Autumn fog: what to know and how to stay safe

With high pressure bringing calm, clear nights this weekend, forecasters say mist and fog are likely to form in places by the morning. Here is what causes autumn fog and how to stay safe in it.

Why fog forms in autumn

The most common type of fog in the UK is radiation fog. It forms on clear, calm nights when the ground loses heat quickly into the sky. The air just above the ground cools, and if it is moist enough, the water vapour condenses into tiny droplets that hang in the air.

Autumn is a prime time for fog because the nights are getting longer, giving the ground more time to cool, while the air often still holds a lot of moisture after a warm summer. Fog is most likely in valleys and low-lying areas, near rivers and canals, and in the countryside around the city.

It usually clears during the morning as the sun warms the ground, but in late autumn and winter it can linger all day.

Fog and the city

Built-up areas such as Birmingham city centre are often slightly warmer than the surrounding countryside, because buildings and roads store heat. This urban heat island effect can mean less fog in the centre than on the outskirts, but fog can still be thick along the Rea and Tame valleys and near the canal network.

Driving in fog

• Slow down and leave a much bigger gap to the vehicle in front.
• Use dipped headlights. Use fog lights only when visibility is seriously reduced, generally below 100 metres, and switch them off when it improves so you do not dazzle other drivers.
• Do not use full beam, which reflects off the fog and makes it harder to see.
• Be aware that fog can be patchy, and conditions can change suddenly.
• On motorways such as the M6, M5 and M42, watch for warning signs and speed limits.

Walking and cycling

People walking and cycling should wear bright or reflective clothing, and cyclists should use lights, as drivers may struggle to see them in fog.

Flights and trains

Thick fog can occasionally delay flights at Birmingham Airport, although modern landing systems allow aircraft to land in very poor visibility. Passengers should check with their airline if fog is forecast.

Fog and health

On still, foggy days, air pollution can build up near the ground, because there is little wind to disperse it. People with asthma and other lung conditions may notice their symptoms getting worse, and should keep their inhalers with them.

The weather this weekend

High pressure is expected to dominate the UK on Saturday, bringing dry conditions for most, with cooler nights after the very mild start to the week. Next week is expected to become more changeable, with rain returning at times.

Forecasts and warnings

The Met Office can issue yellow warnings for fog when it is expected to cause disruption, particularly to travel. Check the latest forecast before setting off on a journey.

A note on frost

As nights lengthen, clear skies can also bring the first ground frosts of the season, especially in rural areas, so gardeners should be ready to protect tender plants.