Over the past few days thick fog has settled across many parts of the northern hemisphere.
Fog has caused major disruption to transport, bringing traffic to a standstill and causing major disruption at airports.
In China, more than 100 flights were cancelled at Chengdu airport and as visibility dropped below 30 metres, authorities closed a number of motorways.
Meanwhile in India, trains have been severely delayed, leaving numerous passengers stranded.
Fog often forms at this time of year because the weather in the northern hemisphere is cooling down.
Warm air can contain more moisture than cool air can, so as the air cools, the air becomes saturated. The excess moisture in the air condenses to forms tiny droplets.
If the winds are very light, then these droplets will hover near the ground, forming fog. Any wind will lift the fog into low cloud.
During the day, the sun tries to burn through the fog, its heat causing the water droplets to evaporate.
However, at this time of year, the sun is often weak and the fog can be very dense. Therefore it can linger throughout the day.