The year in Jalisco, month by month
The figures come from the Guadalajara station, 1991–2020 series: 1,013.5 millimetres a year over 91.6 days, at 1,566 metres. The scoring crosses heat, rain, humidity and the state of the landscape; the Pacific coast runs on its own and is discussed separately.
The Mexican plateau's calendar works neither like the European one nor like the coastal tropical one, and anyone travelling with either model in mind picks the wrong month. Here the year does not split into cold and hot: it splits into dry and wet, and the heat is where you do not expect it.
01 The heat peak arrives before the water
Guadalajara's highest average maximum is May's, at 31.9 degrees. Then April, at 30.5, and then June, at 30.1. July and August, the months that in half the world are the peak of summer, stay at 27.4 and 27.2.
The extremes tell the same story. The station's absolute record is 41.0 degrees and it was recorded in April, not July. And relative humidity hits its annual minimum, 46 per cent, in April too.
The result is that the worst stretch of the year to be here runs from March to May: rising heat, dry dusty air, not a cloud to break the sun and completely ochre countryside. In April 3.8 millimetres fall over 0.8 days.
Thirty-one point nine degrees in May and twenty-seven point four in July: the rainy month is four and a half degrees cooler than the dry one.
02 How the rain brings the thermometer down
The mechanism is direct and has three parts. The first is cloud: from June onwards, the afternoon clouds over almost daily and the sun stops heating the ground during the hours when it bites hardest.
The second is evaporation. Wet ground spends energy evaporating water instead of heating up, so the same radiation produces less temperature. And the third is the storm itself, which dumps cold air within minutes.
Between them they bring the average high down 4.5 degrees from May to July. But there is a counterpart worth noting: relative humidity goes from 48 per cent in May to 72 in August, so the air becomes far heavier even though the thermometer reads lower.
03 Eighty-six per cent in four months
The other figure that defines the year is the distribution. Guadalajara receives 1,013.5 millimetres of rain a year over 91.6 days, and of those, 869.3 millimetres fall between June and September: 85.8 per cent of the total in a third of the calendar.
In days it is clearer still: 71.7 of the year's 91.6 rainy days fall in that stretch. In July it rains on 21.1 days, two out of three; in April, on 0.8.
For the traveller, that has a very concrete practical consequence: the rain of this region is not a diffuse risk spread through the year, it is a block. Outside that block, you barely need to reckon with it.
04 The dry season, which is the season
From October to February is when the region works best. Rain drops from 58.8 to 7.4 millimetres a month and rainy days from 6.6 to 1.3. Highs move between 24.1 and 26.7 degrees and lows between 10.6 and 15.3.
Within that stretch one month stands out, and it is October. The rain has just withdrawn, from 193.7 to 58.8 millimetres in one step, but the countryside is still green and the streams still carry water. It is the only window when both things coincide.
In January and December the landscape has already dried out, but the weather is unbeatable: clear skies, humidity of 60 per cent and cool nights at 10.6 degrees. The station's absolute cold record, −1.5, gives an idea that winter does exist here, however brief.
05 And the coast, which plays another game
Jalisco also has some three hundred kilometres of Pacific coastline, and there the calendar differs in everything except one thing: the rain also concentrates in summer, and even more so.
In Puerto Vallarta the wettest month is September, with 370.0 millimetres, and between June and September some 1,198 millimetres fall, more than Guadalajara collects in a whole year. And summer highs reach 32.2 degrees against the plateau's 27.4.
The difference is altitude. Guadalajara sits at 1,566 metres and that takes about five degrees off sea level. In winter the relation reverses: the coast holds 26.7 degrees of maximum in January and the plateau drops to 24.1, with nights at 10.6.
06 How to choose the month
If the trip is inland, October and November. The rain has gone, with 58.8 and 12.3 millimetres, highs stay at 26.7 and 25.6 degrees and the countryside keeps the green of the wet season. There is no better combination on the calendar.
If the criterion is avoiding the worst, run from April and May before July. They are the months of 30.5 and 31.9 degrees with the driest, dustiest air of the year, and not a cloud to give respite at midday.
And if the trip falls in the wet season, the timetable rule solves almost everything. The storm is an afternoon affair: the morning usually dawns clear and is the reliable part of the day. What you should not do is schedule dirt roads or long excursions for after lunch between July and September.
The year's four stretches
From 7.4 to 58.8 millimetres a month over 1.3 to 6.6 days, with highs of 24.1 to 26.7 degrees and lows of 10.6 to 15.3. Clear skies, kind temperatures and the countryside still green from the rains.
Highs climb from 28.5 to 31.9 degrees while rain falls to 3.8 millimetres in April and humidity to 46 per cent. The station's absolute record, 41.0 degrees, is from April.
183.8 millimetres over 15.4 days and the high dropping from 31.9 to 30.1 degrees. The month when rain enters and cuts the heat: in three weeks the landscape goes from ochre to green.
From 193.7 to 271.3 millimetres a month over 15.9 to 21.1 days, with humidity between 71 and 72 per cent. Highs stay at 26.7 to 27.4 degrees: it is cooler, but it rains two days out of three.
