Solar radiation refers to the overall term for the energy emitted by the sun, the Earth, and the atmosphere. Typically, solar radiation is categorized as shortwave radiation, while the Earth's and atmospheric radiation are classified as longwave radiation. The primary physical quantity measured is the rate of radiant energy flow, also known as irradiance or radiative flux density, with the standard unit being watts per square meter (W/m²). In meteorology, several types of radiation are commonly measured:
1. **Direct Solar Radiation**: This is the solar radiation that reaches a surface perpendicular to the sun’s rays within a solid angle of 0.5° around the sun.
2. **Sky Radiation (or Diffuse Radiation)**: This refers to the scattered and reflected solar radiation from the atmosphere over a 2Ï€ steradian solid angle, reaching the ground.
3. **Total Solar Radiation**: It represents the sum of direct and diffuse solar radiation received at the Earth's surface.
4. **Reflected Solar Radiation**: This is the portion of total solar radiation that is reflected back into the atmosphere from the Earth's surface.
5. **Earth Radiation**: This is the longwave radiation emitted by the Earth and its atmosphere, often referred to as terrestrial radiation.
6. **Net Radiation**: This is the difference between the downward radiation (from the sun and atmosphere) and the upward radiation (from the Earth's surface).
**Instruments for Measuring Radiation:**
- **Direct Solar Radiation Meter**: A standard device used to measure direct solar radiation. It has a light-receiving cylinder with a viewing angle of 10°, and the sensing surface is a blackened manganese-copper plate. The back side connects to the positive terminal of a thermopile, while the front is exposed to sunlight. The electromotive force generated by the thermopile is proportional to the intensity of solar radiation. Some advanced models include an automatic tracking system that aligns the sensor with the sun using a motor and gear reducer.
- **Net Radiation Meter**: Designed to measure the net balance between incoming and outgoing radiation at the Earth's surface. It consists of two sensing surfaces—upper and lower—separated by insulating material and covered with a protective polyethylene film. The upper surface measures incoming radiation, while the lower one detects outgoing radiation. The temperature difference between these surfaces generates a thermoelectric voltage, which is proportional to the net radiation intensity.
These instruments play a crucial role in climate studies, weather forecasting, and environmental monitoring, helping scientists better understand the Earth's energy balance.
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