Weather in Semarang in September

A hard month. September asks something of you. Go in knowing what. Daytime highs average 35.4 °C and the afternoon is not negotiable — outdoor plans move to early morning or evening.

September ranks 8 of 12 in Semarang: past the worst of the year but short of July, which is the month to book if the dates are open.

Rain on 7 days, 3 of them heavy. Enough to need a plan B, not enough to need a plan around it. Pack for heat management rather than style: shade, water, and a schedule that gives up on the middle of the day.

What the averages hide

Averages flatten a month, and this one hides a few things: 20 days of the month clear 35 °C — that is not a heatwave, it is the month. Across the twenty years measured, the hottest September day reached 39.1 °C and the coldest night fell to 21 °C, and the wettest single day dropped 45 mm — roughly 67% of the month's rain in one day.

This is the volatile part. September rainfall has ranged from 0 mm to 368 mm across the twenty years measured — a 368 mm spread around an average of 67 mm. The average is real but almost no individual September looks like it, so treat the figure as the middle of a wide range rather than a forecast.

Wind is light at 8.3 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 62%.

How September sits in the year

Temperatures hold steady either side: 33.9 °C in August, 35.4 here, 34.7 in October. Nothing about the timing within the month matters much. Rain is the thing that moves: October brings 8 days more of it, so the earlier half of a flexible trip is the safer half.

Daylight in September

Semarang sits close enough to the equator that daylight barely moves all year — 12.1 hours in September, against a range of just 0.8 hours across the whole calendar. Sunrise and sunset land at roughly the same time whenever you come.

Sea temperature

The sea sits at about 28.7 °C in September — warm enough that the water offers no relief from the heat. The sea peaks at 29.9 °C in May and bottoms out at 28.5 °C — water lags the air by a month or two, which is why the best swimming often comes after the best weather.

How busy it gets

Quiet. Few other visitors, cheaper rooms, and a real chance some seasonal operators are shut.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in Semarang versus nearby

If Semarang is one option among several, the nearby comparison is worth making. Jepara is a better bet in September — 4.7 °C cooler, 1 day more rain. Surabaya is a better bet in September — within a degree on temperature, 3 days less rain.

So: September is a compromise. July is 19 points better and is what Semarang is actually like at its best — worth moving the trip for if that is possible, and worth adjusting expectations for if it is not.

Every month in Semarang

See all twelve months in Semarang, or where else to go in September.