Residents across Jakarta, Depok, and Bekasi have felt an unexpected chill lately, with temperatures dipping in the early mornings and at night — right as the dry season begins. What’s behind this cool twist in the tropics? The Meteorology, Climatology, and Geophysics Agency (BMKG) points to several causes.
A key factor is the arrival of the Australian Monsoon winds. These winds blow dry, cold air from Australia toward Asia, passing over Indonesian territory and the cooler surface of the Indian Ocean. With little water vapor to hold heat, nights feel especially cold.
“Over the past few days, clear skies have dominated almost all of Java, Bali, Nusa Tenggara, southern Sumatra, southern Kalimantan, and southern Sulawesi,” said BMKG Deputy for Meteorology Guswanto, in a statement quoted from BMKG’s website on Tuesday (1/7/2025).
These dry winds make it harder for clouds to form. Clear skies by day look beautiful — but by night, they let heat escape straight from the ground into the atmosphere. This “radiational cooling” leads to much colder nights.
BMKG explains this is why temperatures drop sharply, especially in areas south of the equator. Java, Bali, and Nusa Tenggara are most affected.
Read also: Towards a Global City, Jakarta Develops New Collaborative Investment Fund
The cold isn’t just felt in cities. Highland regions on Java, like the Bromo Highlands, Sindoro-Sumbing, and Lembang in Bandung, see even lower temperatures. On July 7, 2024, the Dieng Plateau recorded a nighttime low of just 1°C at around 2 a.m.
Besides the Australian Monsoon, Guswanto highlighted other factors: geographic location, topography, altitude, and dry air humidity all contribute. “Highland or mountainous areas tend to be even colder because of lower air pressure and lower humidity,” he added.
From June to August, the sun’s rays also hit Indonesia from a lower angle, especially in areas south of the equator. This seasonal change further reduces daytime heat and deepens the nighttime chill.
At night, another subtle effect comes into play: calm winds slow the mixing of warmer and colder air. As a result, colder air stays trapped near the surface, making mornings and evenings feel even colder.
Even as the dry season usually means sunny days, this combination of natural forces is why Jakarta and nearby cities wake to such crisp mornings. And while it might surprise some, BMKG reminds residents this is part of the annual weather cycle.
So if your mornings feel colder than you remember, now you know why. It’s nature’s way of reminding us that even the tropics can catch us off guard.
