Physical Geography and Climate
The Toluca Valley is a broad highland valley located immediately west of the Valley of Mexico. It is one of the highest valleys in Mexico with an average elevation of 2,570 meters above sea level. The valley is part of the Trans-Mexican Volcanic Belt, with its soils of volcanic origin mostly dating from the Upper Cenozoic. Mountains sides have tertiary volcanic rock, mostly basalt and andesite along with pyroclastic matter and breaches. The valley floor is composed of lacustrine and sedimentary materials . The valley covers an area of about 700km2 running north south with a length of 35 km. Its width is about 20 km. Its northern border is marked by the Guadalupana Volcano, the Cerro el Aguila and the Sierra Monte Alto. Its southern extension is marked by the Tenango and Zempoala Volcanoes. Its eastern boundary is marked by the Sierra de Las Cruces and La Iglesia mountains and the west is defined by the Nevado de Toluca and the Sierra Morelos. These mountains, along with the elevation of the valley in general cause the area to experience cooler temperatures then the rest of the region.
The most distinctive of the surrounding mountains is the Nevado de Toluca (4,690masl) to the southwest. The Nevado is the fourth highest mountain in Mexico at 4,600 masl, formed by successive eruptions with deposited layers of igneous materials . Its peak has snow most of the year and is surrounded by temperate forests. The mountain has important effects on the climate and hydrology of the Toluca Valley The piedmont is characterized by hills covered by broadleaf forests. On the upper slopes there are conifers. The highest elevations have alpine grasslands. While the area is in the tropical latitudes, its high altitudes along with cold are from the Sierra de Las Cruces and Nevado de Toluca gives the area a semi-cold to cold climate. The average monthly temperature varies between 5C at altitudes over 3000m and 20C lower in the valley.
The climate is classified as C(w2)(w)bi(g) under the Koppen classification system which means a sub humid temperate climate with a long summer, winter rain of less than 5%, isothermal with maximum temperatures occurring before the summer solstice . The area has a rainy and dry season with the rainy season extending from mid May to mid October. Most precipitation occurs on the sides of the Nevado de Toluca of up to 1200 mm per year. The valley floor receives and average of 900mm per year. Almost all precipitation occurs during the rainy season with most coming in June. After this, there is a short dry spell in August before rains return in September.
It is part of the upper course of the Lerma River, which is the main source of surface water.
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