Mexico is the largest country of Central America, and is bordered on the north by the USA and on the south by Guatemala and Belize. Almost two thirds of the country consists of plateaux and high mountains with a climate that is warm-temperate; other parts have a tropical climate with temperature reduced by altitude.
There are three important influences which help to determine the character of the climate of different parts of Mexico. The cold Californian Current which sweeps southwards on the Pacific coast has the effect of lowering temperatures and reducing rainfall on the west coast as far south as the tip of the peninsula of Lower California. This and the influence of the North Pacific anticyclone help to make much of the north-west of the country desert or semi-desert; this is a continuation of the dry zone of the USA in California, New Mexico and Arizona.
The warm Caribbean Sea and the influence of the constant north-east trade winds make the eastern coastal region a typical tropical coast with a marked single wet season in summer. The weather and climate of this region have much in common with that of the Caribbean islands.
An important influence is the presence to the north of the great continental land mass of the United States and Canada. This area becomes very cold in winter particularly when cold air sweeps down from the Canadian Arctic, but also very warm in summer. Northern Mexico shares in these extreme temperature conditions. In winter, cold waves ('northers') can bring near-freezing conditions for a few days to the east coast as far south as Tampico. Snow has fallen at Tampico, which is within the tropics. The west coast is protected from such cold waves by the mountains and plateaux of central Mexico.
As in other South and Central American countries, the climatic zones are described on the basis of altitude: tierra caliente (below 2,000 feet); tierra templada (2,000-6,000 feet); tierra fria (above 6,000 feet). Only a very narrow coastal belt on the Pacific coast falls into the tierra caliente category, but there is a more extensive area on the Caribbean shore, including the whole Yucatan peninsula. The largest part of Mexico falls into the other two categories. In most of the terra fria, frost is frequent at night in winter and snow can occur anywhere, but only lies above 1,000-12,000 feet.
The rainy season over the whole country is the period of high sun from May to October, and the wettest part of the country is the lowland on the Caribbean coast. Annual rainfall in these parts is between 1,000 and 1,500 mm, but some places in northern Yucatan get less than 500 mm. The more northern shores of the Pacific and Gulf of California receive less than 250 mm per year, but this increases southwards to between 1,000 and 1,500 mm. Where the coast is backed by high mountains rainfall is heaviest.
Most of Mexico has sunny weather for a large part of the year. The cloudiest regions are the wetter parts of the east coast and the northern part of the Pacific coast where fog and low cloud are formed over the cold ocean current. The drier regions of the interior and much of the tierra templada have high sunshine amounts - as much as eight hours a day in the drier months.
In some parts of the southern lowlands, the combination of heat and humidity can be rather uncomfortable during the wet season, but otherwise most of Mexico has a healthy and reasonably pleasant climate for most of the year.
At the altitude of Mexico City and above, visitors may take a few days to adjust to the lower atmospheric pressure as sudden exertion can lead to breathlessness. On the higher parts of the plateau the sun may at times feel very powerful by day, followed by a rapid drop of temperature at night.
Both the west and east coasts of Mexico are occasionally affected by tropical storms which develop in the Pacific or the Caribbean and bring two or three days of heavy rain. These are most likely to occur in the months August to October. Very few of these reach the strength of fully developed hurricanes, but if they do, the east-coast districts are more liable to severe damage.