Mexico extends between 14 ° and 32 ° of lat. N., that is, for 9 degrees south of the Tropic of Cancer and therefore within the limits of the torrid zone, and for 9 degrees north of the tropic itself, within the temperate zone. But, if due to the effect of latitude the country should have a tropical and subtropical climate, the considerable altitude of most of the territory decreases strongly; and in part also cancels the effects of latitude. And since the height above the sea increases as you go south, so that the greater heights correspond to the southernmost area of the plateau, the average annual temperature varies little from the far north, on the border of the United States, to the extreme south in the basin of Mexico; only in
According to top-medical-schools, the influence of the sea on the other hand is exerted almost exclusively on the low coastal lands, since the steep escarpments of the sierra stop the marine action towards the interior; the sea, however, is of great importance to the effects of rainfall. In fact, Mexico owes its fertility to the prevalence of the Gulf winds, since these vaporous air currents, forced to climb the external slopes of the sierras, not only wet the slopes themselves abandonedly, but also bring sufficient rainfall into the basins and in the valleys of the central plateau, feeding sufficient or luxuriant vegetation even in the tropic area, which would otherwise be condemned to aridity.
On the contrary, the cold winds from the north, which attracted by the low winter pressure area of the Gulf, descend until they hit the coasts and slopes of Veracruz with great violence, making the temperature differences between the summer months and the winter months more sensitive. the same tropical zone. It is also necessary to remember the action of tropical or extra-tropical storms, which often alter the influence of climatic factors and which, like the hurricanes of the Caribbean Sea, can hit the southern parts of the country causing serious damage.
But, we repeat, the prevailing factor of Mexico’s climate is altitude; the mountains and plateaus transform much of the country from a region with a tropical climate to a region with a truly temperate climate, from an arid territory to a territory of abundant rainfall. In fact, Mexico has all kinds of climates, from the glacial one of the highest peaks covered with eternal snow, to the equatorial one of the palm regions in the coastal lowlands of the Pacific and Gulf. Just take a look at the temperature distribution map to see the enormous influence of altitude. The average vertical gradient of the temperature is one centigrade every 200 m., Hence the city of Mexico, located at over 2200 msm, has an average annual and monthly temperature about 10 degrees lower than that of Veracruz and Manzanillo, localities located at roughly the same latitude. The popular voice has for a long time already distinguished these differences with the three expressions oftierras frías, tierras templadas, and tierras calientes, expressions certainly elastic and on the limits of which there is no precise agreement neither among popular judges nor among scholars, but which denounce the existence of thermal zones of altitude already reported by Humboldt and clearly visible to those who go back, for example, from Veracruz to Mexico: tierras calientes are those that have an average annual temperature above 22 °, 5, i.e. the flat coastal sections and the low territories of the isthmus of Tehuantepec, Tabasco, Campeche and of Yucatán; le tierras templadasthey cover most of the country, that is, the whole plateau and a good part of Baja California and their average annual temperature oscillates between 15 ° and 22 °; the tierras frías are limited to the highest mountainous regions, with an average annual temperature of less than 15 °, that is to say to rather restricted areas that can hardly be represented in an overview map. Of course, the area of the tierras heladas is also smaller, consisting of the highest peaks, some of which are covered by perpetual snow such as the peaks of Orizaba, Popocatepetl, Iztaccihuatl, Nevado de Colima and a few others. Still with regard to the temperature, it is noted that the coasts of the Pacific are warmer than those of the Gulf, both because the surface waters of that sea are warmer than the waters of this sea, and because the coasts of the Gulf are beaten, during the winter., of the north winds. Likewise, both on the coasts and on the plateau, the temperature decrease gradient is small proceeding from south to north and this in the coastal lowlands, thanks to the uniform temperature of the sea waters and on the plateau because, as we have said, at most high latitude corresponds to a lower altitude. L’
Various climatic regions can be distinguished in Mexico, bearing in mind that the types of warm climate prevail in the lower regions, while in the high plateau regions the tropical mountain climate and the temperate climate without cold season dominate.
The equatorial type region (climate of Guinea, according to De Martonne) includes in Mexico a rather restricted area of Tabasco, corresponding to the Usumacinta basin and extending along the Gulf coast to Puerto México, an area indicated with the letter A on the map. The average annual temperature is everywhere 25 ° or higher and that of the coldest month never drops below 21 °, 8, while the rainfall remains above 2000 mm. and in some stations it reaches and exceeds 3000 mm. The subequatorial climate region(or Sudanese: letter B) with a dry season and with more accentuated variations in temperature, it embraces the area of the Tehuantepec isthmus, a large part of Chiapas and a part of Campeche: a small section of the eastern side of the state of Veracruz in correspondence with the centers of Orizaba and Córdoba, where the exposure and the relief make rainfall more abundant. Here, too, the average annual temperature exceeds 20 °, except in Orizaba where it is lowered by altitude, but the temperature of the coldest month drops below 20 ° in the higher sections, while it remains higher along the coasts. The rainfall varies from 1100 mm. of Salina Cruz to 2117 of Córdoba, and everywhere there is a dry season.
The tropical climate region (or Senegalese: letter C) characterized by a dry period of 4 to 6 months, with rainfall of less than 1500 mm. and with strong annual variations in temperature, it includes most of the Yucatán peninsula, the coastal plain of the Gulf from Veracruz to Tampico, the Río Las Balsas valley and the entire Pacific coast from the southern border of Guerrero to Culiacán. The average annual temperature remains around 25 °, but while that of the hottest month rises to 28 °, in the coldest month it drops to 19 ° as in Mazatlán and Culiacȧn: rainfall is everywhere below 1500 mm. and even drops to less than 600 in some parts of the Pacific coast.
North of Tampico, in the coastal plain and in the outer slope of the Sierra Madre Oriental, there is a monsoon type climate (D) with a dry winter period and a rainy summer period, with strong temperature variations and a lower rainfall in general than 1000 mm.
The altitude factor prevails on the plateau, which in conjunction with the rainfall gives rise to three different types of tropical mountain climate, corresponding to the regions indicated respectively with the letters I, H, L. The highest part of the Sierra Madre Occidental (I) forms a region of mountain climate, excessive, with the temperature of the warmest month exceeds 20 ° and that of the coldest month that is about 5 °: the average annual temperature is about 13 °, while the rainfall does not usually exceeds 500 mm. Around this region extends another (letter H) which includes the two sides, western and eastern, of the Sierra Madre itself and which we can call a region with a high plateau climate, excessive, with rather strong annual variations in temperature and with rainfall almost always less than 500 mm. The average temperature of 16 ° results from maximum averages of 21 ° and minimum averages of 8 °.
Finally, the section of the southern plateau (letter L) also enjoys a subtropical plateau climate, but of the Valley of Mexico type. The average temperature is around 15 °, and while the average of the hottest month does not usually exceed 20 °, that of the coldest month does not drop below 12 °; the rainfall then varies from 900 mm. of Guadalajara to 340 of Zacatecas, remaining overall above 500 mm. The San Cristóbal area in Chiapas also has a similar climate.
In the northern part of the plateau and the Pacific coast, due to the extreme scarcity of rainfall, a desert climate predominates which in the largest section can be considered steppe (E) while in Baja California, in the north-western coastal section of Sonora and in Bolsón de Mapimí we note the characteristics of the Saharan desert climate(F). In the first of these regions the average annual temperature oscillates between 17 ° (Saltillo) and 24 ° (Laredo and Guaymas), the very sensitive annual excursion varies from 15 ° to 20 °, and the rainfall goes from a maximum of 500 mm. to a minimum of 200. The Saharan climate region has a higher average annual temperature (23 °) with equally strong excursions, and a minimum rainfall, usually less than 100 mm.
The small northern region of Baja California stands out from all the others, which opens onto the Pacific from San Quintín to the border, with a Mediterranean climate (G) similar to that of American California.
