Cold nights when you have warm days usually means very low humidity. Around here in July/Aug the dew point climbs into the 70s and the temp will not drop much at all overnight. In those conditions three things happen: 1) Air temp is close to pool temp. 2) Very little wind. 3) Extremely high humidity.
All three of those lead to almost no evaporation.
Once the dew point drops in the fall, we get more breeze at night, much lower humidity, and much cooler nights. Assuming your pool is heated (still the same temp) each of those three things lead to much higher evaporation.
I notice it when it rains with our autocover, the cover pump leaves 1/2 inch puddles in places, in July/Aug those are still there days later if I haven't done anything with the pool, in the fall they are gone by morning (along with most of the pool's heat).
