Question 10.4: An isolated flower bud can be treated as a sphere, diameter ...
An isolated flower bud can be treated as a sphere, diameter 10 mm. The bud is exposed to a cloudless night sky with L_{d}=230\; W\; m^{-2}. Ground temperature is 273 K and air temperature is 275 K. The bud is susceptible to damage if its temperature falls below 273 K. Calculate the extra heat flux that it would be necessary to supply to the bud in order to maintain its temperature at 273 K. Suggest ways in which the microclimate could be modified to protect the bud.
The net radiation balance of the bud, temperature T_{b} , is R_{n} = 0.5\times 230+0.5\times \sigma \times 273^{4} -\sigma T^{4}_{b} . If T_{b} is to be maintained at 273 K, it follows that R_{n} = 115+158-316=-43\; W\; m^{-2} . This radiative loss must be balanced by heat gain of 43 W m^{-2} if the bud is to maintain thermal equilibrium. The heat gain could be from convection (fans warming the air near the ground) or from latent heat (spraying the bud with water).