What Is Causing the Hot Weather in Java? Is It Because the Sun Is Overhead?
Hot, sunny weather has affected several areas of Java recently. On social media, users have widely linked the high temperatures to the Sun being directly above the island. So what is actually happening?
The Meteorology, Climatology and Geophysics Agency (BMKG) has explained the phenomenon. According to the agency, the Main Culmination phenomenon, associated with the Sun’s apparent daily motion, is indeed taking place over Java in mid-October.
The Main Culmination phenomenon in Java
BMKG explains on its official website that the Sun being directly overhead is known as the Main Culmination, or ‘Shadowless Day’. It occurs when the Sun reaches its highest point in the sky, or the zenith, at midday.
The phenomenon is caused by the fact that the Earth’s equatorial plane, or plane of rotation, does not coincide exactly with its ecliptic plane, or plane of revolution. As a result, as seen from Earth, the Sun’s position appears to shift throughout the year between 23.5° north and 23.5° south. This is known as the Sun’s apparent daily motion.
Nationwide, the second period of Main Culmination takes place progressively, from Sabang in Aceh on 7 September to Baa in East Nusa Tenggara on 21 October. In Java, the Sun’s apparent movement passes over the island in mid-October.
According to a BMKG release, the following major cities in Java experience Main Culmination on these dates:
Central Jakarta and Serang: 9 October 2026
Bandung and Semarang: 11 October 2026
Surabaya: 12 October 2026
Yogyakarta: 13 October 2026
The intense heat in mid-October is not solely due to the Sun’s apparent position; it is also connected to seasonal weather patterns. Java is generally still in the dry season, with very little cloud cover.
End of the dry season and start of the rainy season
BMKG notes that the peak of the dry season in most parts of Indonesia had in fact passed in August. Nevertheless, scant cloud cover and a dry season expected to last longer than usual mean that the Sun’s radiation has felt particularly intense at the Earth’s surface throughout October.
As for when the dry season will end, BMKG forecasts that the rainy season will begin later than the climatological average across most of Indonesia. Most regions are expected to enter the rainy season gradually between November and December.
The timing varies by area. The rainy season is forecast to begin in November in a small number of areas in Java and Sumatra, before spreading across most of Java, Bali and Nusa Tenggara in December. One factor influencing these conditions is a global climate indicator: El Niño is in a Very Strong phase.