CHMI Forecast API
Weather forecasts from the Czech Hydrometeorological Institute
Data Sources
The Czech Hydrometeorological Institute (CHMI) runs the ALADIN limited-area weather model and publishes forecasts as open data. Both domains provide hourly forecasts for 3 days and are updated every 6 hours. The Central Europe domain additionally includes atmospheric data on 17 pressure levels. The CHMI Aladin Seamless model automatically uses the 1 km Czech Republic domain where available and falls back to the Central Europe domain otherwise. After 3 days, Open-Meteo combines forecasts with the ECMWF IFS HRES 9 km model to provide up to 15 days of forecast.
| Weather Model | Region | Spatial Resolution | Temporal Resolution | Forecast Length | Update frequency |
|---|---|---|---|---|---|
| CHMI Aladin Central Europe 2km |  | 2.3 km | Hourly | 3 days | Every 6 hours |
| CHMI Aladin CZ 1km |  | 1 km | Hourly | 3 days | Every 6 hours |
Derived Variables
ALADIN natively provides temperature, humidity, wind, pressure, cloud cover, precipitation, radiation, CAPE, visibility and sunshine duration. Several convenient API variables are not predicted directly by the model. Open-Meteo derives them from the native fields for every forecast step.
| Derived Variable | How it is derived? |
|---|---|
| Weather code | Computed from cloud cover, precipitation, snowfall, wind gusts, CAPE and visibility. ALADIN does not publish a usable WMO weather symbol. |
| Snowfall | Converted from the native snowfall water equivalent, assuming 1 mm of water equals 0.7 cm of snow. |
| Surface pressure | Calculated from mean sea-level pressure, 2 m temperature and terrain elevation. |
| Apparent temperature | Combines 2 m temperature, relative humidity, wind speed and shortwave radiation. |
| Diffuse solar radiation | Difference between the native shortwave (global) and native direct solar radiation. No radiation separation model is required. |
| Direct normal irradiance DNI and global tilted irradiance GTI | Calculated from direct and diffuse radiation using solar geometry and, for GTI, the selected panel tilt and azimuth. |
| Reference evapotranspiration ET₀ | FAO-56 Penman-Monteith equation using temperature, humidity, wind speed and solar radiation. |
| Vapour pressure deficit and wet bulb temperature | Calculated from 2 m temperature, relative humidity and dew point. |
| Terrestrial radiation and day-or-night flag | Astronomical calculations for every grid cell and timestamp. |
| Pressure-level wind speed and direction | Calculated from the native U and V wind components (Central Europe domain only). |
| Pressure-level dew point and cloud cover | Calculated from pressure-level temperature and relative humidity (Central Europe domain only). |
Unlike most regional weather models, ALADIN natively provides direct solar radiation and sunshine duration, so these are not estimated by Open-Meteo.
API Documentation
For a detailed list of all available weather variables please refer to the general Weather Forecast API. Only notable remarks are listed below
- Pressure Level Data: Forecasts on pressure levels are only available for the Central Europe 2km domain on 17 levels (1000, 950, 925, 850, 800, 700, 600, 500, 450, 400, 350, 300, 275, 250, 200, 150 and 100 hPa), e.g. temperature_850hPa or wind_speed_500hPa.
- Snow Depth: ALADIN provides the snow cover as water equivalent (snow_depth_water_equivalent). A snow depth in metres is not available.
- Forecast Length: Forecasts cover 3 days. Selecting more forecast days will return empty values for the additional days.