Shillong — Seasonal Pollution Patterns

February is the most polluted month in Shillong on average, with a long-run AQI of 89 — firmly in the Satisfactory band. This is drawn from 2 CPCB monitoring stations across 6 years of daily readings. Through February, residents should expect elevated PM2.5 and PM10, reduced visibility on cooler mornings, and strong recommendations from doctors to limit outdoor exertion, wear well-fitted N95 masks, and run indoor purifiers through evening and overnight hours when pollutant accumulation typically peaks.

August is the cleanest month of the year in Shillong, averaging AQI 22 in the Good band. The months immediately before and after also tend to sit well below the annual mean of 44, so a visit window centred on August is the safest choice for outdoor sightseeing, marathons, school trips and wedding events. Mornings are usually the crispest time to head out; pollution tends to creep up slightly during the evening commute even in the cleanest months. Always cross-check the day-of live AQI before any high-exertion outdoor plan.

Shillong shows a clear monsoon-cleansed signature — rain and deeper atmospheric mixing drop AQI to a seasonal trough, and everything else relative to that trough looks elevated. The specific February spike combines pre-monsoon dust, post-rain rebounds and the arrival of cool-season trapping effects.

Across the CPCB record, the week around Diwali in Shillong averages AQI 24 — 0.85× the normal October baseline of AQI 28, a shift of 4 AQI points. Firework particulates combine with a cooler, more stagnant late-October atmosphere to produce some of the worst air-quality days of the entire year. Sensitive groups should treat Diwali eve and the two days after as peak-alert days: stay indoors, close windows by evening, run purifiers on high, and reserve any outdoor celebrations for daytime hours when mixing is strongest.

Yes — meaningfully. Shillong's core monsoon window (Jul 15 – Sep 15) averages AQI 23, a 48.9% improvement on the annual mean of 45. Rain removes airborne particulates by wet deposition, and the deeper monsoon boundary layer disperses what remains vertically. Across 337 measured monsoon days we see 100% in the Good-or-Satisfactory band.

Between 2019 and 2024, Shillong's annual average AQI moved from 34 to 75 — a change of +120.6%. In the most recent year-on-year comparison, the winter season specifically worsened by 77.8%. The long-run direction is worsening — rapid urbanisation and emissions growth appear to be outpacing efficiency gains.

August is the single best month at AQI 22. Based on the 12-month averages, the three cleanest months in Shillong are August (AQI 22), September (AQI 24), July (AQI 25). These are the safest choices for outdoor itineraries, long walks, open-air concerts and day-trips. Sensitive groups can treat these months as near-normal activity windows but should still check live AQI for the specific date. Avoid planning outdoor-heavy trips in February, when the baseline jumps into Satisfactory territory.

Shillong is classified as monsoon-cleansed. Based on a 12-month cosine-similarity index computed across all monitored Indian cities, the city whose seasonal signature most closely resembles Shillong's is Aizawl (Mizoram), with its own worst month in March. Cities with similar signatures often respond to similar policy levers — if a neighbouring peer has demonstrated improvements through specific interventions (construction-dust controls, bus electrification, brick-kiln regulation), they are likely candidates for Shillong too. The peer-city panel on this page lists the closest four additional matches.