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Weather Forecasting - The Dark Art

In your day-to-day life, have you ever paused to wonder how weather forecasts are made? We all rely on them, whether it’s to decide what to wear, whether to take an umbrella, or more crucially, to cover up tender plants or put down rock salt to prevent ice on paths. But how are they produced?

Here’s an astonishing statistic: the Met Office receives and uses over half a million observations of weather every day, including temperature, pressure, wind speed and direction, and humidity. These come from weather stations around the world, including buoys and satellites. The Met Office is responsible for maintaining the UK’s weather stations, and they share data with other countries. Data come from 36,000 km above the earth and 2,000m below sea level!

The Met Office uses computers to add together all the data. Based on that information, the computer model estimates what is going on for areas with no observations, such as large areas of sea, places which don’t have many weather stations and so on. This gives the Met Office a picture of the current conditions around the world, which is the essential starting point for a weather forecast. In fact, it takes longer for the computers to build this picture of ‘now’ than it does for them to make a forecast based on it!

The forecast is developed using various models, depending on whether it is for the next few hours, days, or longer-term, and for what area. Each type of model uses and gives different levels of detail. Short-range forecasts can of course provide greater detail with more accuracy than longer-range forecasts.

The system that forms the basis of modern weather forecasting is called numerical weather prediction. It uses a mathematical model that averages weather over space and time to develop the forecast as a system of equations. So the greater the time or space to average, the longer it takes and the  lower the ‘resolution’ of the answer, which explains why a five day forecast is produced for the UK, and not for your town! Short-range forecasts can generally get down to a level of about 10-15km, which also explains why they can’t predict the location of thunderstorms and showers with precision, as these tend to be over a smaller area, although the Met Office is experimenting with new models which would allow greater resolution.

So now, as you follow the forecaster’s advice, and sprinkle some rock salt, also called grit salt or de-icing salt (buy rock salt here), on your path to prevent it from freezing, you will know how they were able to tell you what the temperature would be overnight. You can also amuse yourself by buying a maximum and minimum thermometer and testing how often the forecast was correct!