An animated map showing frequency of lightning strikes over the earth in the year 2002 provides striking evidence of the seasonal patterns. You can view this map on the web either in animated form or as freeze-frame at any particular point during the year.
Lightning activity builds from the Gulf Coast in late winter, spreading rapidly northward toward Wisconsin. Later in the year the pattern reverses and lightning becomes more common over the Amazon River and southward.
This map comes courtesy of the National Space Science and Technology Center. The display does not reflect individual lightning strikes, rather a compilation of lightning activity that illustrates average trends.
More information: Animated map of lightning frequency in 2002