NET R values are positive
a. during the night when +LW is at a maximum
b. after dusk due to a lag effect of outgoing infrared radiation
c. during daylight hours, peaking just after noon with the peak of insolation
d. during the daylight hours, peaking just after sunrise

Answer is C. during daylight hours, peaking just after noon with the peak of insolation

Explanation:

Net R-values can be described or explained as the total amount of energy that is available in influencing the climate or amount of energy available to the land surface for different nonradioactive processes such as biological synthesis.

The Net R value can be measured by radiometer.

It should be noted that Net radiation can be mathematically calculated, For example, the Net radiation for row of crop can be calculated using

Rn = { 1 - ∝ }Rs + Rnl

where, Rn = Net radiation,    ∝ = Albedo, Rs = Short wave radiation, Rnl = Longwave radiation.

Albedo can be calculated as,

∝ = C∝c + { 1 - C}∝s,    where

∝ = Albedo,  ∝c = Plant albedo, ∝s = Soil albedo,  C = Relative fraction.

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