Agronomy module - Agro meteorology 2


  • The sun is the primary source of heat to the earth and its atmosphere. 
  • The distance that separates the earth from the sun is about 1,49,000, 000 kilometers. 
  • The surface temperature of the sun is estimated between 5500oC and 6100oC. 
  • Solar radiation provides more than 99.9 percent of the energy that heats the earth.
  • The word 'insolation' is contraction of "incoming solar radiation". Radiant energy from the sun that strikes the earth is called insolation.
  • Light travels with a speed of 2,97,600 km/ sec. It takes 8 minutes to 20 second. Reach the earth



  • Radiation is the process of transmission of energy by Electro magnetic waves and is the means by which energy emitted by the sun reaches the earth.

  • Conduction is the  process of heat transfer through matter by molecular activity.  In this process heat is transferred from one part of a body to another or between two objects touching each other. 
  • Conduction occurs through molecular movement.

  • Convection is the process of the transfer of heat, through movement of a mass or substance from one place to another. 
  • Convention is possible only in gases or fluids, for they alone have internal mass motions.

  • It is the capacity of any surface to reflect the incoming radiation (light) OR it is the ratio of incoming radiation to the outgoing radiation. 
  • Average albedo value for earth is 34%. 

Solar Constant:  
  • Solar constant is defined as the rate at which solar radiation is received outside the earth's atmosphere on a surface perpendicular to the sun's rays when the earth is at an average distance from the sun.
  • Every minute, the sun radiates approximately 56 x 1026 calories of energy. 

  • Visible solar radiation is called as light. 
  • The visible portion of the solar spectrum is the light with wave length ranging from 0.4 to 0.7 m. 
  • Light is essential for the synthesis of the most important pigment i.e, chlorophyll.  The cholorphyll absorbes the radiant energy and converts into potential energy of carbohydrates (Photosynthesis

Light Intensity:  

  • The intensity of light is measured by a standard unit called candle. 
  • The amount of light received at a distance of one meter from a standard candle is known as "Metre Candle or Lux". 
  • Very low light intensity reduces the rate of photosynthesis and may even result in the closing of the stomata detrimental to plants in many ways.  This results in reduced plant growth. 
  • Very high light intermities are it increases the rate of respiration.  It causes rapid loss of water, i.e., it increases the transpiration rate of water from the plants resulting in closure of stomata. 
  • The most harmful effect of high intensity light is that it oxidise the cell contents which is termed as "Solarisation". 
  • This oxidation is different from respiration and is called as 'Photooxidation".

Based on the response to light intensities the plants are classified as follows:

  1. Sciophytes: (Shade loving plants) 

  • The  plants that grow better under partially shadded (low light) conditions e.g., betevines, buckwheat, turmeric etc.,

  1. Heliophytes: (sun loving Plants )

  • Many species of plants produce maximum dry matter under high light intersities when the moisture is available at the optimum level, e.g. maize, sorghum, rice etc. 

Duration of light: The response of plants to the relative length of the day and night is known as photoperiodism.

i)                   Long day plants:
  • The  plants which develop and produce normally when the photoperiod is greater than the critical minimum (greater than 12 hours) e.g. cereals, potato, sugar beet, wheat, barley etc.

ii)                 Short day plants:  
  • The plants which develop normally when the photoperiod is less than the critical maximum (less than 12 hours) e.g. tobacco, soybean, millets, maize, sugarcane, etc.

iii)               Indeterminate or day neutral plants:
  • Those  plants which are not affected by photo period, e.g., Tomato, Cotton, Sweet potato, pineapple etc.,

Orientation of leaves:

Phototropisum :
  • The change of position or orientation of organs of plants caused by light is usually called as "Phototropisum".

Photomorphogenesis: Change in the morphology of plants due to light.  This is mainly due to ultra violet and violet rays of the sun.

Instruments used for measuring solar radiation

1.      Bellanis pyranometer
2.      Sunshine recroder
3.      Line quantum sensor
4.      Photometer
5.      Lux meter measures the light intensity
6.      Radiometer