There is always some moisture in the air, and most of the time you cannot see it. When the air becomes colder it cannot hold the same amount of water, and small drops of water can then appear as condensation. You can see this moisture for example on a mirror when you have a bath, or when you see your clouds of breath on a cold day.
Condensation occurs mainly during cold weather, whether it is raining or dry. It does not leave a ‘tidemark’, but appears in places where there is little or no movement of air. Look for black mould in corners and on or near windows, in or behind wardrobes and cupboards. Mould may also be found on clothing or soft furnishings. It more commonly forms on north-facing walls which don’t get direct heat from the sun.