ed sky at dawn and dusk is part of weather lore and apparently a cause for delight or warning among shepherds and sailors. But, as huge areas of forest burn, the crimson colour of skies over the western US is a red alert for our air pollution and climate emergency. Even in New York and Washington DC the sun turned orange. This is because smoke from wood burning strongly absorbs purple and blue light. Longer wavelengths pass through and the sky takes on shades of red. This curiosity can be useful. In 2005 Swiss scientists noticed that the particle pollution from wood stoves that filled Alpine villages in winter strongly absorbed ultraviolet light, opening a way to routinely measure wood smoke in our air. Smoke from other solid fuels can strongly absorb purple and blue light too. This may explain the colours in the pea-souper smogs that enveloped London up to the 1960s. In 1855, the Times described the November sky as having the changing colours of a bad bruise. Even in 1947, Arnold Marsh wrote of red and yellow smog-filled skies. It is these tricks of the light that inspired Claude Monet’s iconic paintings of the UK Houses of Parliament.