In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to go into hibernation or to migrate, and plants to be dormant.
In temperate and subpolar regions, generally four calendar-based seasons (with their adjectives) are recognized: spring (vernal), summer (estival), autumn (autumnal) and winter (hibernal).
Hot regions have two or three seasons; the rainy (or wet, or monsoon) season and the dry season, and in some tropical areas, a cool or mild season.
In the United Kingdom the spring season starts on the first day of the month of March and ends at the end of May. In some years, the spring weather does not start until the last week of March and thus it is not uncommon to experience icy winter weather in March contrary to the warm weather that spring is associated with.