The appropriate value will be entered into the weather file when a selection is made from the drop-down box.
The wind exposure index is used to calculate the external surface resistance of walls, windows, roofs etc. when a value is not explicitly entered in APcdb. In most cases, the peak summertime conditions will occur for a sheltered site. This is because the higher surface resistance levels give rise to higher sol-air temperatures on external surfaces. Higher surface resistance levels also reduce the conduction of heat out of the building at night.
For winter heat loss design calculations, the peak conditions occur when the exposure index is set to 3, i.e. severe (but only if your constructions are defined without typing in surface resistance levels). This index is only used for the CIBSE Heating Loads calculation.
