A study of cosmic rays and thunderstorm frequency has shown a decrease in thunderstorms at the time of high cosmic rays and an increase in thunderstorms 2--4 days later. This was done by superposed epoch analysis of thunderstorms over the eastern two thirds of the United States for 1957--1976. When data for spring and fall months were used, the minimum deepened. When high cosmic rays near full and new moon for these months were key days, the minimum deepened again and was significant at less than the 0.01% level. It is believed that when the Sun, Earth, and Moon are aligned, particulate matter in the lower stratosphere is modulated and acted upon by cosmic rays, bringing about an immediate decrease in thunderstorms. ¿ American Geophysical Union 1990