"Meteorology, or weather forecasting, is a science that played a seldom-acknowledged role in World War II. Knowing future wind and weather patterns, even if only a few days in advance, allowed for better planning of shipping and airplane routes and for spying and reconnaissance. Fog and rain could be used to conceal tactical movement."
- so sayeth the Science History website on their page about the roll meteorology played in the Second World War. Understanding this explains why so many Axis spies were found with weather data among their possessions.