The year in Xi'an, month by month
All the figures are Xi'an's, at 445 metres of altitude in the Wei river valley: 672.4 millimetres of rain a year over 96.7 days and 1,870.9 hours of sunshine. The scoring crosses three things: how much water falls, how much light there is and how warm it is.
Almost any guide to Xi'an recommends the same thing: spring or autumn. It is sensible advice for half of China and here it is only half right, because this city's autumn is not what the word suggests.
September receives 117.2 millimetres of rain over 12.3 days, and both are the year's highest figures. It rains more, and on more days, than in July, which has 103.8 millimetres over 10.8 days, and than in August, with 96.7 over 10.4. At the same time sunshine falls to 135.2 hours and humidity rises to 80 %.
01 One hundred and seventeen millimetres in September
It is worth seeing the whole list, because the shape of the year is not what you expect. Xi'an's wettest months are, in order: September with 117.2 millimetres, July with 103.8, August with 96.7, June with 85.6, May with 68.0 and October with 66.6.
In other words: the wet season runs from June to October and the peak is at the end, not in the middle. Across those five months 469.9 millimetres fall, almost 70 % of the annual total, which is 672.4 millimetres over 96.7 days.
Winter, by contrast, is extremely dry. December receives 6.5 millimetres over 3.5 days and January 8.8 over 4.0. Those are by far the year's lowest figures, and they help nobody, because winter's problem here is a different one.
02 The golden autumn that is not quite
The second column to look at is light, and it is the one that dismantles the autumn advice. Sunshine hits its annual low in October, at 119.8 hours, followed by January with 120.2, November with 120.6 and February with 121.4. September is not much better, at 135.2.
Compared with July, which gives 209.3 hours, or June, with 201.2, an October of 119.8 hours is practically half the light. Add to that humidity of 80 % in September and October, the year's highest.
The practical consequence is that September and October combine the worst of two columns: the year's most frequent rain and its scarcest light. November, on the other hand, does work: 27.9 millimetres over 6.1 days, though with highs of 12.6 degrees and only 120.6 hours of sunshine.
03 April and May are the real window
The other half of the usual advice does hold, and with room to spare. April receives 49.6 millimetres over 7.8 days, with 185.1 hours of sunshine and average highs of 22.1 degrees. It is the year's best crossing of the three variables.
May rains somewhat more, 68.0 millimetres over 9.8 days, but compensates with 200.1 hours of sunshine and highs of 26.9 degrees. March is the alternative if you want fewer people: 28.4 millimetres over 7.2 days, though average highs drop to 15.5 degrees and nights stay at 3.9.
And there is an altitude factor worth bearing in mind if the plan includes the range. Xi'an sits at 445 metres, but the Qinling reaches 3,771 at Mount Taibai. What is spring in the city may still be winter up there.
The year's four stretches
From 6.5 to 13.3 millimetres a month over three or four days: by far the driest stretch of the year. But average lows sit between −4.4 and −1.1 degrees, there are 11.2 snow days across the three months and sunshine never exceeds 121.7 hours.
28.4, 49.6 and 68.0 millimetres, with sunshine climbing from 160.2 to 200.1 hours and highs from 15.5 to 26.9 degrees. The only stretch of the year that combines high light with moderate rain and comfortable temperature.
85.6, 103.8 and 96.7 millimetres over nine to eleven days. Highs of 30.7 to 32.8 degrees, with an absolute record of 42.0, and humidity rising from 63 % to 76 %. Walking open-air sites with those figures is hard.
117.2, 66.6 and 27.9 millimetres over 12.3, 11.0 and 6.1 days. Sunshine hits its annual low in October, at 119.8 hours, and humidity stays at 80 % for two months. November is the only one of the three that gets away with it.
04 A record of forty-two degrees
Xi'an's summer has a bad reputation and has earned it. July gives average highs of 32.8 degrees and August 30.7, with absolute records of 42.0 and 41.7 degrees. The Wei valley is closed by mountains to the south and plateau to the north, and the hot air stays put.
Add humidity, which goes from 63 % in June to 70 % in July and 76 % in August. And the rain, which in those months arrives as afternoon storms: 103.8 and 96.7 millimetres over 10.8 and 10.4 days.
Summer's advantage is light: July accumulates 209.3 hours of sunshine, the year's maximum. If you have to travel in those months, the obvious strategy is the same as in any hot city, invert the day, plus one alternative peculiar to here: go up into the Qinling.
05 And winter drops below zero
Winter is the year's dry stretch, but that does not make it a good season. January gives average highs of 5.0 degrees and lows of −4.4, with a record of −17.5. December and February stay at 6.6 and 9.3 for highs.
There are also 4.7 snow days in January, 3.7 in February and 2.8 in December. It is not snowfall that blocks the city, but it is a sustained continental cold with very little light: between 120.2 and 121.7 hours a month across the three.
And if the plan includes the province's north, expect more. On the Loess Plateau winter is harsher than in the Wei valley, and the landscape, on the other hand, reads better: with no vegetation, the shape of the gullies is laid bare.
06 How to choose the month
If you can go only once, go in April: 49.6 millimetres over 7.8 days, 185.1 hours of sunshine and average highs of 22.1 degrees. It is the month when all three variables coincide at their best and the heat has not yet arrived.
May works almost as well and has more light, 200.1 hours, in exchange for 68.0 millimetres of rain. If you prefer cold to water, November is a reasonable option, with 27.9 millimetres over 6.1 days and highs of 12.6 degrees.
And if your only window is September, go knowing what it is: the month with the most water and the most rainy days of the year, with 135.2 hours of sunshine and 80 % humidity. Save the covered visits — the Terracotta Army pits, the museums — for the worst days, and leave the open-air routes for the clear ones.
