Air Quality in Rancho Cucamonga, California — Live

Live readings from PurpleAir sensors across Rancho Cucamonga — with free alerts the moment the air turns.

Live Rancho Cucamonga AQI from nearby PurpleAir sensors loads when this page opens in a browser, along with a live sensor map covering Rancho Cucamonga and Upland, Ontario, Fontana, Claremont.

Rancho Cucamonga is built up the alluvial fan at the base of the San Gabriel Mountains, sitting around 1,768 feet, several hundred feet higher than the valley floor towns below it. That position is the whole story of its air. The mountain wall is the city's northern boundary rather than a distant backdrop, which means Rancho Cucamonga gets the fires that start above it and the winds that pour down out of the canyons, and it sits right where the basin's afternoon pollution runs out of flat ground and starts climbing.

Canairy watches the sensors around your home, your kids' school, and everywhere else you care about, and tells you in plain English when the air changes enough to matter.

Fire here comes downhill, and it does not wait for fire season

On 21 October 2003 the Grand Prix Fire started in the foothills near the Rancho Cucamonga and Fontana border. Fanned by Santa Ana winds it grew past 12,000 acres within days, and after nightfall it crept south out of the foothills, jumped fire breaks and burned homes in the Alta Loma neighbourhood. It ended at 69,894 acres and 194 houses. Together with the Old and Padua fires that same late October, the complex burned 125,000 acres, forced about 100,000 people out of their homes, destroyed 787 properties and damaged 3,860 more, and cost over 1.276 billion dollars, according to the US Forest Service accounting summarised by Headwaters Economics.

The other date locals remember is 30 April 2014, when the Etiwanda Fire started around 8 in the morning in the national forest just north of the city, driven by Santa Ana winds in what should have been spring. It burned roughly 2,190 acres before it was contained on 5 May. Between 1,500 and 1,650 homes were evacuated along with a growing list of schools, and three firefighters were hurt. April is the point. On this side of the valley a wind event and dry brush are enough on their own, and the calendar does not have much to do with it.

Living where the basin's air starts climbing

Researchers studying ozone in the San Gabriel Mountains describe the mechanism plainly: daytime heating on the slopes drives polluted air up out of the Los Angeles Basin and lifts it into the free troposphere, a process they call the mountain chimney effect. The timing shows it clearly. At a monitoring station high in the same range, surface ozone peaks in the early evening and can hang around into the following day, roughly four hours after ozone has already peaked in central Los Angeles.

That is the air moving past Rancho Cucamonga on its way up. In practical terms, the worst part of the day here tends to be later than people expect, and a hot, still late afternoon in July is worth more caution than the same temperature at noon. It also means the sharp mountain view you get on a clear morning is not much of a guide to what the air will be doing at six in the evening.

The wind that arrives from the wrong direction

Santa Ana winds are katabatic, meaning they flow downhill, pushed by high pressure over the Great Basin and the upper Mojave. As the air drops it warms by around 5 degrees Fahrenheit for every thousand feet it descends and its relative humidity often falls below 10 percent, with speeds regularly over 40 miles an hour. Rancho Cucamonga is directly below the canyons those winds come down, which is why the city's own emergency pages are largely about reporting wind damage.

Those events scrub the smog out of the basin for a day or two, so the readings can look excellent. What they add instead is dust lifted off open ground and, if anything is burning, smoke moving very quickly. Both the Grand Prix and Etiwanda fires ran on exactly this wind. It is worth checking the number rather than assuming a windy day is a clean one.

Rancho Cucamonga air quality FAQ

Why is there smoke in Rancho Cucamonga today?

The first thing to rule out is a fire in the foothills or the national forest directly north, because that is where this city's smoke has historically come from and it arrives fast. The Grand Prix Fire in October 2003 started at the Rancho Cucamonga and Fontana border and burned into Alta Loma; the Etiwanda Fire in April 2014 started in the forest just above town. If it is windy, hot and dry, take a local fire seriously. If instead the haze is even across the whole sky on a still, hot afternoon with no plume and no smell, that is more likely the regional smog layer, which tends to be at its worst here later in the day than most people assume.

What is the air quality in Rancho Cucamonga right now?

This page shows the current AQI for Rancho Cucamonga, calculated from PurpleAir sensors across the city and updated continuously. The map below shows every sensor behind the number. The city climbs several hundred feet from its southern edge to the foothills, so a reading in Alta Loma or Etiwanda near the mountains can differ from one down by the 10, and it is worth picking the sensor closest to where you actually are.

Is the air better up against the mountains?

Not automatically. Being higher up gets you away from the freeway and warehouse traffic on the valley floor, which helps with fine particles. But the foothills are exactly where the basin's polluted air is being pushed upslope through the afternoon, and they are also the first thing a foothill fire reaches. The honest answer is that it is different, not simply better, and the only way to know on a given day is to look at a sensor near your own house.

When is the air usually worst here?

Two separate patterns. Through summer the problem is ozone on hot, still days, and it peaks later in the day than people expect, because the mountain slopes keep drawing basin air upward into the evening. Through autumn the problem is fire and wind, when a Santa Ana event can put heavy smoke over the city within an hour of an ignition in the foothills. Winter mornings after a rain are generally the cleanest air the city gets.

What AQI level is unhealthy?

Under the US EPA scale, AQI 0–50 is Good and 51–100 is Moderate. From 101 the air is Unhealthy for Sensitive Groups — kids, older adults, and people with asthma or heart conditions — and above 150 it's Unhealthy for everyone. Canairy translates these numbers into a plain-English verdict, like whether to close the windows or move a workout indoors.

Is Canairy free?

Yes. The Canairy app is free to download and includes live air quality for all your saved places, plain-English verdicts, and free push alerts when the air crosses your threshold. A paid plan adds extras like SMS text alerts.

Get Canairy free on Google Play.