The Earth's axis is tilted at 23.5 degrees from perpendicular, which means that when the Earth revolves the Sun over the course of a year, the angle of the Sun's rays varies at any given place. Seasons fluctuate as a result of variations in angle toward the sun and the accompanying solar energy. When the angle between the Earth and the sun is 90 degrees (direct sunlight), winter comes when the temperature drops below 0 degrees Celsius (32 degrees Fahrenheit). At this time of year, the days are short and cold, with little light left after sunset. The polar regions experience four seasons in two years because of the rotation of the earth acting on the sunstroke principle. As soon as spring arrives at the equator, it grows hotter and drier there, while further north it gets colder and more rainy. As for the tropics, they have only one season all year round: the rainy season. During the dry season, there is no water anywhere near the surface of the ground. But just as the rainy season begins, so too does the summer season. It too is short - about nine months - but very hot and humid, with frequent rainstorms. Seeds from plants that grow well with such conditions produce spores in large numbers, so that when the wind blows them around, new plants can grow from these spores far away from their original location.