Why is forecasting low cloud so difficult?

Official blog of the Met Office news team

You may have been enjoying the sun these past few days, and noticed that some website and app forecasts have been somewhat more pessimistic with their predictions of the cloud cover. But why is something as fundamental as cloudy or sunny actually so difficult to forecast?

As with most weather forecasting, computer models are relied upon to produce accurate simulations of the atmosphere to guide cloud-cover forecasts. Because computer resources are finite, the model cannot simulate all the small-scale detail present in the real world, but “sees” the world as a pixellated image. For UK forecasting, the pixel sizes are 1.5km in the horizontal and around 100m in the vertical at the typical altitude of low clouds. Therefore clouds at these scales, or smaller, are very difficult to represent. Their formation, evolution and dissipation is controlled by “parametrizations” – the part of the model used to deal with…

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