UK air pollution: why so high?

You’ve probably heard in the media over the past couple of days that air pollution levels are expected to rise to high, locally very high levels on Friday.

This is the second time in a month that such high levels of pollution will be experienced in some parts of the UK, prompting health warnings from Defra.

The young, elderly and those with respiratory problems are at greatest risk, with advice to completely avoid strenuous activity.

Where are pollution levels expected to be highest?

The latest air pollution forecast map (right) from Defra shows that high (red) and very high (magenta) levels of pollution are expected across central and parts of England on Friday.

As of 10am on Friday, high levels of pollution have already been recorded in these areas, reaching 7-9 on a scale that only goes up to 10.

For the rest of the UK, air pollution is expected to remain low (yellow) to moderate (orange).

Where has the pollution come from?

Being an island, the cause of air pollution in the UK is rarely straightforward – often coming from a variety of sources.

At the moment, there is a gentle south easterly wind which is bringing in polluted air from France, which is combining with our own home-grown pollution to give dirty air.

There’s also a small contribution from Saharan dust, pushed towards us from winds high up in the atmosphere, although this is not as large a contributor as it was this time last year.

Why is the polluted air not moving away?

At the moment, high pressure is sitting over the UK, bringing settled weather to most places with sunshine and light winds.

Lights winds under high pressure means that the air isn’t being mixed very much, allowing polluted air to get stuck in similar areas for longer periods of time.

Also, high pressure does exactly what it says on the tin – it puts pressure on the air from above, pushing it down towards the surface.

This is effectively like putting a lid on saucepan of boiling water and trapping the steam (i.e. polluted air) in a confined space, rather than it being allowed to rise and disperse.

When will the air quality improve?

Thankfully, this spell of high air pollution is going to be very short, with a weather front sweeping across the UK from the west tonight.

This will bring in a feed of clean air off the Atlantic, clearing the dirty air away and leading to air pollution levels falling back to low, locally moderate this weekend.

As for the weather, following a cooler weekend with a little rain, the sunshine and warmth will return for most of us next week.

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