• Solar constant is defined as the energy falling in one minute on a surface area of one square centimeter at the outer boundary of atmosphere held normal to the sunlight at the mean distance of the earth from the sun.
• Value of solar constant is 1.94 cal /cm2/min.
• Direct solar radiation is the radiation received directly from sun by a surface normal to the incident radiation.
• Diffuse radiation or sky radiation is the radiation scattered by suspended particles in the atmosphere.
• Quantity of diffused radiation depends on latitude, season and cloudiness.
• In high latitudes diffused radiation is important source of solar radiation and forms substantial part of solar radiation during winter months.
• Cloudiness increases ratio of diffused to direct radiation.
• Before sunrise and after sunset all the solar radiation received is diffused or sky radiation.
• In plant canopies diffused radiation penetrates effectively than direct radiation.
• Diffused radiation contains 65% photosynthetically active radiation (PAR).
• Direct radiation contains 42% photosynthetically active radiation (PAR).
• Global radiation is the total of direct and diffused solar radiation received on a horizontal surface from the sun directly and from the sky as scattered radiation.
• Albedo or reflected solar radiation is the solar radiation that is reflected without any change in its quality.
• Clouds are effective reflectors of solar radiation.
• Snow is a very effective reflector, particularly when it is fresh.
• Water surfaces and sea are poor reflectors.
• Rocks, sand, soil and vegetation reflect from 10 to 30 percent of the incident solar radiation.
• Reflected solar radiation is important in remote sensing.
• Terrestrial radiation is the thermal radiation emitted by the earth.
• Earth absorbs solar radiation in short waves and emits back in the form of long waves.
• Terrestrial radiation heats the atmosphere.
• Net radiation is the radiation balance between global and reflected solar radiation.
• SI unit of solar radiation is watts/m2
• 1 watt = 1 joule/second
• CGS unit of solar radiation is Calories/cm2/min
• 697.93 W/m2 = 1 cal/cm2/min
• Lux is oldest unit of solar radiation
• 10,000 lux = 3.47 Mega-joules
• Photosynthetic active radiation (PAR) is measured in Einstein units.
• 1 Einstein unit = 1 mole of photons
• Total incoming radiation measured by pyranometer.
• Diffused radiation measured by shading pyranometer.
• Reflected radiation is measured by albedometer.
• Net radiation is measured by net radiometer.
• Photosynthetically active radiation is measured by quantum sensor.
• Direct solar radiation is measured by pyrheliometers.
• Pyrgeometer is used to measure terrestrial radiation.
• Sunshine hours recorded by sunshine recorder.
• Height of sunshine recorder from ground surface is 3.08 m.
• Campbell stokes sunshine recorder is used to measure sunshine hours in India.
• Height of Stevenson screen from ground surface is 1.22 m.
• Duration of the day and night are equal all over the earth on March 21 which is called vernal or spring equinox.
• From March 21, the day length increases gradually in the northern hemisphere till June 21, which is called the summer solstice.
• From June 21 onwards, day length decreases in the northern hemisphere till September 23, which is called the autumn equinox.
• Day and night are equal all over the world, with the sun right over the equator.
• From September 23, sun moves southward and and the days continue to get shorter and shorter in the northern hemisphere till December 21, which is called the winter solstice.
• Solar radiation in narrow wave bands is measured by Spectroradiometer.
• Infrared thermometer measures temperature of canopy without contact.
• Infrared thermometer senses radiation in the range of 8-14 micro meters wave band (infrared region).
• Infrared thermometer is used to estimate water status of plants and also to schedule irrigation.
• Temperature is the degree of hotness or coldness of a substance determined by its molecular activity.
• Temperature is measured by thermometer.
• Depth of water table is measured by Piezometer.
• Conduction is transfer of heat by molecular activity.
• Convection is process of heat transfer within liquids and gases resulting from the motion of fluid.
• Convection is faster than conduction.
• Radiation is transfer of energy by electromagnetic waves moving at the speed of light.
• Conduction and convection requires medium, whereas radiation does not require medium.
• X-rays are short.
• Radio waves are long
• On an average cloudy day 43% solar radiation absorbed by earth, 22% by atmosphere and 35% reflected into space from earth.
• Reflected solar radiation is unchanged in character.
• Solar radiation absorbed by earth or atmosphere is converted into thermal energy.
• Solar radiation is more at 12 noon, whereas highest temperature is noticed after 2 P.M.
• At equator seasonal temperature variation is less.
• Seasonal variation in temperature is more at higher latitudes.
• Isotherms are the lines that connect points of equal temperature.
• Vertical temperature variations influence cloud formation, condensation and precipitation.
• Lapse rate is vertical temperature decrease or gradient.
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