Because of the use of air conditioning during those hot summer months, windows especially the older and draftier ones, tend to condensate.
The warm air meeting the cooler surface of the window causes differences in the temperature and humidity levels on the inside and outside of the glass, causing condensation to build up.
Condensation can be prevented by making the interior and exterior conditions similar:
Reduce indoor humidity levels
Install a dehumidifier
Check for ventilation issues
Use kitchen and bathroom exhaust fans
Add extra insulation around windows