Blistering:Bubbles resulting from localized loss of adhesion and lifting of the paint film from the underlying surface.
Possible Cause:
Painting a warm surface in direct sunlight
Application of an oil-based or alkyd paint over a damp or wet surface
Moisture escaping through the exterior wall (less likely with latex paint than with oil-based or alkyd paint)
Exposure or latex paint film to dew, high humidity or rain shortly after paint has dried, especially if their was inadequate surface preperation
Solution:
If blisters go down to the substrate, try to remove the source of moisture. Repair loose caulking; consider installing an exhaust fan. Remove blisters
If blisters do not go all the way down to the substrate, remove them by scraping, then sanding; prime any bare wood and repaint with a quality exterior paint.

Note: Images provided by The Rohm + Hass Paint Quality Institute