A period of one year, synchronized with the natural progression of the hydrological seasons by specifying the calendar date of its first day. Generally in glaciology the hydrological year is found to be convenient because it begins near the start of the accumulation season and ends near the end of the ablation season. For example the appropriate dates are 1 October to 30 September in the mid-latitudes of the Northern Hemisphere. The concept of the hydrological year is most useful where the accumulation and ablation seasons are well differentiated, as on mid-latitude glaciers and most high-latitude glaciers, but it is less well suited to those regions in which there are more than two hydrological seasons, as in the tropics, or in which most of the accumulation occurs in the same season as most of the ablation, as in monsoon climates (see summer-accumulation type.).