Aviation Weather Instruments Dictionary
Note.- These explanations are generally based on established scientific
definitions, some of which have been simplified to assist non-specialist readers.
Approved ICAO definitions
are marked with an asterisk (*) and published WMO
definitions1 with a double asterisk (**). The
units, where appropriate, are indicated in brackets.
Air temperature. The temperature indicated by a thermometer
exposed to the air in a place sheltered from direct solar radiation (degree
Celsius, ° C).
Allard's law. An equation relating luminance (E) produced by
a point source of light of intensity (I) on a plane normal to the line of
sight, at distance (x) from the source, in an atmosphere having a transmissivity
(T).
Note.- Applicable to the visual range of lights.
Atmospheric pressure. Pressure (force per unit area) exerted
by the atmosphere on any surface by virtue of its weight; it is equivalent
to the weight of a vertical column of air extending above a surface of unit
area to the outer limit of the atmosphere. (hectopascal, hPa)
Ceilometer. Instrument for measuring the height of the base
of a cloud layer, with or without a recording device. Measurement done by
calculating the return time of laser light pulses reflected by the cloud base.
Cloud amount. The fraction of the sky covered by the clouds
of a certain genus, species, variety, layer, or combination of clouds.
Cloud base. The lowest level of a cloud or cloud layer. (meter,
m (or foot, ft)).
Convective cloud. Cumuliform cloud which form in an atmospheric
layer made unstable by heating at the base or cooling at the top.
Dedicated display. A display connected to a sensor, designed
to provide a direct visualization of the operational variables.
Dew-point temperature. Temperature to which a volume of air
must be cooled at constant pressure and constant moisture in order to reach
saturation; any further cooling causes condensation. (degree Celsius, °C)
Disdrometer. A device used for catching the drops of liquid
hydrometeors and for measuring the distribution of their diameters.
Extinction coefficient** (s). The proportion of luminous flux
lost by a collimated beam, emitted by a incandescent source at a color temperature
of 2,700 K, while traveling the length of a unit distance in the atmosphere.
(per meter, m-1)
Note.- The coefficient is a measure of the attenuation due to both absorption
and scattering.
Luminance** (E). The luminous flux per unit area (lux, lx).
Koschmieder's law. A relationship between the apparent luminance
contrast (Cx) of an object, seen against the horizon sky by a distant observer,
and its inherent luminance contrast (C0), i.e. the luminance contrast that
the object would have against the horizon when seen from very short range.
Note. - Applicable to the visual range of objects by day
Lightning detection network. Network of lightning detectors
transmitting in real time to a central computer, locating lightning flashes
by combining information received from each detector.
Luminance (photometric brightness) (L). The luminous intensity
of any surface in a given direction per unit of projected area (candela per
square meter, cd/m2)
Luminance contrast (C). The ratio of the difference between
the luminance of an object and its background to the luminance of the background
(dimensionless).
Luminance intensity (I)**. The luminous flux per unit solid
angle (candela, cd).
Magnetic wind direction. The direction, with respect to magnetic
north, from which the wind is blowing. The magnetic wind directions are used
in aircraft operations, necessitated by the magnetic frame of reference applied
to air navigation facilities. (degree)
Meteorological optical range (MOR)**. The length of the path
in the atmosphere required to reduce the luminous flux in a collimated beam
from an incandescent lamp, at a color temperature of 2 700 K, to 0.05 of its
original value, the luminous flux being evaluated by means of the photometric
luminosity function of the International Commission on Illumination (CIE)
(meter, m or kilometer, km).
Note. - The relationship between meteorological optical range and extinction
coefficient (at the contrast threshold of e = 0.05) using Koschmieder's law
is : MOR = In(0.05)/s " 3/s. MOR = visibility under certain conditions.
(see visibility)
Precipitation intensity. An indication of the amount of precipitation
collected per unit time interval. It is expressed as light, moderate or heavy.
Each intensity is defined with respect to the type of precipitation occurring,
based on rate of fall.
Present weather. Weather existing at a station at the time
of observation.
Present weather sensor. Sensor measuring physical parameters
of the atmosphere and calculating a limited set of present weather, always
including present weather related to precipitation.
Prevailing visibility*. The visibility value, observed in accordance
with the definition of "visibility", which is reached or exceeded
within at least half the horizon circle or within at least half of the surface
of the aerodrome. These areas could comprise contiguous or non-contiguous
sectors. (meter, m or kilometer, km)
Note: - This value may be assessed by human observation and/or instrumented
systems. When instruments are installed, they are used to obtain the best
estimate of the prevailing visibility.
QFE. Atmospheric pressure at aerodrome elevation (or at runway
threshold) (hectopascal, hPa) .
QNH. Altimeter sub-scale setting to obtain elevation when on
the ground (hectopascal, hPa).
Runway visual range (RVR)*. The range over which the pilot
of an aircraft on the center line on a runway can see the runway surface markings
or the lights delineating the runway or identifying its center line (meter,
m).
Scatter meter. An instrument for estimating extinction coefficient
by measuring the flux scattered from a light beam by particles present in
the atmosphere.
Transmissivity (or transmission coefficient) (T). The fraction
of luminous flux which remains in a beam after traversing an optical path
of a unit distance in the atmosphere (dimensionless).
Transmissometer. An instrument that takes a direct measurement
of the transmittance between two points in space, i.e. over a specified path
length or base line.
Transmittance (tb). Transmissivity within an optical path of
a given length b in the atmosphere (dimensionless).
True Wind Direction. Direction from which the wind blows, measured
clockwise from true north.
Visibility*. Visibility for aeronautical
purposes is the greater of:
a) the greatest distance at which a black object of suitable dimensions,
situated near the ground, can be seen and recognized when observed against
a bright background; and
b) the greatest distance at which lights in the vicinity of 1,000 candelas
can be seen and identified against an unlit background.
Note: -The two distances have different values in air of a given extinction
coefficient, and the latter b) varies with the background illumination. The
former a) is represented by the meteorological optical range (MOR).
Visual threshold of illumination (ET). The smallest luminance
required by the eye to make a small light source visible (lux, lx).
Weather radar. An adaptation of radar for meteorological purposes.
The scattering of electromagnetic waves, at wavelengths of a few millimeters
to several centimeters, by raindrops and cloud drops is used to determine
the distance, size, shape, location, motion, phase (liquid and solid), as
well as the intensity of the precipitation. Another application is in the
detection of clear-air phenomena through scattering by insects, birds, etc.
and fluctuation of the refractive index.
1 Guide to Meteorological Instruments and Methods
of Observation
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