The climate of Bolivia varies drastically from one eco-region to the other, from the tropics in the eastern ''llanos'' to a polar climate in the western Andes. The summers are warm, humid in the east and dry in the west, with rains that often modify temperatures, humidity, winds, atmospheric pressure and evaporation, yielding very different climates in different areas. When the climatological phenomenon known as ''El Niño'' takes place, it causes great alterations in the weather. Winters are very cold in the west, and it snows in the mountain ranges, while in the western regions, windy days are more common. The autumn is dry in the non-tropical regions.