## Why weather models can get it wrong

The data observations from weather measuring instruments are collected and processed into a model that covers all areas and the change in weather over several days. Quite often, not all data you'd like is available or as recent as you would wish, but the models fill in the gaps.

*A map showing predicted air
pressures made using a numerical weather prediction model for S4C
weather*

Complex mathematical equations are used to predict the physics
and movement of the atmosphere and to determine the rates of change
in the atmosphere over time. The rates of change predict the state
of the atmosphere a short time in the future. The equations are
then applied to this next atmospheric state to find new rates of
change, and these new rates of change predict the atmosphere at a
yet further time into the future. This is called **time
stepping**.

This time stepping is continually repeated until the desired forecast time in the future is reached. Computation time for a regional weather model is from a few seconds to a few minutes.

Forecasts cannot accurately predict the state of the atmosphere in two weeks time. Tiny errors in the initial data input, such as for temperature and wind, double for every 5 days of prediction within numerical models. This means that a forecast for the next few hours is more accurate than a forecast for next week.