Solar Radiation, Heat Transfer and Measurement Instruments

Forms of Energy Transfer

Radiation

A body at a certain temperature transmits energy in the form of electromagnetic waves; the radiation emitted varies according to the absolute temperature of the body.

Black Body

Black body: A body that absorbs all incident radiation and emits the maximum possible radiation for its temperature. It is also known as an ideal body. The amount of energy radiating from a black body is given by the Stefan–Boltzmann law.

Conduction

Conduction: Heat transfer that occurs due to a temperature gradient (dT/ds). The relationship describes the amount of heat transferred per unit area of cross section per unit time. Conduction is more effective in solids and also propagates in liquids and gases.

Convection

Convection: A form of heat transfer that is important in liquids and gases, where bulk motion of the fluid transports heat.

Albedo and the Greenhouse Effect

Albedo of the Land (α)

Albedo (α): The ratio between the amount of reflected light energy and the total incoming energy.

Greenhouse

Greenhouse effect: A process by which energy emitted by the Earth is absorbed and re-emitted by the atmosphere, contributing to global warming. Human activities that introduce particles and greenhouse gases into the atmosphere increase the Earth's temperature.

Measurement of Solar Radiation

Actinometry

Actinometry: The study and measurement of radiant energy arriving at the Earth's surface, including thermal, chemical and luminous components.

Pyrheliometer

Pyrheliometer: An instrument that measures the intensity of direct solar radiation at normal incidence.

Pyranometer

Pyranometer: Instruments that measure the radiation received from the entire sky; they are designed to measure global (total) solar radiation.

Actinograph of Robitzsch

Actinograph (Robitzsch): An instrument using bimetallic elements, typically consisting of three strips—two of them white and a blackened center strip—to record variations in incoming radiation.

Campbell–Stokes Heliograph

The Campbell–Stokes heliograph measures insolation, i.e., the duration of sunshine at a given place during the day. The device consists of the following components:

• A glass sphere (approximately 10 cm diameter) that concentrates the sun's rays into a focal point; the focal position varies with the position of the sun.
• A semi-metal ring that partially surrounds the glass sphere and contains three channels for different types of recording paper: short curved band, long curved band, and straight band.
• A serrated metal clip (or barbed wire clip) that secures the recording band in position.
• A horizontal metal support base and an adjusting ring that slides on a latitude scale to orient the instrument correctly.

The heliograph should be positioned so that its axis aligns with the instrument's support points and is oriented for the local latitude and the sun's apparent path.