Step 8: some of this energy will go towards evaporating surface water (what we care about), but some will go into directly heating the near-surface air via conduction. the proportion that actually evaporates water is called the evaporative fraction or ef. from theory i teach in boundary layer meteorology class, there is the following rough relationship between ef and temperature: surface temperature typical evaporative fraction 0°c (32f) 10°c (50f) 20°c (68f) 30°c (86f) 0.52 0.71 0.87 1.00 _use this to given this what do you think is the average surface temperature of earth? guesstimate an average evaporative fraction for earth. evaporative fraction, how many w/m2 of energy actually go towards evaporating surface water and powering the water cycle?