Heat rash occurs in babies when the pores of the sweat glands become blocked. This generally happens when the weather is hot or humid. As the infant sweats, little red bumps, and possibly tiny blisters, form because the blocked glands cannot clear the sweat.
Heat rashes and babies; Prickly heat rash; Red miliaria
To avoid heat rash, keep your baby cool and dry during warm weather.
Some helpful suggestions:
For details, see: Rash, child under 2 years
Infant heat rash