🌫️ Haze & the night sky

What the smoke is doing to Singapore's air right now — and what it costs the view from Jurong. Science Centre Observatory.
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Right now, over Jurong

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Today so far — is it clearing or closing in?

The 24-hour PSI is a rolling 24-hour average, so it turns slowly and lags the real air by many hours. The 1-hour PM2.5 is what the sky looks like this hour. When the two disagree, the PM2.5 is telling you where the PSI is heading.

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Where the smoke is — and whether it is coming here

A ground sensor in Jurong can only tell you what has already arrived. Whether tonight gets better or worse depends on two things it cannot see: where the fires are burning, and which way the wind aloft is carrying their smoke.

Satellite view of Southeast Asia showing smoke and active fires
Imagery: the NEA / MSS layer is Singapore's own haze satellite product from the Meteorological Service Singapore; the other four are NASA Worldview / GIBS. Transport wind: Open-Meteo. For hotspot counts by country and the regional haze map, see the ASEAN Specialised Meteorological Centre ↗ (operated by MSS Singapore) and MSS's haze information page ↗ — both are interactive map viewers, so they are linked rather than embedded.

What it costs tonight's sky

Estimated, not measured. The model converts the PM2.5 concentration into an aerosol optical depth (allowing for how smoke particles swell in Singapore's humidity), then applies the loss along the actual path length to each altitude, and separately adds the sky brightening as the haze scatters Jurong's streetlights back down. Good to a few tenths of a magnitude on a normal night; treat it as guidance, not photometry.

The next few nights — averaged across the 7–11 pm observing window

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Why haze hurts astronomy twice over

1 · It dims the stars

Smoke particles absorb and scatter starlight out of the beam before it reaches the eyepiece. Crucially this scales with how much air you look through — an object 20° above the horizon is seen through roughly three times the atmosphere of one overhead, so it loses three times as much. This is why haze finishes off a low Sagittarius globular long before it touches something at the zenith, and why "wait for it to rise higher" is even better advice on a hazy night than a clear one.

2 · It brightens the sky

Every particle that scatters starlight away also scatters Jurong's streetlights back down at you. Under a Bortle 8–9 sky there is a great deal of light to scatter, so the background gets brighter at the same time as the stars get fainter — the floor rises while the ceiling drops. On a bad night this is the larger of the two effects, and it is why a hazy Singapore sky looks orange rather than merely dim.

The practical rule for the deck: haze is a contrast problem, not a brightness problem. Anything that beats the sky by a wide margin — the Moon, Jupiter, Saturn, Venus, bright double stars, tight clusters — sails through almost untouched, though it will soften at high power. Anything faint and spread out — nebulae, galaxies, the Milky Way — loses first and loses most. On a hazy Friday, run the session on the Solar System and the doubles and save the faint fuzzies for a clear week.

Does the Seestar's dual-band filter help? Partly. The Hα/OIII filter rejects most of the added sky glow, so an emission nebula still stacks usefully in haze that would defeat the eye. But no filter can recover light that was scattered out of the beam on the way in — the target itself is genuinely fainter, so exposures need to be longer for the same result. Broadband targets (galaxies, reflection nebulae) get no help at all.

Does haze affect seeing? Not directly. Seeing is about turbulence — the steadiness of the air — and a smoke layer is not necessarily turbulent. In practice hazy Singapore evenings are often calm, so planetary detail can hold up well even as the deep sky disappears. That is another reason a hazy night is a Solar System night.

The bands, and what they mean for a public session

A Friday night at SCOB puts visitors of every age on an open deck for a couple of hours. That is a health question as much as an observing one, and it is keyed to the 24-hour PSI, because that is the index NEA's official advisories use — the 1-hour PM2.5 bands are published for information only and carry no advisory of their own.

Health advice is the National Environment Agency's; the session guidance is SCOB's own reading of it and is not a substitute for a duty officer's judgement or any Science Centre policy. Check haze.gov.sg for the official advisory and forecast on the night.
Live readings: 24-hour PSI and 1-hour PM2.5 from the National Environment Agency via data.gov.sg, refreshed on load and every 15 minutes. Forecast: Open-Meteo air quality (global model — coarse resolution, use it for the trend across nights, not as a nowcast). Official advisories and the 24-hour PSI forecast: haze.gov.sg. Airmass after Kasten & Young (1989); the aerosol extinction and sky-glow figures are SCOB's own estimate and are described in haze-core.js.
Companion pages: where did all the stars go? · plan ahead — best Fridays · tonight's dashboard.
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