Most of the time, moisture in the oxygen is invisible. However, it might type fog or mist that makes it difficult to see even a short way ahead.

The atmosphere contains water nearly all of the time. But most of this is within the form of drinking water vapour – that is, water which has completely evaporated to form a gas. Like most gases, drinking water vapour is invisible. For the moisture to become visible, it should be in liquid form, as water drops. Drinking water vapour turns to liquid (condenses) when the air cools and contracts to the point where there is not sufficient room between the oxygen molecules for all of the vapour. This is called the dew point. High within the oxygen, moisture condensing into drinking water types clouds. Near the ground, it forms fog and mist.

Aerosols

Water vapour can only condense into drinking water droplets if there are surfaces for it to condense on to. In mid-air, the only surfaces obtainable are minute particles, known as aerosols. A litre of air might contain hundreds of these, in the form of grains of dust and sea salt. They form tiny surfaces, known as condensation nuclei, around which the vapour liquefies. Occasionally, the air is so clean and free of aerosols that there are no nuclei for water to condense on to, even when it is below the dew point.

Clean oxygen produces good visibility. It is water drops in air that make distant hills look hazy – when you will find few, distant objects are clear and sharp. By contrast, when the air is dirty and full of aerosols, there are so numerous condensation nuclei that the drinking water vapour condenses readily, and may type drinking water drops before the dew point is reached. As a result, visibility in dirty oxygen is poor, and places with dirty air, such as large cities, are prone to fog and mist. However, visibility over even the most polluted cities may be quite good. If the oxygen is filthy but contains little moisture, fog and mist aren’t able to type.