How the weather forecast works

There are thousands of weather stations around the world. Valuable data from these stations and also from planes, ships, buoys and satellites are gathered together, so we have an idea how the earth’s atmosphere looks like at a certain time. Of course, all the data must be observed at the same time and this time is called “reference time”.

All these data are then put into a computer. Rather supercomputer, with huge computation power, usually owned by states or meteorology agencies.

After few hours of computation, the computer outputs the result, which is called “forecast model”. The process of computing this model is called “running the model”.

Forecast model is a bunch of huge computer files in weird formats, containing just numbers, and numbers and again numbers.

Meteorologists and companies all over the world download these files on a regular basis, and based on these data, create those beautiful icons that you are familiar with or Windy.com animations and visualisation you like so much.

Some “forecast models” can be downloaded for free, some must be paid for.

Some forecast models cover the whole world. These are called “global models”. “Local models,” on the other side cover just a smaller area.

There are two major global forecast models: GFS and ECMWF, called also Euro.

Most of the weather sites use only the GFS model because it's free, while ECMWF must be paid. On Windy.com we use both and you can choose which one you want to use. We have decided to pay for ECMWF because we consider it much more advanced and precise than a GFS. Studies show, that ECMWF is the most accurate forecasting model in the world.

How the forecast model looks like

Forecast model is a bunch of big computer files in weird data formats. We download these files from meteorology agencies on regular basis, whenever a new version of files are available. Each download is called “update”. For example, GFS model is updated every 6 hours, while ECMWF is updated every 12 hours. See the exact times on the following link.

Forecast model contains numbers, that describe how the earth’s atmosphere will look in the next days in each three hours intervals. Some local models forecast weather even in hourly intervals.

But computing forecast for each place in the world would be computationally very difficult. Therefore model is computed just for some places in the world. These places are called “grid” and density of such a grid is called “resolution”.

And what happens if you want a weather forecast just between these points in a grid? We just look at the closest points in the grid and make some average out of it. This process is called “interpolation”.

If you click on a map on Windyty and move picker around the map, the numbers you see are interpolated numbers for any place in the world.

On Windy we use models GSF with 22km resolution grid, ECMWF with 9km resolution grid and a lot of local models with resolution even 3km.

(Special thanks to @ajf_hodgson for the grammar check)

A shortened version of this link: on.windy.com/ForecastModel