The year in Yogyakarta, month by month
The figures are for the city, at 121 metres: some 2,157 millimetres of rain a year, with a monthly mean running from 16 to 392 millimetres, and mean temperatures between 25.4 and 27.1 degrees. The scoring crosses rain, humidity and visibility of the volcano.
Tropical destinations are almost always summed up the same way: there is a dry season and a wet one, and you should go in the dry one. That is true, but it is a lazy answer, because it does not say what you specifically gain or lose.
Here what you gain and lose is very concrete, and it has a name: a 2,930-metre cone twenty-eight kilometres away that, depending on the month, you either see or do not.
01 Sixteen millimetres against three hundred and ninety-two
The difference between the driest and the wettest month is twenty-fourfold: 16 millimetres in August against 392 in January. July adds 29 and September 49, so the heart of the dry season is three months with almost no water.
At the other end, between November and March 1,569 millimetres accumulate, almost three quarters of the annual total. This is not rain spread out in light showers: it is five months of abundant, daily water.
That distribution explains the landscape you will see. In March the countryside is a green that does not look real and the rice fields are full; in September the same fields are yellow and dry, the earth cracked between harvests.
Sixteen millimetres in August, three hundred and ninety-two in January. The same place, two different countries.
02 The thermometer only moves 1.7 degrees
The mean temperature of the coolest month, July, is 25.4 degrees. That of the warmest, April, 27.1. A whole year fits inside 1.7 degrees of difference, something that at mid latitudes would be an afternoon's variation.
Highs move a little more, from 29.8 to 31.6 degrees, and lows from 20.6 to 23.0. Those two and a half degrees in the lows are, in practice, the difference between sleeping well and not: July and August have the year's coolest nights.
And what you really feel is humidity, which runs from September's 69 per cent to 82 in January, February and December. Thirty degrees at 69 per cent and thirty degrees at 82 are not the same temperature for the body, even if the thermometer says so.
03 What the month decides is visibility
Here is the variable almost no guide mentions. A cone of nearly three thousand metres twenty-eight kilometres from the city generates its own cloud, and in the wet season that cloud covers it practically all day, every day.
In the dry season that changes, but not entirely: even in August the summit is usually clear at dawn and starts clouding over by mid-morning. The good window is from five to eight, and it ends while you are having breakfast.
The same applies to the air. In September, with humidity at 69 per cent, the view is clean and long; in January, at 82 per cent, haze eats the horizon even when it is not raining. If your trip depends on looking into the distance, the month matters a great deal.
04 And in the wet season the channels run
There is a second reason to prefer the dry season, and it is specific to this place. The loose material the volcano deposits on its flanks mobilises with heavy rain and comes down the ravines as flows of mud and stones.
It happened very visibly in November 2010, when the intense rains following the eruptions generated flows that caused significant damage downstream, after the worst of the eruption had already passed.
For an ordinary trip this does not mean danger, it means logistics: in the rainy season there are cut tracks, impassable fords and excursions on the flank cancelled without notice. In August, those same channels are paths of dry sand.
05 Rain is an hour, not a season
It helps to understand how it rains here, because it changes how you plan the day. Even in the wet months, inland tropical rain tends to concentrate in the afternoon: the morning dawns clear, the heat loads the atmosphere and by mid-afternoon it discharges.
That means a trip in January is not a lost trip, it is a trip with the hours inverted: everything important happens between six and one, and the afternoon is reserved for whatever has a roof. The city, moreover, is perfectly equipped for that.
What does change irreparably is the ground. With 392 millimetres in a month, the dirt tracks of the karst south turn to mud and the caves visited for their underground rivers can close for safety when it rains hard.
06 How to choose the month
If you can choose, May to September, and within that stretch, July and August. They are the months with least water (29 and 16 millimetres), the coolest nights (lows of 20.6 degrees) and the best odds of seeing the cone clear at dawn.
September deserves a separate mention. Rain rises a little, to 49 millimetres, and heat reaches highs of 31.5 degrees, but humidity falls to 69 per cent, the year's minimum. It is the month of cleanest air and longest views.
And if your window falls in the wet season, come anyway, but with the plan inverted: get up very early, write off the outdoor afternoons and do not build the trip around a photograph of the volcano. In January, with 392 millimetres, that photograph is a matter of luck.
The year's four stretches
From 237 to 392 millimetres a month, 1,569 across the five: almost three quarters of the year. Humidity of 77 to 82 per cent and the volcano covered nearly every day.
149 millimetres, half of March's, with humidity at 78 per cent and highs of 31.5 degrees. The countryside is still green and whole afternoons are already clear.
From 16 to 141 millimetres a month, with July and August at 29 and 16. Lows of 20.6 degrees and humidity down to 69 per cent: the best air and the best visibility of the year.
136 millimetres and the year's highest highs, 31.6 degrees, with humidity rising to 73 per cent. Afternoon storms become frequent again.
