Fer & Vent Climb forecast · Paris
Visibility

What limits the view from the summit

The theoretical horizon from 276 metres above sea level is about 59 kilometres. The practical horizon on a Paris day is usually much shorter. This section explains why.

The geometric horizon

From the top observation deck of the Eiffel Tower, at 276 metres above the mean level of the Seine (and about 310 metres above sea level counting the pole), the geometric horizon over a sphere the size of Earth lies at 59.5 kilometres. In other words, from the summit a reader with perfect vision looking straight at the surface of the Earth from that height would, in a vacuum, see as far as 59.5 kilometres before the curve of the globe hid the next feature from view.

Why the practical view is usually shorter

On a real Paris day the air column between the viewer and the horizon reduces the view by three mechanisms:

  1. Molecular scattering. The light that reaches a distant tree is scattered on its way to our eye by the molecules of air itself. On a clean day this limits the view to about 300 km — which is more than the geometric horizon, so molecular scattering is rarely the binding factor.
  2. Aerosol scattering. Dust, pollen, soot, sea-salt droplets and condensation nuclei scatter light far more effectively than molecules. Aerosol is the usual binding factor on a Paris day.
  3. Low cloud and fog. When the cloud base drops below 300 metres the summit itself enters the cloud and visibility collapses to a few hundred metres. This happens on about ten mornings a year in Paris, usually in autumn and winter.

What a typical reading looks like

From our notebooks, visibility from the summit falls into roughly three regimes:

  • A clean post-frontal day in October or February: 45 – 60 km. You can see the Basilique de Saint-Denis to the north and the Fontainebleau forest edge to the south-east.
  • A quiet high-pressure summer day: 15 – 25 km. The outline of La Défense is softened; Montmartre is sharp. The horizon fades into a pale blue–grey rather than ending on a sharp line.
  • A Saharan dust episode in spring: 8 – 15 km. The sky is tinted orange, Montmartre is barely visible, and the sun looks like a disc you can look at.

Reading our visibility figure

The headline figure in our daily note is a horizontal visibility at summit level around solar noon. It does not describe the view at ground level — ground level visibility is often shorter because of road dust and traffic exhaust. On most days the summit view is a few kilometres clearer than the Champ-de-Mars view.

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