Summer heat haze
When ground temperatures rise above 25 °C and dew-point spreads sit under 10 °C, Paris produces a shallow boundary-layer haze that extends up to about 1,500 metres. The summit of the Eiffel Tower is well below the top of this layer and the view suffers accordingly. On such days visibility drops from a normal 30 km to about 15 km, and the colour of the sky softens from a saturated blue to a pale, milky blue-grey.
Heat haze is self-limiting. A thunderstorm or a frontal passage breaks it. After a thundery evening the following morning often produces one of the clearest views of the summer.
Saharan dust episodes
Three to six times a year, a plume of mineral dust lifted from the Sahara drifts over the Île-de-France. The dust layer is typically between 1,000 and 5,000 metres and carries fine particles with a diameter of 1 to 10 microns. The visible effect at summit level is unmistakable — the sky takes on an orange tint, visibility can collapse to 8 kilometres, and the sun becomes a disc.
The Copernicus Atmosphere Monitoring Service publishes a free three-day forecast of dust transport. Dust forecasts have improved considerably in the last decade and are, in our experience, correct about four days in five.
Winter pollution episodes
In cold anticyclonic weather, especially in late January and early February, a thermal inversion can trap wood-stove smoke and traffic exhaust under a stable boundary layer. On such days the Paris agglomeration is sometimes placed under circulation différenciée (restricted traffic). From the summit the view is usually clear above the inversion — one can see Montmartre and the Palais de Chaillot sharply — and poor below it, with a grey band of smog obscuring the lower rooftops.
Fog
Fog on the Champ-de-Mars is rare — maybe ten mornings a year. When it does happen, usually in October or November under a damp high pressure, the summit is sometimes above the fog layer, which gives one of the most memorable views one can have of Paris: a cotton sea with Montparnasse and Montmartre poking out of it. The window is short, often less than two hours, and we try to flag it on the forecast page as soon as the Montsouris 06:00 reading suggests it.
Air-quality note
None of the aerosol events listed on this page is a direct health concern for a short visit to the summit. For sensitive readers (children, elderly, respiratory conditions) the City of Paris publishes a daily air-quality index at AirParif; on days where the index is above the “degraded” threshold we prefer not to publish a visit recommendation at all.