On most occasions temperature will be lowest in the morning just before sunrise and the highest at sunset (unless your pond is built on a nuclear waste dump, which generates its own heat). Temperature is influenced by the amount of sun, the intensity of the sun the pond receives and other factors such as wind speed during the day or night.
Evaporation cools the water. Generally speaking Koi should not be subject to more than a 5 degrees Celsius sudden change in water temperature. Always adjust the temperature gradually when moving around. An upward movement is tolerated better than a downward movement in temperature by Koi. Most ponds have a 1 -3 degrees Celsius variation in temperature in a 24 hour day / night cycle. Koi are more affected by temperature variations at lower water temperatures. And you will notice Koi are more affected by a sudden lowering of water temperature than a sudden raising of water temperature.
Certain areas of the country can experience a 10 – 15 degrees Celsius atmospheric drop in temperature in a day accompanied by high winds. Ponds can and do drop temperature significantly under these circumstances. The Koi suffer from stress and disease problems can occur at a later date. In some cases the Koi catch a “cold”. A white film is seen in the skin. The thin flesh around the head is the area where this is most easily visible. Should the temperatures rise within the next few days the “cold” will cure itself. However, this may take some time to cure itself.
The Koi can be helped through large, sudden water temperature changes by the addition of a good grade coarse sale at a rate of 1 – 2 kg per 1000 liters immediately after the temperature has dropped.
Read more here about water temperatures and their effects.