Terrestrial biomes are large geographic regions on land defined by their average temperature, average rainfall, and dominant plant types. The geographic locations of terrestrial biomes are loosely correlated with latitude because of global patterns of air circulation. There are also patterns in ocean circulation based on latitude as a result of differential exposure to the Sun. These ocean currents influence the climate of coastal biomes.Biomes surrounding the equator tend to be warm with regular, year-round precipitation. This is because the equator constantly receives steady, direct sunlight. The warm air rises at the equator and loses water as precipitation. Dry biomes exist around 30 degrees north and south latitude because the air has already lost its moisture. The dry air falls at these latitudes and becomes warm as it descends. At 60 degrees north and south latitude, air picks up moisture and rises again, increasing the precipitation in the area. However, the temperatures are cooler because there are fewer direct rays from the Sun at these latitudes. At Earth's poles, the dry air falls again, leaving the polar regions dry and cold. Variation in average temperature and rainfall leads to differences in the primary vegetation found in various terrestrial biomes. For example, a desert cactus adapted to an arid climate is unlikely to be found in a tropical rainforest, and maple trees that require a lot of water are unlikely to be found on the Arctic tundra. Similarly, heat-adapted and cold-adapted plants are found in biomes that match their physiology. As British naturalist Charles Darwin, who came up with the theory of evolution, observed during his travels, organisms tend to be found in environments to which they are well suited.