Tham khảo tài liệu 'the earth’s atmosphere contents part 2', khoa học tự nhiên, vật lý phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Incoming Solar Energy 47 22 the earth will have moved so that the sun is directly above the equator. Except at the poles the days and nights throughout the world are of equal length. This day is called the autumnal fall equinox and it marks the astronomical beginning of fall in the Northern Hemisphere. At the North Pole the sun appears on the horizon for 24 hours due to the bending of light by the atmosphere. The following day or at least within several days the sun disappears from view not to rise again for a long cold six months. Throughout the northern half of the world on each successive day there are fewer hours of daylight and the noon sun is slightly lower in the sky. Less direct sunlight and shorter hours of daylight spell cooler weather for the Northern Hemisphere. Reduced sunlight lower air temperatures and cooling breezes stimulate the beautiful pageantry of fall colors see Fig. . In some years around the middle of autumn there is an unseasonably warm spell especially in the eastern two-thirds of the United States. This warm period referred to as Indian Summer may last from several days up to a week or more. It usually occurs when a large high pressure area stalls near the southeast coast. The clockwise flow of air around this system moves warm air from the Gulf of Mexico into the central or eastern half of the nation. The warm gentle breezes and smoke from a variety of sources respectively make for mild hazy days. The warm weather ends abruptly when an outbreak of polar air reminds us that winter is not far away. On December 21 three months after the autumnal equinox the Northern Hemisphere is tilted as far away y pH I r Contrary to popular belief it is not the first frost that causes the leaves of deciduous trees to change color. The yellow and orange colors which are actually in the leaves begin to show through several weeks before the first frost as shorter days and cooler nights cause a decrease in the production of the green pigment .
