The Importance of Solar Radiation Among Other Meteorological Parameters

The sun is our source of life, providing over 99.98% of all energy to the Earth’s surface, the rest is internal geothermal energy. Solar radiation has a vital impact on our climate and weather. Monitoring solar radiation thus is a key discipline of meteorologists and climate researchers. To get valuable results, professionals rely on highly specialized instruments.

It has been common practice to perform routine meteorological measurements as part of weather forecasting and climatology studies for centuries. Evaluating and interpreting this statistically robust data is vital as part of tracking medium-term and long-term atmospheric conditions.

Without this data, it would not be possible to maintain contemporary transport and communications infrastructures such as ground, air and sea traffic. This data is primarily acquired by measuring and observing the atmosphere close to the ground – an area known as the ‘boundary layer.’

The most significant meteorological parameters in this field are:

  • Wind direction and speed
  • Air temperature
  • Air humidity
  • Air pressure
  • Precipitation
  • Haze and other air contents
  • Terrestrial and solar radiation

These parameters are also key to monitoring and addressing issues such as air pollution, sun simulation, avalanche warning, renewable energy, forestry, water supply and distribution, agriculture, town and regional planning.

For example, it is only possible to thoroughly evaluate and interpret gas emission measurements by comparing these to simultaneously acquired meteorological data.

The structure of the atmosphere close to the ground is central to the local climate. It is necessary to determine and monitor parameters such as solar radiation, air humidity and temperature to understand and assess the chemical reactions of pollutants present in the air.

Any meteorological parameter can be affected by short-term variations, typically triggered by atmospheric turbulences. Meteorological parameters are also directly or indirectly impacted by solar radiation, resulting in common daily or yearly trends.

Evaluating these trends requires the calculation of mean values from the actual values measured over a specific period.

The daily cycle of some meteorological parameters is easy to understand; for example, the temperature cycle is typically a normal curve with a minimum value just after sunrise and a maximum value in the early afternoon.

A meteorological parameter’s yearly cycle can be determined by taking daily measurements, with a specific climatic region’s average yearly cycle normally determined by taking measurements over a minimum 30-year period.

As meteorological measurements must be collected outside, sensors and related electronics must be designed to accommodate the local climate, from deserts to arctic conditions.

The temporal and spatial characteristics of radiation values in the atmosphere close to the ground are influenced by the characteristics of the ground surface itself.

A number of factors can impact the received radiation in any particular locale, including:

  • Location on the Earth
  • Time and date
  • Precipitation type, for example, cloud, rain, fog, or snow
  • Constriction of the horizon (field of view)
  • Air pollution, for example, gases and aerosols
  • Albedo

The physical effects of these factors mean that some application fields are only able to measure ‘global radiation’ throughout the measurement location.

Some applications may require a measurement of the ‘direct radiation’ emanating from the sun and ‘diffuse radiation’ – radiation that is not emanating directly from the sun. It may also be necessary to determine the ‘radiation balance’ of incoming to outgoing radiation in the short-wave and long-wave.

The physical characteristics of solar radiation are affected by the atmosphere before it reaches the ground.

Its absorption in different wavelength ranges is also an essential parameter, while albedo can be influenced by surfaces with varying reflective characteristics, for example, water, ice, snow, grass, crops, stone or woodland.

These atmospheric properties, different wavelength ranges and the qualities of the ground’s surface must be considered when measuring solar radiation. It is, therefore, necessary to develop highly specialized sensors for each individual - often complex - measuring task.

Developing an appropriate sensor requires prior determination of the meteorological value to be measured and how this will be defined.

For example, parameters within the short-wave spectral range include:

Table 1. Source: OTT HydroMet

 
Direct Solar Radiation  S 
Diffuse Sky Radiation  H 
Global Radiation   G (= S + H) 
Reflected Global Radiation  R
Albedo (Reflection Factor)   R / (S + H) 
Short-wave Radiation Balance                 (S + H) - R

 

Table 2. Source: OTT HydroMet

 
Atmospheric Radiation  A
Emission of Ground Surface
(including reflected atmospheric radiation)
 E
Downward Global Radiation S + H + A 
Upward Global Radiation R + E 
Long-wave Radiation Balance A - E 

 

Radiation balance across the complete spectrum can be understood as the difference between received and returned radiation:

Table 3. Source: OTT HydroMet

 
Radiation Balance Q (= S + H – R + A - E)

 

These parameters represent the most critical climatological factors. A number of sensors have been developed to accommodate these.

Table 4. Source: OTT HydroMet

Direct Solar Radiation
Sensor  Pyrheliometer Illustration direct solar radiation
Spectral Range   300 nm - 3,000 nm 
View Angle   <5º 
Neccessary Accessory   Sun Tracker 
Global Radiation
Sensor   Pyranometer Illustration of Global Radiation
Spectral Range  300 nm – 3,000 nm
View Angle  180°
Necessary Accessory   None
Reflected Global Radiation
Sensor   Pyranometer Illustration reflected global radiation
Spectral Range  300 nm – 3,000 nm
View Angle   180°
Necessary Accessory   Glare Shield to keep out low-angle non-reflected radiation
Diffuse Sky Radiation
Sensor   Pyranometer
Spectral Range   300 nm – 3,000 nm
View Angle   180°
Necessary Accessory Shadow Ring
Shading Device:  Shadow Ring
Shadow Band
SunTracker/Shadow Ball
Sun Tracker/Shadow Disk
Atmospheric Radiation
Sensors   Pyranometer and Pyrgeometer Illustration Atmospheric Radiation
Spectral Range   300 nm – 50 µm
View Angle   180°
Necessary Accessory   None
Emission of Ground Surface
Sensor   Pyranometer and Pyrgeometer Emission of Ground Surface
Spectral Range   300 nm – 50 µm
View Angle   180°
Necessary Accessory   Glare Shields to keep out low-angle non-reflected radiation
Radiation Balance
Sensor   Net Radiometer
 2 Pyranometers and
 2 Pyrgeometers
Radiation Balance
Spectral Range   300 nm – 50 µm
View Angle   180° 
Necessary Accessory   Glare Shields for downward facing sensors to keep out low- angle non-reflected radiation

 

Combinations of these meteorological measurements are important factors in applications such as weather forecasts and climatology, depending on the task and the accuracy needed.

As part of the widespread goal to monitor and minimize air pollution, measurements must be made that adhere to national standards for evaluating emissions and air quality. These are typically referred to as ‘environmental measurements.’

Environmental measurements tend to involve determining the global radiation, direct radiation and radiation balance.

Global and direct radiation data is combined with specific spectral data to determine the quantities of gases and aerosol particles in the air. This data also helps provide insight into the photochemical formation of any secondary impurities present.

The radiation balance offers valuable data on the vertical exchange and spread of pollution.

Solar sensors are also employed in environmental simulation applications, whereby ‘artificial suns’ are used to test the impact of solar radiation on materials. Very high-intensity artificial radiation sources are used to reduce testing time. These radiation sources far exceed the solar constant - the maximum natural value of 1367 W/m².

It is important that sensors employed in these applications possess measuring ranges that are able to accommodate radiation levels in the range of 2000 to 4000 W/m². These sensors must also be able to work at temperatures higher than 100 ºC.

This information has been sourced, reviewed and adapted from materials provided by OTT HydroMet.

For more information on this source, please visit OTT HydroMet.

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