The dry season is a yearly period of low rainfall in the tropics. The weather in the tropics is dominated by the tropical rain belt, which comes from the northern to the southern tropics over the course of the year. The tropical rain belt lies in the southern hemisphere roughly from October to March, and during this time the northern tropics experience a dry season in which precipitation is more rare, and days are typically sunny throughout. From April to September, the rain belt lies in the northern hemisphere, and the southern tropics experience their dry season. Under the Köppen climate classification, for tropical climates, a dry season month is defined as a month where average precipitation is below . The dry season is characterized by its low humidity, and some watering holes and rivers drying up. Because of the lack of these watering holes, many grazing animals are forced to migrate due to the lack of water and feed to more fertile spots. Examples of such animals are zebras, elephants, and wildebeest. Because of the lack of water in the plants, bushfires are common. Data shows that in Africa, the advent of the Dry Season coincides with a rise in the cases of measles—which researchers believe might be attributed to the higher concentration of people in the dry season, as agricultural operations are all but impossible without irrigation. During this time, some farmers move into cities, creating hubs of higher population density, and allowing disease to spread more easily. The rain belt reaches roughly as far north as the Tropic of Cancer and as far south as the Tropic of Capricorn. Near these latitudes, there is one wet season and one dry season annually. On the equator, there are two wet and two dry seasons as the rain belt passes over twice a year, once moving north and once moving south. Between the tropics and the equator, locations may experience a short wet and a long wet season. Local geography may substantially modify these climate patterns, however. New data shows that in the South American Amazon Rainforest, foliage growth and coverage varies in between the dry and wet seasons-with approximately 25% more leaves and faster growth in the dry season. Researchers believe that the Amazon itself has an effect in bringing the onset of the wet season; by growing more foliage, it evaporates more water. However, this growth appears only in the undisturbed parts of the Amazon, where researchers believe roots can reach deeper and gather more rain water. It has also been shown that the ozone levels differ between the dry and wet seasons in the Amazon, with the level being substantially higher in the dry season than in wet season.