Map from Google - Mexico

Mexico is a vast country and has different types of climate, however, it's generally arid on the west coast and the inland highlands of the center-north, it becomes moderately rainy in the mountain ranges and the southern plateaus, while it's very rainy in some tropical southern areas. Summer is the rainiest season almost everywhere.
The temperature increases gradually from north to south, so that the south has a purely tropical climate. The role of altitude deserves attention too: there are many cities located at high altitude, therefore colder, starting from the capital.
During winter, a cold wind from the United States, the "Norte" can blow, able to lower the temperature abruptly, especially in the north-central. Because of this wind, the northern inland areas (and a little less the northern coastal areas) during winter may experience cold waves, especially in hilly and mountainous areas.

1- The mild coast

Baja California, coast
In the peninsula of Baja California, nearly a continuation of California, the climate is very mild on the west coast, where daytime temperatures go from 66/68 degrees Fahrenheit in winter, to 75/77 °F in summer in the northern part. The sun shines throughout the year, and the rains, poor, fall from November to March; the total rainfall is around 8/10 inches per year. Here, as in California, a cold current flows, which makes the sea cool and causes some fog, but in return it provides sun, little rain and a pleasant climate. However, the temperatures can suddenly increase, surpassing even 105 °F, when the winds blow from the interior. A city that is located in this area is Tijuana, just south of the border with the United States, a short distance from the greater San Diego area.
Here are the mean temperatures of Rosarito, in the coast near Tijuana (in summer the center of the city is slightly warmer, because of its distance from the coast).
Average temperatures - Rosarito
Min (°F)505050525557616361595450
Max (°F)666666687072737775737068

The rainfall is scarce and amounts to 10 inches per year, and follows a Mediterranean pattern, being concentrated in autumn and winter. Here is the average precipitation.
Average precipitation - Rosarito
Prec. (in)

The temperature of the sea in this part of the coast is very cool even in summer, as we can see in the following table.
Sea temperature - Rosarito
Sea (°F)595959596163666868666461

Moving towards the south, the sea becomes progressively warmer. In the southern part of Baja California, the sea temperature reaches 70/72 °F from August to October, while in Baja California Sur, the sea becomes decidedly warmer.
East of the coast, in mountain ranges of Sierra Juárez and San Pedro Mártir, the winter temperature decreases due to the altitude, with frequent night frosts, while summer gets hot, because the distance from the sea (which is cooled down by the cold current) is even more important than altitude, although above 3,300 feet, the nights get cool.

2- Sonoran desert

Climate of the Sonoran Desert
The Sonoran Desert, the westernmost of the Mexican deserts, is characterized by an increase in rainfall in the latter part of the summer, a kind of "little monsoon" from July to mid-October, which may bring some thundersdtorms and an increase in humidity. The central-southern part of Baja California (administratively divided in the states of Baja California and Baja California Sur) may be classified in this area, although the climate along the west coast is milder than the shores of the gulf and the inland areas.
In fact, continuing south on the aforementioned west coast, the temperature rises gradually, until in Baja California Sur highs arrive at 68/72 °F in winter and 86 °F in summer in the northern part, and reach 77 °F in winter and 90 °F in summer in the far south.
In the eastern side, the coast of the Gulf of California in summer is much warmer than the western coast, because the cold current doesn't arrive here, and being a closed gulf, in summer it heats up a lot. Here, the daytime summer temperatures are around 95/97 °F, while in winter they are around 68 °F in the north, 71.5 °F in the center, and 77 °F in the south.
Baja California is a barren, almost desert peninsula: along the coasts the annual precipitation is around 4/6 inches. But in the hills of Baja California Sur, the little monsoon brings additional rainfall, so that the annual amount exceeds 16 inches per year. This area, known as Sierra de la Laguna, is covered by woods (and therefore should be excluded from the desert area, as indicated by the small circle in the map).
Within the little monsoon, especially in August and September, Baja California Sur (more rarely Baja California) can be affected by low pressure areas and tropical cyclones that form in the Pacific Ocean off the southern coast of Mexico. These disturbances, usually less intense than those that form in the Gulf of Mexico, can then penetrate in the continent, affecting the northern part of Sinaloa and the state of Sonora.
Cabo San Lucas is located at the southern tip of Baja California. Here, as in the rest of the peninsula, the climate is arid; winter is sunny with cool nights, around 54/55 °F, and pleasantly warm days, around 79/81 °F. Spring is warm and sunny, and daytime temperatures reach 84 °F in April. Summer is hot, with highs around 93 °F, but in the second part, from mid-July to October, there's some chance of showers and thunderstorms, sometimes intense, especially in September, as part of the little monsoon.
Average temperatures - Los Cabos
Los CabosJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)555555596366737573706357
Max (°F)777981848690919391908479

In this southern part of the Baja California the rainfall pattern is opposite to that of the northern part, because the rains are concentrated in late summer. Here is the average precipitation.
Average precipitation - Los Cabos
Los CabosJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (in)0.50.100000.

Here the sea is very warm in summer, while in winter it is a bit cool, but not impossible for swimming.
Sea temperature - Los Cabos
Los CabosJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°F)727070727377818484847975

In the north-east of the state of Baja California, we find the Yuma Desert, where the town of Mexicali is located: it is an area of reclaimed lands and recent settlements, where summer temperatures reach extremely high values: even more than 130 °F! This is the hottest place in Mexico, as well as one of the hottest places in the world.
Average temperatures - Mexicali
Min (°F)434650556370797972615043
Max (°F)6873798695104108106100907768

Rainfall is very low, below 4 inches per year, and even the increase in rainfall between mid-summer and autumn, here is very low. Here is the average precipitation.
Average precipitation - Mexicali
Prec. (in)

Southeast of this desert, we find the Altar Desert, a sand desert which looks like the vast areas covered by dunes of the Sahara.

Sonoran Desert

The western part of the state of Sonora is part of the Sonoran Desert itself, and as we have seen with regard to Baja California Sur, it experiences an increase in precipitation from July to October. The wet winds from July to October, slightly lower the temperature to the point that the hottest month is often June, but they raise the relative humidity. In this area we find the important city of Hermosillo, which has a dry winter, pleasantly warm during the day (but nights can be cold), and a very hot summer, with average highs around 103 °F already in June.
Average temperatures - Hermosillo
Min (°F)485254596675797775665550
Max (°F)757982909510410210099938275

From July to October, the little monsoon brings rain and thunderstorms, sometimes strong, which, however, soon leave room for the sun. The rainiest month is August, with almost 4 inches of rain. Even the relative humidity increases, rising from 33% in June to 55% in July and 51% in August and September, so that the heat becomes sweltering. Here is the average precipitation.
Average precipitation - Hermosillo
Prec. (in)

3- Chihuahuan Desert

The mountain range called Sierra Madre Occidental, which separates the state of Sonora from that of Chihuahua, and continues south to the states of Sinaloa, Durango and Nayarit, experiences a fairly long period of monsoon rains (mid-June to early October) even quite heavy, so that precipitation exceeds 15.7 inches per year, with July as the rainiest month, making the climate of these mountains semi-arid rather than desert. Above 3,300 feet, rainfall even exceeds 27/31 inches, so that the landscape becomes green. In the southern part of the Sierra Madre Occidental, even winter becomes quite rainy, so that rainfall exceeds 39.5 inches per year.

To the east of the Sierra Madre Occidental and to the west of the Sierra Madre Oriental, ie between the two ranges, we find the Chihuahuan Desert, an area of highlands located in the states of Chihuahua and Coahuila as well as the north-east of Durango, characterized by a wide daily temperature range, sunny and dry winters, with nights that can be frosty, and hot summers which become moist from mid-June to mid-October, when the sun frequently shines, but it alternates with rains and thunderstorms. Because of the altitude, the summer heat is less intense than in the lower areas of the Sonoran Desert, however sometimes the temperature can reach or exceed 105 °F.
An important city of this region is Ciudad Juarez, in the far north, at 3,600 feet on the border with Texas, with an average in January of 46.5 °F, and in July of 84 °F.
Average temperatures - Ciudad Juarez
Ciudad JuarezJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)343643505768726863523934
Max (°F)596472798897979388796861

The annual precipitation is 10.5 inches, with September as the rainiest month. Here is the average precipitation.
Average precipitation - Ciudad Juarez
Ciudad JuarezJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (in)

Other cities are Chihuahua, 4,600 feet, where the average is 48.5 °F in January, the hottest month is June with an average of 79 °F, 17.7 inches of rain per year fall, with August as the rainiest month; Torreon, 3,600 feet above sea level, January average equal to 58 °F, June average of 82.5 °F, 9 inches of rain per year, with September as the rainiest month; Saltillo, 5,600 feet, January average of 53.5 °F, June average of 73.5 °F, 14.5 inches of rain per year, with September as the rainiest month. During winter, cold spells can be intense especially in the north: in Ciudad Juarez the temperature can drop below 5 °F.
Average temperatures - Torreon
Min (°F)454854616670706866595045
Max (°F)727779919595939390867973

4 - Eastern plains

Eastern plains
In this eastern area, overlooking the Gulf of Mexico (including the the state of Tamaulipas, the east of Nuevo Leon, the south-east of San Luis Potosi, the north-east of Hidalgo, northern Puebla and northern Veracruz), along the coast, plains and hills below 1,600 feet of altitude, winter is very mild, sometimes pleasantly warm, but we can not exclude some brief cold spells, while summer is hot and humid. The rainfall ranges generally from 20 to 40 inches per year, and increases gradually from north to south and as near the highlands, where it even exceeds 40 inches per year. The rainy season runs from May to October. In this area there are cities like Monterrey, 1,600 feet above sea level, which has an average in January of 58 °F, and in July of 84 °F, 23 inches of rain per year, including more than 2 inches per month from May to October, and September as the rainiest month; and Tampico, on the coast, which has an average in January 66 °F, in August of 83 °F.
Average temperatures - Tampico
Min (°F)596166707577777775726661
Max (°F)737581848890909088868175

Tampico receives 45 in of rain per year, including more than 4 inches per month from June to October. Here is the average precipitation.
Average precipitation - Tampico
Prec. (in)

At Tampico, the Gulf of Mexico is warm enough for swimming all year round, although in winter it drops to 73 °F.
Sea temperature - Tampico
Sea (°F)737373778182848484827975

In south-central Mexico, the temperature variations are caused mainly by the altitude, so that we can distinguish among the tierras calientes, from the plain to 3,300 feet, the tierras templadas, betwen 3,300 and 6,500 feet, and the tierras frìas above 6,500 feet.

5 - Tierras frías

In the plateau there is a considerable difference in temperature between day and night, especially in the dry season, with the tropical sun that burns the skin, even though the temperature is not so high. As for the rains, there is a dry season from November to April, and a rainy season from May to October.

Tierras frìas
The vast capital, Mexico City is located in the south, at an altitude of 7,200 feet in the southern part of the plateau; administratively it belongs to the Federal District. Winter is dry and sunny, with cold nights: in January and February lows are around 39/43 °F, but sometimes they can drop a few degrees below freezing, while days are pleasant, so that highs are around 70/73 °F. In spring the temperature rises gradually until April and May, which are the months when the daytime temperatures are higher, around 80 °F, while the night-time temperature is still pretty cool, around 50/54 °F. In June, the increase in the daytime temperature stops, in fact it comes down by a few degrees, to around 73/75 °F, because of the summer monsoon, which brings clouds and rains from June to September. In a typical year, 28 inches of rain fall, with a maximum of 6.3 inches in July.
The Mexican capital is polluted, and there is an almost constant mist.
For further information: see the climate of Mexico City.
Average temperatures - Mexico City
Mexico CityJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)434548525454545454504645
Max (°F)707379818177737372727270

6 - Tierras templadas

Climate of the Tierras Templadas
In the lands of south-central Mexico which are located at intermediate altitudes, the average temperatures are mild, although sometimes it can get cold on winter nights, and sometimes during the day it can get hot. In this area, there are cities like Guadalajara, at 4,900 feet above sea level, in the state of Jalisco, with an average in January of 63.5 °F, and in May of 76.5 °F. Here are the average temperatures.
Average temperatures - Guadalajara
Min (°F)505255596364636363595552
Max (°F)778184889088818181818177

Rainfall amounts to 38 inches per year, with a very dry winter, and a rainy season from May to early October. Here is the average precipitation.
Average precipitation - Guadalajara
Prec. (in)

In the southern highlands, the temperatures are slightly higher and rainfall is more abundant, as happens in the Chiapas highlands, where in addition, also October and November are rainy.

7 - Tierras calientes

Climate of the Tierras Calientes
In the plains and hills of the south, the climate is hot all year round, with a dry season (December to April or November to May, depending on area) and a rainy season (May to November or June to October). The warmest period is the one that precedes the rainy season, usually the months of April and May. But then with the arrival of the rains, the humidity increases and thus the feeling of sultriness. Usually the rains come in the form of thunderstorms, sometimes strong, in the afternoon. Normally, the rains become more abundant moving south. In inland areas of the southern states, precipitation exceeds 80 inches per year, but in some cases even 120 in. The south of the state of Veracruz, the south-east of Oaxaca, the state of Tabasco and the northern Chiapas (see Palenque, Yaxchilan) are very rainy areas: here there are moderate rains also in winter, and therefore there is not a real dry season.
In Acapulco, along the coast of the state of Guerrero, the climate is hot all year round, but the dry season, from November to May, is sunny and it almost never rains. The sea is always warm, not going below 82 °F in winter.
Average temperatures - Acapulco
Min (°F)737575757777777777777775
Max (°F)868686889090909090909088

The mountains located north of the city protect this coast from cold winds, while this does not happen in the coast on the Gulf of Mexico (see below). However, in the rainy season, from June to October, the rains are abundant and sometimes torrential. Here is the average precipitation.
Average precipitation - Acapulco
Prec. (in)

Here, and in general along the southern coast of Mexico, during the rainy season tropical cyclones can form, causing gale force winds and heavy rains, the cordonazos. Traditionally El cordonazo de San Francisco is a storm that occurs on October 4, the day of St. Francis.

Further north, in Puerto Vallarta, in the state of Jalisco, the climate is similar to Acapulco, but during winter the nights are cooler, and they can occasionally get pretty cool, around 50 °F, and this can still happen, though more rarely, in March and April.
Average temperatures - Puerto Vallarta
Puerto VallartaJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)636163636873737373726864
Max (°F)848484868890919393939086

The sea in Puerto Vallarta is warm enough for swimming even in winter, since it doesn't go below 75 °F.
Sea temperature - Puerto Vallarta
Puerto VallartaJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°F)777575757982848684848279

In the east, in the Yucatán Peninsula (states of Campeche, Yucatán and Quintana Roo), the climate is generally warm or hot all year round, but winter is not as dry and sunny as in the south coast: the months of December and January, when highs are around 82/86 °F, are quite rainy; besides in winter, from December to March sometimes the cold wind from the United States, the Norte, can blow, so that the minimum temperature can drop to 50 °F, and even below. In Cancun, the least rainy period runs from February to April, and the driest month is April, so it is a suitable place for Easter vacations, when the Norte is less likely to blow.
Average temperatures - Cancun
Min (°F)707072737577777777757370
Max (°F)828486909191919391888682

The Gulf of Mexico in Cancun and the Riviera Maya is warm enough for swimming all year round.
Sea temperature - Cancun
Sea (°F)797979818282848484848281

The rainy season runs from May to November, although the rains are abundant especially in June, and between September and mid-November; this is a hot and sultry period, during which there's no shortage of sunshine, because the rains generally occur in the form of downpour or thunderstorm, so they don't last long. Similar situation is found in Isla Mujeres and the Mayan Riviera south of Cancun (Playa del Carmen, Tulum, Cozumel, etc..): here March and April are the driest and sunniest months, with April as the best month.
During the rainy season, Yucatán and coastal areas bordering the Gulf of Mexico may be affected by hurricanes, especially in the period from August to October, however the hurricane season runs from June to November. Hurricanes come from the south or east, and sometimes can be destructive.

8 - Mountains

In high mountains and volcanoes of south-central Mexico, the climate is cold, although given the low latitude, this happens at very high altitudes. For example, in the volcano Nevado de Toluca, southwest of Mexico City, at 14,000 feet above sea level, the average temperature in January is 35.5 °F, while that of April and May, the warmest months, is 41 °F.
Average temperatures - Nev. de Toluca
Nev. de TolucaJanFebMarAprMayJunJulAugSepOctNovDec
Min (°F)272830323234343234323028
Max (°F)454648505048464646464545

The rainy season runs from May to October, and July and August are the rainiest months, but with these temperatures it can snow even in summer. Here is the average precipitation.
Average precipitation - Nev. de Toluca
Nev. de TolucaJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (in)

The three highest volcanoes of Mexico, Citlaltépetl, Popocatepetl and Iztaccihuatl exceed 16,400 feet, and at the highest elevations perennial glaciers are found.

When to go

To explore the northern regions of Mexico (coasts and deserts), October is a good month, however, spring and autumn are generally the most recommended (March-April and September-October).
Even in the tierras frìas, spring and autumn are preferable, but here the main purpose is to avoid the summer rains rather than the heat: here you can also avoid September, which is still a rainy month. The tierras frìas can be visited also in winter, bearing in mind that it can get cold at night.
The tierras calientes can be visited preferably from December to February, to avoid both the heat and the summer rains, and because in some areas the months of October and November are still very rainy. In spring, in April and May, the rains have not arrived yet, but it's usually hot.

As regards the sea, for a beach vacation the southern coast (see Acapulco) is warm and sunny in winter (eg Christmas), with a warm sea, around 84 °F. In Acapulco the good season is long, from November to April.
The coast of the Gulf of Mexico and Yucatán (see Veracruz, Cancun), during winter do not have an ideal climate, at least for a beach vacation: the sky is often cloudy, sometimes it can rain, and the Norte can bring a little cool. So the spring is better, from March to May, with a preference for April: in March sometimes some cool air can still arrive, especially in the first half of the month, while in May it begins to get hot, with the first thunderstorms.
Spring is fine also in Cabo San Lucas and the southern part of Baja California. In the northern coast of Baja California, where the air is mild even in summer, you can swim in the sea in summer, even though the water is a little cold.

What to pack

In winter: for the Chihuahuan desert and the mountains, warm clothes, jacket, hat, gloves. For Baja California, eastern plains and tierras templadas, clothes for spring and autumn. For Mexico City and tierras frìas, clothes for spring and autumn during the day, warm jacket, wool cap for the evening. For tierras calientes, light clothing, a sweatshirt for the evening in Yucatan and Riviera Maya; for the reef, equipment for snorkeling, water shoes or rubber soled shoes. For the highest mountains, warm clothes, down jacket, gloves, cap.

In summer: light clothing, sunglasses, sunscreen, a light raincoat. For the Chihuahuan Desert, a sweatshirt for the evening. For Mexico City and tierras frìas, sweatshirt or sweater for the evening, a jacket and umbrella. For the highest mountains, warm clothes, warm jacket, wind jacket, gloves, hat, hiking shoes.