Fer & Vent Climb forecast · Paris
About · methodology

How the site is built

This page is the mirror of our forecast methodology, written for a wider audience. If you have landed here from a search engine, this is where to start.

Sources

We rely entirely on publicly available sources:

  • Météo-France. The national meteorological service of France publishes free bulletins for the Paris region twice a day. These are our primary source for the next twenty-four hours.
  • Observatoire de Paris–Montsouris. The 14th-arrondissement station has been reading pressure and temperature continuously since 1872. Its data is a reliable check on the shorter-horizon Météo-France bulletin.
  • SETE anemometer. When published, the summit anemometer reading is our best check on the Météo-France wind forecast.
  • Copernicus Atmosphere Monitoring Service. For Saharan dust and ozone episodes, the Copernicus three-day forecast is free and reliable.
  • AirParif. The regional air-quality agency's daily index is our check on pollution episodes.

Reasoning

Our daily note is a reading of these sources, not a model. We weight them by proximity to the tower and by stability:

  • Visibility — 40 % Météo-France, 40 % Montsouris observations, 20 % SETE.
  • Wind — 60 % Météo-France, 20 % SETE, 20 % Montsouris adjusted for altitude.
  • Light — strictly geometric from the IMCCE almanac for Paris.

Where the method falls short

  • Rapid-onset Saharan dust episodes can escape the morning bulletin. We update at 13:00 and 19:00 precisely to catch these.
  • Cold-front timing is accurate to about one hour on the three-hour horizon. A front that was due at 14:00 and arrives at 15:30 can turn a good afternoon into a middling one.
  • Convective showers in summer are impossible to predict at the hour level. We flag "possible shower" but we cannot say when.

Correction policy

When we are wrong we publish a short correction note on the top of the daily forecast page the next day. Corrections stay there for seventy-two hours. We think this is the simplest form of transparency a hobbyist site can offer.

The source code

The site is a flat set of HTML pages. There is no CMS, no database. Updates are made by hand through a terminal, which is why there are only three updates a day — not because we lack opinions, but because we lack time.