El Salvador has a tropical climate with pronounced wet and dry seasons. Temperatures vary primarily with elevation and show little seasonal change. The Pacific lowlands are uniformly hot; the central plateau and mountain areas are more moderate. The rainy season extends from May to October; this time of year is referred to as ''invierno'' or winter. Almost all the annual rainfall occurs during this period; yearly totals, particularly on southern-facing mountain slopes, can be as high as 2170 mm. Protected areas and the central plateau receive less, although still significant, amounts. Rainfall during this season generally comes from low pressure systems formed over the Pacific and usually falls in heavy afternoon thunderstorms.