When attending a summer funeral, it is best for men to wear suits and women to wear slacks or dresses. Although the temperature may be hot, a funeral is still a formal occasion and so the standard of dress should match that.
For men, if wearing a suit is too uncomfortable, then a dress shirt and slacks will suffice. Neckties may also be worn in place of ties. If part of the service is situated outside, then a hat is best worn. For footwear, dress shoes are appropriate. For women, dresses should not be revealing or detract from the mood of the occasion in any way. Sandals can be worn by women so long as the sandals are plain.