Seasonal cycles of dengue disease are observed in every Thai province. Weather patterns vary geographically providing a broad range of conditions for potential virus transmission. Annual monthly temperature ranges 10–42°C and varies north to south. In February–May temperatures rise and humidity is lowest. The rainy season (May–October and later on the southern-east coast) brings rising humidity and gently falling temperatures. Temperatures drop in winter (October–February) particularly in northern Thailand, whereas the south remains mild year round. Annual rainfall is 1–2 m or more throughout. Northern winters are mostly dry. Figure 2 illustrates the space-time variation in DHF cases and weather patterns of three geographically distinct provinces: Udon Thani (northeast), Bangkok (central), and Trang (south). Of particular importance with respect to DENV transmission is the phase difference between annual temperature and humidity cycles, the contrast in annual temperature and humidity ranges from north to south and geographic variations in the timing of the rainy season.