The year in Chiapas, month by month
The figures come from three stations of the Mexican national weather service, 1991–2020 series: Tuxtla Gutiérrez (532 metres, 1,053.6 millimetres a year), San Cristóbal de las Casas (2,113 metres, 1,141.3) and Tapachula (coastal plain, 2,057.8). The scoring crosses rain, heat and how usable the roads are.
The rule everyone learns about the tropics is that there is a dry season and a wet one, and that within the wet season the month makes no difference because it rains every day. In Chiapas the first part is true and the second is not.
01 The gap in the middle of the season
The Chiapas rainy season starts in May and ends in October, and inside it the curve does not rise continuously: it rises, dips and rises again. The low point is July.
In Tuxtla Gutiérrez, the capital, June leaves 227.4 millimetres over 16.9 days and July 167.8 over 15.5. That is almost sixty millimetres less, a 26 per cent drop, precisely in the month when you would expect the opposite.
The phenomenon is known throughout Mesoamerica as the canícula, and it is a pause in the wet season lasting a few weeks in high summer. It does not stop the rain, but it spaces it out: less rain and less violent rain.
02 Three stations, the same dip
What turns the figure into a rule rather than a coincidence is that it appears on all three levels of the state at once, even though their annual totals look nothing alike.
In San Cristóbal de las Casas, at 2,113 metres, June leaves 238 millimetres and July 141.6: a 41 per cent drop, and from 22.2 to 17.6 rainy days. It is the sharpest dip of the three.
In Tapachula, on the coastal plain, June leaves 327.8 and July 257.6, 21 per cent less. That station's annual total is 2,057.8 millimetres, almost double the capital's, and even so the July step is there.
Three stations at 532, 2,113 and 170 metres, with annual totals ranging from 1,053 to 2,057 millimetres, and the same dip in July.
03 The dry season, which is the easy one
Even so, the best part of the year remains the other half. From November to February, Tuxtla records between 1.4 and 12.3 millimetres a month, with fewer than three rainy days. San Cristóbal, between 4.4 and 32.8.
Within that window it pays to distinguish. November arrives with the landscape still green from the wet season and the waterfalls still flowing: the best combination of the year. January and February give the clearest skies and the coldest nights up top, with average lows of 4.9 and 5.2 degrees in the Altos.
March and April are the trap of the dry season. It still does not rain, but Tuxtla's average high climbs to 34.5 and 36.1 degrees, with 42 as the absolute record, and the air fills with haze. Dry yes, comfortable no.
04 September, the month worth avoiding
If July is the window, September is the closed door. It is the wettest month of the year at all three stations at once: 220.7 millimetres in Tuxtla, 229.3 in San Cristóbal and 403.6 in Tapachula.
That coastal figure deserves a second look: 403.6 millimetres over 23.6 days is almost a fifth of the annual total concentrated in one month, and it rains almost four days out of five.
The consequences are practical. Dirt roads become unreliable, rivers run heavy with sediment and the waterfalls, blue in March, come down brown. And October, although the total already falls to 71.1 millimetres in Tuxtla, is still a transitional month.
05 And the correction for altitude
There is a nuance the monthly total hides and that matters a great deal if the trip is a mountain one. San Cristóbal collects 1,141.3 millimetres a year, barely ninety more than Tuxtla, but spreads them over 150.5 days against 92.6.
In other words: up there it rains a little more, but almost twice as often. In the Altos water arrives as drizzle and persistent fog rather than short downpours, and that changes the clothes you need and the margin you leave in each day.
The other correction is temperature. San Cristóbal's average high moves only between 20.5 and 24.0 degrees over the whole year, a range of 3.5 degrees. Up there the month barely changes the thermometer: it changes the water.
06 How to choose the month
If you can choose without constraints, November. It is the year's first dry month and arrives with the landscape still loaded from the rain that has just left: 12.3 millimetres in Tuxtla and waterfalls still flowing.
If the calendar forces you into summer, July rather than August. The canícula's drop is real and measured at all three stations, and the difference between 141.6 and 238 millimetres in the Altos shows in practice.
And in any month, the rule that organises the day is the same: in the wet season the morning is the reliable part. Field outings, dirt-road journeys and open-air visits happen before midday; the afternoon is left for whatever has a roof.
The year's four stretches
From 1.4 to 12.3 millimetres a month in Tuxtla and 4.4 to 32.8 in San Cristóbal. Firm tracks, low rivers and clear skies. November adds the green the rain left behind; January and February, the clearest days.
Average highs of 34.5 and 36.1 degrees in the Central Depression, with only 2.7 and 11.3 millimetres of rain. Still dry, but midday down below is unbearable and the air fills with haze.
Rain goes from 117.8 to 227.4 millimetres in Tuxtla and from 138 to 238 in San Cristóbal. The heat eases and the state turns green, but June already brings up to 22.2 rainy days a month in the Altos.
July opens a parenthesis of 167.8 millimetres, but August and September climb to 215.5 and 220.7 in Tuxtla and to 403.6 in Tapachula. October drops to 71.1. This is the water stretch, with July as the only window.
