Never mind the 90s, when it comes to air pollution, the 50s are back. Yesterday marked 65 years since the Great London Smog, and the UN Environment Assembly commemorated the occasion by announcing their first ever international Smog Day as December 5th 2017.

It’s a timely event, as December falls right in the middle of the winter smog season. Many cities are already feeling the effects. In New Delhi for example, an international cricket match between India and Sri Lanka was halted for 20 minutes due to crippling pollution conditions.

Air quality variations have always been seasonal. Ground-level ozone requires sunlight to form, and so is more hazardous in summer. High particulate matter levels are more common in cold weather due in part to residential heating peaks, as houses turn up the thermostat to keep warm through the winter months.

However, more extreme pollution requires particularly unfavourable weather conditions – that’s where a phenomenon known as a temperature inversion comes in.

A temperature inversion occurs when warm air in the upper atmosphere traps a layer of colder air closer to the ground, the reverse of the conditions you would usually expect.

This “lid” of warm air prevents pollutants from dispersing, trapping particulate matter at ground level and causing pollution levels to spike in areas with already high emissions. This leads to a situation in which additional pollution only adds to the smog-like conditions.

Using historical data from our global environmental data platform, we created a timelapse showing this inversion phenomena affecting Northern Europe in December 2016. The darker the circle, the higher the impact of ambient pollution on your health.

At the highest levels indicated, air pollution becomes unhealthy after less than an hour of exposure.

Tackling air pollution effectively means recognising that exceptional events occur. Even low emissions rates become dangerous when weather conditions are not ideal, so severe health impacts can still occur in cities with otherwise clean air.

That’s why accessible air quality information is more important than ever to help us all #BeatPollution.