An old-fashioned wool suit would never be appropriate in 100-degree weather. Formal-wear doesn't necessarily mean wool (which also doesn't mean heavy. Wool can

An old-fashioned wool suit would never be appropriate in 100-degree weather. Formal-wear doesn't necessarily mean wool (which also doesn't mean heavy. Wool can be very light and summery). The point is less to dress more formally and more to dress more appropriately for the occasion.