Autumn is the season which marks the transition from Summer, the warmest season to Winter, the coolest season. During the Summer, either the Northern Hemisphere or Southern Hemisphere is exposed to more direct sunlight. In Autumn, the given hemisphere is gradually exposed to less and less of this direct sunlight, until it is exposed to very little in Winter. Thus, throughout Autumn the temperature generally becomes cooler and cooler. The season typically begins in September in the Northern Hemisphere and March in the Southern Hemisphere. It then occurs throughout October and November or April and May in the Northern Hemisphere and Southern Hemisphere respectively. The effects of Autumn are felt in the more temperate places of each hemisphere, whilst places closer to the equator experience very little or none of the changes.
The onset of Autumn drastically occurs in the last meteorological month of Summer (August in the Northern Hemisphere, February in the Southern Hemisphere), as it is then that the change in the Earth's tilt is marked. However, due to seasonal lag, the meteorological Autumn does not begin until a month later as the ground and oceans are warm after being heated throughout the Summer. Thus, mild to warm temperatures may be common as late as September and October in the Northern Hemisphere or March and April in the Southern Hemisphere, due to the ground and oceans being at their warmest, even though very little direct sunlight is being received. Meanwhile, snow is unlikely to occur until near the end of the final month of Autumn in temperate regions, although the first frosts after the Summer are likely to occur before that.
In some places, major windstorms are common during the Autumn and early part of Winter, due to the warmer oceans. Warm nights can be a factor of Autumn, with the daily maximum and minimum values generally closer together due to the warm oceans keeping a region in warm air. This is reversed in Spring.
The cooler weather experienced in Autumn has an adverse effect on trees - it causes leaves from Deciduous trees to change color, die out and finally fall.
- Autumn is home to the Halloween season.