How Weather Forecasts are Produced

By Katie Denham

This article attempts to go under the hood of weather forecasts to take a look at how they are produced using Numerical Weather Prediction (NWP) and atmospheric models. It’s not intended to be a comprehensive guide to the subject but to provide an overview of how forecasts are produced so that the reasons for their strengths and weakness are more widely understood.

In the beginning:

One of the pioneers of Numerical Weather Prediction was a guy called Lewis Fry Richardson who in 1910 devised one of the very first atmospheric models. At the time weather forecasts were produced by forecasters trying to match weather sequences up with patterns from the past. This approach was a bit hit and miss to say the least.

Lewis worked out if he divided the atmosphere into sections and imputed observational data, then by running a set of calculations he could generate a forecast, effectively modelling the atmosphere.

Lewis began by dividing the UK and part of northern Europe up into 200km squares, with each square being 5 layers in depth. His chosen date was the 20th of May 1910. He set out to take the weather observations from 7am and use them to predict the weather 6 hours later using his model. He used a number of equations for each sector to calculate changes in such variables as temperate and pressure, this technique is at the heart of all Numerical Weather Prediction.

As this was long before even the most basic computer had been invented, poor Lewis had to do all the calculations by hand with just a slide rule to help. Unfortunately this meant the forecast took 1000 hours to produce so it was way out of date before it was finished.

Way, way out of date in fact as it was not published until…1922!

Not only was the forecast rather late, it was also quite badly wrong as it predicted an alarming rise in pressure of 145 millibars in 6 hours. This was because Lewis had not applied modern smoothing techniques to remove random variations in the model, but give the poor guy a break, this was 1910 and when smoothing techniques were applied the forecast was shown to be fairly accurate.

Fast forward in time….

Although the potential of atmospheric modelling was realised it was not until the advent of computer technology that further progress was made. From the 1950s onwards atmospheric models have been experimented with and refined, leading to the sophisticated models and NWP in use today.

As in Lewis Fry Richardson’s early experiment, atmospheric models today are based on dividing some or all of the Earth’s surface into squares. The more squares there are per area covered by the model the higher resolution the model is said to be. And, just as Lewis pioneered each square is then divided up in into layers representing different altitudes. The temperature, wind, pressure, density and humidity data from surface weather stations, weather balloons and satellites is then entered into the model and the computer program is run. The computer, or more often a bank of computers known as a supercomputer do their job of crunching the numbers. They do all the equations and output the data for what the new values for each square should be for each step forward in time.

So now we have computers things so be easy… right….?







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