All fog occurs when there are so numerous water droplets in the air that visibility is hampered. But you will find different kinds of fog, which form in various ways. On cold, clear, calm autumn and winter nights, the ground quickly radiates the heat it absorbed during the day back into the oxygen. As the ground cools, it also cools the air directly above it in a layer a few metres deep. If this layer cools to its dew point, a blanket of water droplets types. This low-level fog is known as radiation fog. Such fogs are usually densest just after dawn, when temperatures are lowest then burn off as the sun warms the ground.

Radiation fogs usually type in sheltered valleys, exactly where cold air drains off the hills and there is little wind to stir up the atmosphere. In mountain places, you frequently see a fog on the valley floor, while mountain tops are clearly visible. Dense and persistent radiation fogs can happen in urban and industrial places, where there are plenty of aerosols to act as condensation nuclei. Fogs can also type when warm, moist oxygen flows over a cold surface.

The cold surface cools the air to its dew point so that big numbers of water droplets condense, forming an advection fog. Advection fogs are more widespread than radiation fogs and, though they too could be burned off through the morning sun, are much more likely to persist until the wind changes direction. Advection fogs tend to type over land in the winter months, when cosy air from the tropics flows more than frozen ground. Typical winter advection fogs occur within the snow-covered upper Mississippi valley from the USA, when warm air from the Gulf of Mexico flows over it.

In the summer, when the land is warm, advection fogs form instead over large lakes for example Lough Neagh in Northern Ireland. They also type over oceans, particularly where cosy air flows over cold currents. The sea off Maine and Newfoundland within the American northeast is often blanketed in thick advection fogs. These are formed by moist oxygen warmed through the Gulf Stream present flowing north over the icy Labrador Current, which sweeps south of the Arctic.

One more typical kind of fog, called steam fog, occurs when cold air flows over water that’s several degrees warmer. The water’s warmth causes it to evaporate, but the cold oxygen makes it condense, producing fog. This kind of fog frequently appears over quiet rivers or small lakes. A specific type of steam fog, arctic sea smoke, forms more than cracks in the ice around the Poles.

A fourth kind of fog is upslope fog. This happens when warm, moist air is forced up into cold air through the presence of mountains in its path. The vapour it contains then condenses to type mist around the tops from the mountains.