Consider the following statements regarding the temperature of the Earth's surface : 1. The temperature of a surface is determined by net radiation 2. Net radiation produces a radiant energy flow that can heat or cool a surface 3. Net radiation balance is usually positive during the daytime and negative during night time 4. Soil surface loses energy as temperature increases during the daytime Which of the statements given above are correct?

Correct — C, 1, 2 and 3 only. Net radiation is the difference between all the radiation a surface receives and all it loses, and it is what sets the surface temperature, so statement 1 holds. Statement 2 follows from it: a positive net radiation supplies energy that heats the surface and the air and soil next to it, while a negative net radiation drains energy and cools them. Statement 3 is the daily pattern — during the day incoming solar radiation greatly exceeds the outgoing longwave loss, so net radiation is positive, and at night with no solar input the surface radiates away more than it receives, so net radiation is negative. Statement 4 reverses this. During the day the soil surface is gaining energy, which is exactly why its temperature rises; it is at night, as the surface cools, that it is losing energy on balance.

• (a)1 and 2 only — This accepts only the first two statements and drops the third, but the daily rhythm of positive net radiation by day and negative by night is the most directly observable part of the whole balance.
• (b)3 and 4 only — This keeps statement 4, which is the incorrect one, and drops statements 1 and 2, which set out what net radiation is and what it does.
• (d)1, 2, 3 and 4 — This accepts all four, including the reversed statement 4. A surface whose temperature is rising must be taking in more energy than it gives out, so it cannot be described as losing energy at that time.

The surface energy balance is an accounting exercise. Net radiation equals incoming shortwave minus reflected shortwave, plus incoming longwave from the atmosphere, minus outgoing longwave from the surface. Whatever is left over is spent in three ways: heating the air above by sensible heat transfer, evaporating water as latent heat, and conducting heat down into the soil. By day net radiation is positive and all three fluxes run outward from the surface; at night the sign reverses and heat flows back up from the soil towards a cooling surface.

A four-statement item is easiest to settle when one statement contradicts another, and that is the case here. Statement 3 says net radiation is positive during the day, and statement 4 says the soil surface loses energy during the day. Both cannot hold, so one of them is the intended error, and it is the fourth, since positive net radiation by day is the securest fact of the four. The word to be careful with is 'loses': the surface does radiate more strongly as it warms, and that outgoing longwave loss grows through the day, but the net flow is still inward, which is why the temperature climbs.

• Net radiation is the balance of all incoming and outgoing radiation at a surface, and it sets the surface temperature.
• Net radiation is normally positive by day and negative at night.
• The surplus by day is spent as sensible heat to the air, latent heat in evaporation and conduction into the soil.
• At night the flows reverse and heat moves up from the soil towards the cooling surface.
• Maximum air temperature comes after local noon because the surface goes on gaining while net radiation stays positive.

• Accepting all four statements because each sounds plausible on its own.
• Confusing the growth of outgoing longwave radiation with a net loss of energy.
• Overlooking that two of the statements cannot both be true.

A four-statement code item containing an internal contradiction. Spotting that two statements conflict is faster than testing each one separately.

• practice — not a real PYQ

  Net radiation at a surface during the night is normally

  • (a)positive
  • (b)negative
  • (c)zero
  • (d)the same as during the day

  Answer(b) negative — with no solar input the surface radiates away more than it receives.

• practice — not a real PYQ

  The highest air temperature of the day usually occurs

  • (a)at sunrise
  • (b)exactly at local noon
  • (c)in the early afternoon, after local noon
  • (d)just after sunset

  Answer(c) in the early afternoon, after local noon — the surface keeps gaining while net radiation stays positive.