Bubbles are caused by poor adhesion of the paint to the foundation.
Causes:
Painting a surface that has been heated up by the sun, or painting in direct sunlight.
Applying oil or alkyd paint on a wet or humid foundation. Penetration of humidity through the external wall (less probable in case of latex paints, since latex paint has better moisture permeability).
Formation of bubbles due to dew or rain on a recently painted latex surface (particularly if the foundation has not been prepared sufficiently).
Solution:
Remove the damaged paint with a scraper or by polishing, prime and paint the surface using a high-quality outdoor paint. Optimally painting should be done during cloudy, cool weather with no wind. Do not paint late in the evening or if rain is likely, since the paint might not be dry before the rain or dew.