The landscape is mainly flat, with two low mountain ranges; "Sierra de la Ventana" (near Bahía Blanca) and "Sierra de Tandil" (Tandil). The highest point is "Cerro Tres Picos" ( amsl; ) and the longest river is "Río Salado" ( long). As part of The Pampas, the weather of the province is strongly influenced by the ocean, with hot summers and temperate winters. Humidity is high and precipitation is abundant and distributed over the year. The Western and Southwestern regions are drier. The climate of the province of Buenos Aires is extremely benign for human activities: it is temperate, with four marked seasons and reliable rainfall on most regions. The province can be divided into four main climatic regions: the southwestern, drier region; the cool Atlantic region; the northern and eastern humid region, and the Delta region, with the warmest, wettest climate. The northern region has warm, humid summers, with days between and nights between , pleasant falls, cool, drier winters with highs between and nights between , and windy, variable springs. Heat waves may bring days with temperatures over , but these do not usually last very long, as cold fronts bring thunderstorms and cooler days, with night temperatures often falling down to . Winter cold waves may bring days with highs about , and lows below , with extremes down to . Snow is uncommon, but there have been accumulations on several occasions in the past. Precipitation ranges from per year. The Delta region is slightly warmer, especially at night, due to the presence of water and the northerly location. Summer nights tend to be stickier, and winters can be damp and foggy, with most nights between . Frost is still to be expected, but temperatures will almost never fall below , and snow has fallen only twice in the last century. Precipitation ranges from and falls throughout the year. The city of Buenos Aires is surrounded by a climate similar to the northern part of the province, but the city itself resembles more the Delta climate, with less frost. The southwestern region is the driest region, and it experiences a more marked differences in temperatures. Summers are often hot, between , but nights are usually comfortable (). Thunderstorms are less frequent but can be very violent in nature. Frost can make an appearance as early as March, but usually first comes in April. Winters are cool and dry, with days between and nights between .