Mountain Weather Data Sources & Methods

Weather data

Forecasts come from MET Norway Locationforecast 2.0 (Norwegian Meteorological Institute), licensed CC BY 4.0. For each mountain we request the forecast at the summit coordinates with the summit altitude parameter, so temperature is adjusted to that elevation. Data is fetched automatically every 3 hours, following MET Norway's caching rules (Expires/If-Modified-Since headers).

Limitations: for Indonesia, MET Norway uses a global model (ECMWF). Wind and rain represent the model grid cell, not the exact summit point; wind on an exposed summit can be stronger. Wind gust data is not available from the global model, so it is not shown. Forecasts several days ahead are available in 6-hour steps and are therefore less detailed.

Feels-like temperature

We calculate feels-like temperature with the wind chill formula (Environment Canada/NWS) using the elevation-adjusted temperature and model wind speed, for temperatures ≤ 10 °C and wind > 4.8 km/h. Outside those conditions, feels-like equals air temperature.

Hiking condition rating

Each day is rated with the following simple rules, based on daily values at the summit:

Poor if any of: maximum wind ≥ 50 km/h, total rain ≥ 20 mm, a thunderstorm symbol, or lowest feels-like temperature ≤ −10 °C.
Caution if any of: maximum wind 30–49 km/h, rain 5–19.9 mm, or lowest feels-like temperature ≤ 0 °C.
Good if none of the criteria above apply.

This rating is about model weather only. It does not take into account volcano status, trail closures, hikers' fitness or technical difficulty.

Ranking score (weekend & regions)

To rank mountains we use the score: rain (mm) + 0.5 × max(0, max wind − 15) + 25 if there is a thunderstorm signal + 0.05 × average cloud cover (%) + 0.3 × max(0, −lowest feels-like temperature). A lower score means friendlier weather.

Sunrise & sunset

Calculated with a standard astronomical algorithm (horizon −0.833°) for the summit coordinates. From the summit, the sun may be visible a few minutes earlier. Clear-sunrise chances use the model's total cloud cover at the hour closest to sunrise.

Mountain data

Names, elevations and coordinates of the 113 mountains come from Wikidata (CC0) and OpenStreetMap summit points (© OpenStreetMap contributors, ODbL). The province is derived from the summit coordinates via the Nominatim geocoder, so mountains on a border may be listed in only one province. The sources for each mountain are listed on its page.

Elevation classes (A < 1,500, B 1,500–2,499, C 2,500–3,499, D ≥ 3,500 m) are based purely on summit elevation and are not trail difficulty. We only show difficulty when there is a verified source; trail data comes from OpenStreetMap (see below); otherwise it says "no verified data yet".

Elevation & coordinate verification

For each mountain we compare the elevation from Wikidata, Indonesian and English Wikipedia, the OpenStreetMap ele tag, and the highest Copernicus GLO-30 elevation model value around the summit. If sources differ by more than 20 m, we choose the value supported by at least two independent sources and consistent with the DEM (a 30 m DEM tends to be slightly lower than a sharp summit, but the real summit cannot be far below the DEM). Alternative values are shown on the mountain page. If the source coordinates turn out to point to a crater or another spot, we move the point to the highest DEM cell within 3 km and note it on the mountain page.

Hiking routes (estimated from OSM data)

Route cards are built from OpenStreetMap route=hiking relations with an end within 500 m of the summit. The geometry is joined in relation member order, densified every 25 m, then given elevations from Copernicus GLO-30 (smoothed with a 5-point moving average). Statistics are calculated from the lower end of the route to its highest point.

Tobler: speed = 6 × e^{−3.5 × |gradient + 0.05|} km/h, calculated per ± 100 m segment (gradient capped at ±100%).
Naismith: 5 km/h + 1 hour per 600 m of ascent.

Both are pure moving time without breaks, without a heavy pack and without overnight stays, so real times are usually much longer. The OSM route start is not necessarily the official basecamp or registration post. The trailhead forecast is requested at the coordinates and DEM elevation of that point. The derived dataset is available at /gunung/data/trails.json under the ODbL 1.0 licence.

Hidden small mountains

The hidden small mountains page lists mountains we added by written rules: summit below 2,000 m; a hiking route or trailhead mapped in OpenStreetMap; elevation cross-checked (OSM tag, Wikidata, Copernicus DEM); and "little-discussed" means no Indonesian Wikipedia article or an article averaging fewer than 200 views per month (October 2025–September 2026, Wikimedia Pageviews API). The "suitable for beginners" reasons are numbers only (elevation, distance, total ascent, time estimate), not subjective judgement.

SIMAKSI, quotas & official contacts

SIMAKSI is the Indonesian hiking/entry permit. Each mountain falls into one of three categories, each with the date it was last checked:

Live data: figures fetched automatically at most once an hour from official sites whose robots.txt allows it. If a fetch fails, we show the last successful data with its time and a "data may be out of date" warning.
Official online booking: we link to the official booking/quota system without copying its figures (for example because robots.txt disallows automated access or the site cannot be reached from our server).
No online data: no verifiable official source yet; ask the basecamp.

We never infer a trail's open/closed status ourselves: pages always point to the manager's official announcements. Contacts shown are only institutional contacts published by the manager or a government site, not personal numbers. All official links are checked automatically every month and flagged if they fail to open. Volcano status is not fetched automatically from MAGMA Indonesia because that site's robots.txt disallows it; we only link to it. We do not use data from third-party hiking aggregator sites.

Language

The English pages are translations of the Indonesian pages generated from the same data. Mountain, place and institution names, official announcement titles, and some notes copied from official sources stay in Indonesian. If a translation differs from the Indonesian page, the Indonesian page applies.

Attribution

Weather data: MET Norway, CC BY 4.0 — processed (aggregated daily, feels-like temperature and rating calculated).
Rinjani hiker counts: official website of Balai TN Gunung Rinjani (rinjaninationalpark.id).

Disclaimer

Important: the forecasts on this site are automatically processed numerical model output (MET Norway), not official BMKG warnings and not volcano status. Always check BMKG, activity status at MAGMA Indonesia (PVMBG), and trail opening/closure announcements from the national park or basecamp before you climb.

Found an error in the data? Email [email protected].

Data sources & methods