Glossary

Solar radiation & meteorological terms (A–Z)

Absorption
Process during which incoming energy is absorbed by an object.

Airmass
Ratio of the actual solar radiation path through the atmosphere to the direct vertical path.

Albedo
The proportion of incident radiation being reflected by an object or surface.

Albedometer
Sensor for measuring incoming and reflected radiation such that the Albedo can be calculated.

Angle of Incidence (AOI)
Angle between incoming solar radiation and the normal (perpendicular) to a surface.

Anemometer
Sensor for measuring wind speed.

Atmosphere
All the air surrounding the earth.

AWOS (Aviation Weather Observation System)
Automated system providing meteorological and surface condition data to support airport operations and flight safety.

Barometer
General term for sensors measuring the atmospheric air pressure.

Bifacial Gain
Additional energy produced by a bifacial PV module due to irradiance collected on the rear side.

Black Ice
Thin, transparent layer of ice on a road surface that is difficult to detect visually and poses a significant traffic hazard.

Blowing Snow
Snow lifted from the surface by wind, reducing visibility and causing drifting even without active snowfall.

BSRN (Baseline Surface Radiation Network)
Global network of high‑quality radiation monitoring stations using standardized instruments and methods.

Calibration Factor
Numeric coefficient applied to a sensor output to obtain a traceable measurement value.

Ceiling
Height of the lowest cloud layer covering more than half the sky (broken or overcast), critical for aviation operations.

Celsius
Temperature scale named after the Swedisch scientist Anders Celsius. 0 ºC represents the freezing point of water, 100 ºC represents its boiling point; both under standard conditions.

Chemical Wetness
Road condition where de‑icing chemicals are present in solution, lowering the freezing point of surface moisture.

Climate
Statistical characteristics of the weather during a certain time period in a certain region.

Coefficient of Friction (Road)
Dimensionless value expressing the available grip between vehicle tires and the road surface under given conditions.

Condensation
The changeover from the gaseous state to liquid state.

Corona
The coloured ring around the sun or moon caused by the refraction of light through watervapour in the atmosphere.

Cosine Response
How closely a radiometer’s angular response follows the ideal cosine law.

Dew Point
The temperature point to which air has to cool down to saturate with water vapour.

Diffraction
Change in direction of light waves at the edge of objects or through small apertures.

Diffuse Radiation
Scattered and reflected solar radiation falling onto a horizontal surface, with the direct radiation excluded.

Direct Radiation
Solar radiation falling directly onto a surface normal to the sun, with the diffuse radiation excluded.

DHI (Diffuse Horizontal Irradiance)
Diffuse sky irradiance measured on a horizontal surface.

DNI (Direct Normal Irradiance)
Solar irradiance received by a surface always kept perpendicular to the sun’s rays.

Dobson Unit
Unit of measurement for the total amount of ozone between the measuring instrument and the sun. 100 DU = 1m column of ozone at ground level under standard atmospheric conditions. The typical yearly mean value in Europe is 330 to 360 DU.

Drift
Slow temporal change of the measuring value of a sensor.

Equinox
The date when day and night have the same length.

Extraterrestrial Irradiance
Solar irradiance at the top of Earth’s atmosphere on a plane normal to the sun.

Far Infrared Radiation (FIR)
Long-wave radiation in the range of 3 to 50 µm.

Freezing Fog
Fog composed of supercooled droplets that freeze on contact with surfaces.

Freezing Temperature
Temperature at which surface moisture begins to freeze.

Friction (Road)
Resistance between tires and road surface affecting braking and vehicle control.

Global Radiation
Total of direct and diffuse solar radiation from the hemisphere falling onto a horizontal surface.

GHI (Global Horizontal Irradiance)
Total solar irradiance on a horizontal surface.

Ground‑Reflected Irradiance
Irradiance reflected from the ground onto a surface.

Humidity
The presence of water vapour in the air, usually measured by hygrometers.

Ice Percentage
Estimated proportion of road or runway surface covered by ice.

Irradiation
Temporal integral of illuminance (Wh/m²).

Irradiance
Instantaneous value of radiation (W/m²).

Invasive Road Sensor
Road weather sensor installed flush within the pavement.

Infrared Radiation
Long-wave radiation in the range of 0.7 to 50 µm.

Inversion
When the air temperature rises in relation to the height above ground level instead of falling.

Isobar
On meteorological maps, a line connecting locations of equal barometric pressure.

Isohel
On meteorological maps, a line connecting locations with the same sunshine duration values.

Isorad
On meteorological maps, a line connecting locations with the same solar irradiance.

Isotherm
On meteorological maps, a line connecting locations with the same air temperature.

ISO/IEC 17025
International standard for calibration and testing laboratory competence.

Kelvin
Temperature scale named after scientist Lord Kelvin. Zero Kelvin equals - 273.15 º Celsius. One degree on the Kelvin scale equals one degree on the Celsius scale.

LIDAR
Light Detection And Ranging uses laser pulses to determine the concentration and spatial distribution of features in the atmosphere, such as aerosols and gasses.

Light
Electromagnetic radiation visible to the human eye, wavelengths between 400 nm and 700 nm.

Lightning
Visible electrical discharge in the atmosphere.

Longwave Exchange Error
Measurement bias caused by radiative heat exchange between sensor and sky.

Measuring Error
Deviation of a measuring value from the real value.

Measuring Range
Range of values in which measuring errors do not exceed certain specified limits.

Measuring Instrument
A sensor for measuring a physical parameter.

Measuring Chain
Series of components to measure a physical parameter and to transform the measuring signal into the desired output format.

Measuring Principle
Physical basis of the measurements.

Measuring Signal
Output value of a sensor or measuring device which is directly related to the physical parameter being measured.

Measuring Procedure
Application of a measuring principle and a measuring method.

Measuring Value
A numerical value in specified units derived from the measuring signal of a sensor.

MED
Minimum Erythemal Dose. MED/hr is a measure of skin exposure to UV radiation.

Mesosphere
A layer of the upper atmosphere located between the stratosphere and the ionosphere.

Meteorology
The science that deals with atmospheric phenomena and processes, particularly with respect to weather conditions and forecasting.

Method of Measurement
Special modus operandi of measuring independent from the measuring principle.

Metrology
The science of determining measuring values of physical parameters.

Mobile Road Weather Sensor
Vehicle‑mounted sensor collecting continuous road weather data.

Net Radiation
Difference between incoming and outgoing radiation of the same spectral range.

Non‑Invasive Road Sensor
Road weather sensor installed above the surface without pavement embedding.

Optical Road Surface Measurement
Technique using reflected light to determine road condition and moisture.

Output
Measuring signal coming out of a sensor.

Ozone
Gas naturally occurring in the atmosphere. In higher concentrations it is toxic and corrosive. Its molecules consist of three oxygen atoms and the chemical formula is O3.

Ozone Hole
Term for a decrease of the atmospheric ozone content by more than 50 % from normal in a defined geographical area.

Ozone Layer
The ozone layer is the area in the atmosphere at a height from  20 to 40 km which absorbs a considerable part of the UV radiation coming from the sun.

Performance Ratio (PR)
Normalized indicator of PV plant performance.

Plane‑of‑Array Irradiance (POA)
Irradiance incident on a tilted surface such as a PV module.

Precipitation
Any form of water falling to earth from the atmosphere including rain, snow, sleet and hail.

Present Road Condition
Current state of the road surface (dry, wet, snow, ice, slush).

Present Weather
Observed or detected precipitation or obscuration conditions.

Pyranometer
Sensor for the measurement of global solar radiation in a spectral range from 300 to 3000 nm.

Pyrgeometer
Sensorfor the measurement of global solar and atmospheric radiation in a spectral range from 5 to 50 µm.

Pyrheliometer
Sensor for the measurement of direct solar radiation in a spectral range from 300 to 3000 nm.  

Pyrometer
Sensor for remote measurement of temperature.

RADAR
RAdio Detection And Ranging uses radio signals to determine the position of features in the atmosphere. In meteorology this is usually Weather Radar for mapping clouds and precipitation.

Radiation
Transport of energy by electromagnetic waves. The spectrum extends from very short wavelength cosmic radiation through to X-rays, ultraviolet, visible light, infrared and on to radio waves and very long wavelength microwaves.

Radiation Balance
Difference between upward and downward radiation at both short and long wavelengths.

Resolution
Smallest difference between two measuring values that can be detected by the sensor or subsequent parts of the measuring chain.

Responsivity
Ratio between sensor output and input radiation.

Response Time
Time taken for the output value to reach a defined percentage of the input value after a step-change in the input value.

Road Surface Temperature
Temperature of the road or runway surface itself.

Road Visibility
Distance at which road objects can be clearly seen.

Road Weather Information System (RWIS)
Integrated system combining road and meteorological sensors.

Radiometer Ventilation
Forced airflow to reduce thermal errors and condensation.

Saturation
Condition where the concentration of water vapour in the air has reached its theoretical maximum. This maximum depends on the ambient temperature and pressure.

Sensitivity
Change of the output value in relation to the input value.

Sky
The hemispherical part of the atmosphere that is directed to the sun, it causes diffuse radiation. In space there is only direct solar radiation.

Snow Height
Depth of snow accumulated on a surface.

Soiling Loss
Reduction in solar performance due to surface contamination.

Soiling Ratio
Ratio comparing soiled and clean sensor response.

Solar Constant
Long-term mean solar irradiance (1.368 kW/m²) outside the earth's atmosphere falling onto a 1 m² surface which is oriented normal to the sun's rays whilst the distance between earth and sun is medial.

Spectral Response
Sensor sensitivity as a function of wavelength.

Spectrophotometer
Sensing instrument for the measurement of radiation over a range of wavelengths.

Stratosphere
The stratosphere is a layer in the atmosphere at a height from 10 - 15 km up to 50 km. It is above the troposphere and below the mesosphere. It is nearly cloudless and contains the ozone layer.

Subsurface Temperature
Temperature measured below the road surface.

Thermal Mapping
Spatial representation of road surface temperature.

Thermal Offset
Radiometer error caused by internal temperature gradients.

Treshold
An input value above or below which there is a specific change of the output value.

Troposphere
Lowest layer of the atmosphere. Here is where the weather occurs. The troposhere reaches up to a height of 10 - 15 km. Between the troposphere and the stratosphere is the tropopause.

Tropopause
Junction layer of troposphere and stratosphere, the height varies from 10 to 15 km depending on location and conditions.

Ultraviolet Radiation
Invisible radiation in the spectral range from 100 to 400 nm.

UV Index
Global UV Index of UN / WMO / WHO for the evaluation of values of erythemally weighted UV radiation that is harmful to skin.

Visibility
Degree to which objects can be seen through the atmosphere.

Water Film Height
Thickness of water present on a road or runway surface.

Weather
Condition of the atmosphere at a certain time and location. The weather is mainly influenced by solar radiation, air temperature, air humidity, precipitation, air pressure and air movement.

Wind
Movements of masses of air in the atmosphere. Traditionally regarded as horizontal but nowadays often measured in 3 axes.

Winter Maintenance Decision Support System
Software integrating RWIS data and forecasts for maintenance planning.

WMO
World Meteorological Organization.

WRR
World Radiometric Reference for solar measurements.

Yield Loss
Reduction in expected energy production.

Zenith
The highest point of the sky, vertically above the observer.

Zero Offset
Non‑zero sensor output when true radiation is zero.