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