Why is it tricky to forecast cloud trapped under high pressure?

Over the past few days, it’s been a bit of a headache for us weather folk to try and predict where and when it will and won’t be cloudy.

You’re probably thinking that this should be a really easy task with an expensive supercomputer churning out detailed weather information. However, there’s a bit more to it than that.

Cloud and moisture trapped under a big area of high pressure over the UK can give some of the greatest challenges to weather forecasters, as well as the weather computer models that we use.

Weather computer models work by dividing our atmosphere into grid boxes, within which complex calculations are done to determine weather conditions at each point around the earth.

In simple terms, these calculations concentrate on the state of three fundamental elements that influence the weather on our planet: pressure, temperature and humidity.

Why are pressure temperature and humidity so important?

Air pressure is a measure of the weight of air above a particular location. If the pressure is high, then the air sinks and the weather is likely to be settled. If the pressure is low, then air rises and forms clouds, with unsettled weather more likely.

As well as affecting the nature of what falls from the sky, temperature also drives the jet stream. Contrasts in temperature horizontally and vertically in the atmosphere help create wind and help transfer energy from one place to another.

Humidity is a measure of how much moisture is present in the atmosphere. Whilst for some parts of our planet, such as the tropics or deserts, it doesn’t change much over time, in temperate latitudes like ours, it varies a lot. This is what makes the weather across the UK some of the most changeable in the world.

Why do weather computer models struggle to get the cloud detail right?

Cloud cover in weather computer models is parameterised, which means that a number of assumptions and simplifications are made as to how cloud cover is worked out in each of the grid boxes I mentioned a little earlier.

The Met Office NAE model that covers the UK has grid boxes that are 12km by 12km in size.

In the current weather situation, there’s a lot of cloud in the lower part of the atmosphere and not much wind to stir things up. As are result, the computer models can have difficulty working out precisely what is happening with the cloud cover in each 12km by 12km grid box.

For example, the model may have worked out that in one grid box there’s full cloud cover, when in reality the cloud has broken up, leaving just 30 per cent cloud cover.

Amplified errors

This may not sound significant, but when an incorrect assumption is put into the bigger, evolving picture, it can cause errors to become amplified over time. This can give a very different forecast in comparison to what is happening in reality.

The next few days will see a brisk wind bring colder, drier air to help break up the cloud, before the same issue returns early next week with dense, lingering for likely to become a problem.

Don’t forget, you can get the latest forecast on the Channel 4 Weather website. I also post regular updates on Twitter – @liamdutton