Sea of fog: why it is grey down below and sunny up above

Sound familiar? The sky has been grey for days, the streetlights are still on at eight in the morning, and the air feels damp and cold. Yet the webcam on the Rigi shows something completely different: blue sky, sunshine, and below it a white sea of clouds. Welcome to high-fog season. Here is a simple explanation of how it happens, why the air gets worse at the same time and how you can escape the grey.

Normally it gets colder higher up

As a rule, the higher you climb, the colder it gets. On average, the temperature drops by about 0.6 degrees for every 100 metres. That is why snow arrives on the mountains earlier than in the valleys.

In late autumn, it flips around

In November and December there is often calm, high-pressure weather. This is what happens:

• The ground cools down. During long, clear nights the ground loses a lot of heat. The air right above it turns cold.
• Cold air is heavy. It flows downhill like water and collects in the large basin between the Jura and the Alps, the Swiss Plateau. Experts call this a cold-air pool.
• It is warmer higher up. In a high-pressure area, air slowly sinks and warms up as it does. So a warm layer sits on top of the cold air like a lid.

This reversal is called an inversion: cold below, warm above, exactly the opposite of normal. The cold air below is moist. When it cools further, countless tiny water droplets form, and fog appears. If this cloud layer does not sit on the ground but a few hundred metres above it, it is called high fog.

Why the fog stays for days

In summer, the sun would simply burn the fog away by mid-morning. In November it is too weak for that, and the days are short. On top of that, the warm lid stops the cold air from mixing with the air above. So the high fog can stay for a week or longer. It usually only disappears when the weather changes: a front with wind and rain clears out the cold-air pool, or the foehn wind blows it away.

Did you know? As early as the 19th century, visitors travelled up the Rigi to watch the sun rise and set above the clouds. In 1871, Europe's first mountain railway opened there: the rack railway from Vitznau up the Rigi.

The downside: smog under the lid

The warm lid does not only trap the fog, but also everything we release into the air. Exhaust fumes from cars and smoke from heating, especially from wood stoves and fireplaces, cannot escape upwards. Day after day they build up in the cold air beneath the fog. This is known as winter smog.

The main problem is particulate matter: tiny particles, much finer than a hair, which we breathe in and which get deep into the lungs. On top of that come nitrogen oxides from traffic. In Switzerland, the limit for particulate matter (PM10) is 50 micrograms per cubic metre of air as a daily average. During long spells of high fog, this limit is exceeded again and again on the Swiss Plateau.

Children, older people and anyone with asthma or heart and circulation problems feel it most: coughing, irritated eyes and airways, more symptoms. During severe winter smog, the cantons can order measures, for example a ban on outdoor fires and on fireplace fires that are lit just for cosiness. You can find the current readings on the cantons' websites and in the airCHeck app.

What you can do yourself

• On smog days, postpone strenuous outdoor exercise or head straight up above the fog. The air up there is clear.
• Only burn dry, untreated wood in your fireplace and light the fire from the top. This produces much less smoke.
• Walk or cycle short distances instead of taking the car.

What else the fog does to us

Little light: If you do not see the sun for days, you often feel tired and listless. It helps to go outside every day, even when it is grey. Even a dull November day is much brighter outdoors than an office lit by lamps.

Poor visibility: In fog you see cars, bikes and pedestrians much later. Turn your lights on, drive more slowly and keep a bigger distance. Only switch on the rear fog light in really thick fog, otherwise it dazzles the drivers behind you.

Hoarfrost and slippery surfaces: When the temperature drops below zero, the fog droplets freeze onto trees, grass and fences. The landscape then looks as if it has been dusted with sugar. It is beautiful to look at, but bridges, roads and paths can get slippery.

Out of the grey

The best remedy for the fog blues is a trip up the mountain. Pack a thermos flask of hot tea. The sun may be shining up there, but as soon as you sit still and gaze at the sea of fog, it quickly gets chilly. A hand warmer in your jacket pocket keeps your fingers warm until you have got the perfect photo.

And if you stay in the valley: light a candle, make some tea and look forward to the next change in the weather, because it is sure to come.