It may be required to de ice an aircraft in a number of situations - factors that would allow the formation of ice include high humidity, cold soaked wing, low fuel temperature ( if the aircraft has been cruising at high altitude the fuel temperatures can easily drop to - 20 to - 30 degrees). If you are on the ground with a lot of cold fuel it can form frost on the upper surface of the wing in 'short's and tee shirt weather'.
As the upper surface must be kept free from frost you may well see aircraft get de iced in all types of weather.
The 737-800 suffers this problem. Does anyone else know of other aircraft that are more prone to this kind of icing.