTION The
direction from which the wind is
blowing. Usually measured in degrees azimuth.
WIND GUST The peak
momentary wind velocity within a given
interval of time.
WIND PASSAGE The
distance or length of flow of the air
past a point during a given interval of time.
WIND ROSE A flower-like
diagram indicating the relative
frequencies of different wind directions for a given station and period
of
time.
WIND SPEED The rate of
wind movement in units of
distance/time.
WIND VANE An instrument
used to indicate wind direction.
WIND VECTOR A component
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
proportional
to wind speed.
WIND VELOCITY A vector
term which includes both wind speed
and wind direction.
WINDWARD Situated on
the side from which the wind blows.
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