The smokiest days in Nelson

Smoke here doesn't drift in for an afternoon. It parks. These are the sustained episodes — three days or more of genuinely bad air — ranked by how bad they got at their worst.

Averaging all nine summers smooths the individual events into one curve. It climbs steeply through the back half of July, runs high for two months, then drops off a cliff around the third week of September. Two completely separate sets of instruments — sensors in town and the government monitor down the valley — trace the same shape, which is the best sign it's real.

Take every smoky day on record and sort it by date. They don't pile up in one week — they spread almost evenly across mid-July to mid-September, roughly a quarter of them in each half-month. Outside that window they all but vanish.

One row per summer, one square per day. This chart explains why the season feels so different year to year: smoke arrives in solid blocks, and whether a summer is miserable comes down to whether one of those blocks parks over us. 2019 had none at all. 2018 and 2021 had several. Notice too that the in-town and down-valley rows darken on the same dates — when it's smoky here, it's smoky across the whole region.

One thing this data does not show is a steady worsening. The gap between summers is enormous — 2021 was four times smokier than 2019 at the same monitor — and that swamps any slow drift you might look for. Nine years simply isn't long enough to call a trend. What it is long enough to show is when the smoke comes, and that part has been remarkably consistent.

Two independent sources, both public, cross-checked against each other.

In town — the network of PurpleAir sensors around Nelson, up to five of them, going back to June 2021. These are the same readings behind the purple dots on the PurpleAir map. Their raw numbers are adjusted using the standard US EPA correction for wood smoke, which is what the map itself applies, and each day is the median across whichever sensors were running.
Down the valley — the BC government's air quality monitor at Zinio Park in Castlegar, about 30 km southwest. It's a proper regulatory instrument that's been running continuously since well before 2018, which is what makes the nine-year view possible.
They agree closely. On the 590 days where both were recording, the two track each other almost perfectly. Castlegar tends to read a bit higher — it sits lower in a wider part of the valley where smoke settles — but the timing is the same.
“Smoky” here means a daily average of 35.5 µg/m³ or more, the point where the air quality index crosses 101 and health advisories start applying to kids, seniors and anyone with asthma or heart trouble. A clear day in Nelson is around 4–6.

One gap worth knowing about. Nelson hasn't had its own government air monitor since the Kutenai Place station shut down in April 2015. For a town that spends a good chunk of every summer under smoke, the only local measurements we have now come from sensors people bought and put up themselves.

The in-town record has gaps — 43 usable days in summer 2021 and 87 in 2023, against a full 122 in 2022, 2024 and 2025 — because sensors go offline. One of the original sensors also started reading far too low in late 2025 and has since failed; it's left out from that point on. The Castlegar record is essentially complete for all nine summers. 2026 runs through August 7. Charts cover June 1 to September 30 only.

The smokiest days in Nelson

Six smoke events that defined the last nine summers

It switches on in mid-July and off in mid-September

Average daily PM2.5 by calendar day

Four half-months hold nearly all the smoke

Share of all smoky days (AQI 101+) falling in each half-month

Whether a summer is bad comes down to luck

Every day, coloured by air quality

How each summer compared

Where these numbers come from

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