When air gets warm, it becomes lighter (less dense) as the molecules spread out more. This causes the warmer less dense air to float above cooler more dense air. As a result the hot air rises.
As the warm air rises the cold air above is displaced and sinks to replace the warm air from the bottom. The cold air at the bottom is now heated and this will eventually rise.
Air moves continually in a cycle of being heated and rising
before being replaced by warmer air from below. When
the air moves like this, it is called convection.
Move the pointer over the magnifying glasses in the diagram below.
To start with convection makes a building colder at the bottom and warmer at the top but as it continues it allows the heat to be circulated.
Convection occurs in the stairwells of Bowater House. This along with the heating effect of Passive Solar Gain helps to warm the stairwells. It is possible that the stairwells could get too hot due to excessive heating and so temperature sensors on the windows have been fitted which automatically open them for ventilation if the temperature is too high.