Matterhorn pandas: Matterhorn summit window from 50 years of historical weather + pandas.

Matterhorn (credit<br>Caramelized Onions)

1. Background #

I have been thinking about Matterhorn for a long, long time and this may be the year when it happens.<br>Wish me luck!

There are more than a dozen options to climb Matterhorn, and<br>the "normal routes" on the Italian and Swiss side are probably the easiest.<br>Of these, the Hörnli Ridge<br>on the Swiss side (although crowded) has the simplest logistics and easy access from the resort village of<br>Zermatt.<br>This!

The totally awesome Hörnli Hut sits at its base.

Matterhorn is one of the highest peaks in Europe at 4,478m (14,692ft).<br>Reduced oxygen at high altitude is totally a thing and<br>acclimatization<br>seems like a good idea.

Weather is the main variable: you don't want to be caught in a storm on your way down.<br>Rain makes the rock slippery and snow , especially fresh snow, makes the climb a lot more complicated.

2. Plan #

Go solo (some climbers use a guide) and<br>spend 5 days (4 nights) in Switzerland, with a 3-day summit window:

Day 2: second summit day backup, if both day 3 and 4 look bad. Least desirable: will skip most of the acclimatization.

Day 3: preferred summit day.

Day 4: backup, in case of bad weather on day 3.

Unless you plan to bum around Europe for a couple of weeks (I don't :)<br>waiting for a window of good weather and last minute availability at the hut, you need to commit<br>to flights, book the hut ahead of time and hope for the best.

That leaves two questions:

When to go?

What to expect in terms of weather?

3. Conventional wisdom #

The Hörnli Hut is open between the end of June and mid-September each year. This is a clue.

The optimal summit window is probably mid-July to mid-September: snow before and after that, with likely<br>variations between years.<br>I watched many hours of Matterhorn climbing videos on YouTube:

July: lots of snow.

Early September: sometimes almost snow free.

The summit temperature rarely goes above 0°C.

The recommended summit ascent starts early: 4:30AM ("alpine start"), aiming for a 4-5h ascent<br>and similar descent: be back at the hut by 2PM, beat afternoon showers<br>(convective storms) and make it to the<br>last gondola download from Schwarzsee (between 4:30-5PM).<br>Guides are reportedly very strict with their clients about a 9AM summit cut-off<br>and moving fast throughout.

It would be nice to get historical weather averages for snow , temperature , precipitation<br>(incl. afternoon showers) and pick a good 3-day summit window between July and September.

4. Weather #

The first thing I found is the MeteoSwiss weather forecast.

I have been comparing it (cloud, precipitation) with actual weather conditions on Matterhorn from<br>the Hörnli Hut webcam.<br>It seems pretty accurate.<br>The 8-day forecast<br>(Error band for temperature. Nice!)<br>should be good enough to decide on the best summit day once I'm in Switzerland.

Looking for historical weather, I found Open Meteo.<br>Historical Weather API. JSON. Open Source ❤️. Awesome!

Download 50 years of historical weather for Matterhorn (click)<br># Craft your own query that looks like<br># this (438,312 data points; 19MB).<br># Be considerate of the OpenMeteo API.<br># Do not overload it. Please? Thanks!<br>curl -s "https://archive-api.open-meteo.com/v1/archive?"\<br>"latitude=45.9764&longitude=7.6586&"\<br>"elevation=4478&timezone=Europe/Zurich&"\<br>"start_date=1976-01-01&end_date=2025-12-31&"\<br>"hourly=temperature_2m,precipitation,cloud_cover,cloud_cover_low,snowfall,snow_depth"\<br>> matterhorn-1976-2025.json"

The only missing ingredient was pandas,<br>an excellent Python library for<br>data manipulation and analysis: think Excel + major steroids (minus Copilot and proprietary bullshit).

Matterhorn + panda (credits<br>Caramelized Onions<br>and Sheilalau)

I whipped up a simple python analysis script summi using pandas and matplotlib<br>and dropped it, of course, on CodeBerg.

Usage instructions (click)<br>$ ./summi --help<br>usage: summi [--help] [-b ROLLING_BEST | -m ROLLING_MEAN] [-p MIN_PRECIPITATION] [-h MIN_HOUR] [-H MAX_HOUR] [-y MIN_YEAR] [-Y MAX_YEAR] [-d MIN_MONTH_DAY]<br>[-D MAX_MONTH_DAY] [-o [HOURLY ...]] [-l LEGEND_LOC] [-t TITLE] [-u SUBTITLE]<br>{c,cloud,p,prec,precipitation,s,snow,t,temp,temperature}

Display location-specific weather conditions and charts. Reads input as a JSON file downloaded from Open-meteo.<br>* URL path: "https://archive-api.open-meteo.com/v1/archive"<br>* URL parameters:<br>- latitude, longitude, elevation<br>- start_date, end_date (YYYY-MM-DD)<br>- timezone (timezone name from tz database)<br>- hourly=temperature_2m,precipitation,cloud_cover,cloud_cover_low,snowfall,snow_depth<br>* Docs: https://open-meteo.com/en/docs/historical-forecast-api

positional arguments:<br>{c,cloud,p,prec,precipitation,s,snow,t,temp,temperature}<br>Graph to diplay.

options:<br>--help show this help message and exit<br>-b ROLLING_BEST, --rolling-best ROLLING_BEST<br>Calculate rolling "optimistic" (min/max) aggregates over a number of days. Must be between 2 and 30.<br>-m...