Seasonal Cycle

THERE ARE SEVERAL versions of seasons. The classical concept of season is of the four seasons that divide the year—spring, summer, fall, and winter. Some regions of the globe have weather-based seasons, such as rainy or dry seasons. Certain natural occurrences are more frequent during particular times of year; therefore, we have hurricane season as well as tornado season.

Seasonal cycles for weather patterns occur because of the atmosphere. Although the atmosphere is hundreds of miles thick, weather occurs in the base 7 mi. (11 km.), called the troposphere. Wind patterns in the troposphere are affected by the earths rotation. This pattern is stereotypical and is the cause for seasonal cycles such as monsoon season in the northern Indian Ocean.

Other seasonal cycles include patterned flooding of rivers, such as the Nile River in Egypt. Ancient Egyptians utilized flood patterns for agriculture; today many cultures try to alter flood patterns. For example, the Colorado River floods created the Grand Canyon; today those floods no longer occur, due to dams and rerouting of the river.

A well-known example of a seasonal cycle is El Nino. It usually occurs in late December, around Christmas; hence it was named with the word for “child” in Spanish. El Nino is a warm ocean current, typically based off of Australia, that once every three to five years travels northeast towards Ecuador and Peru. Its opposite cycle, La Nina, contains cooler winds. Additionally, El Nino weakens trade winds, while La Nina brings stronger trade winds.

Tornados typically follow a seasonal cycle as well. They usually occur in the spring, yet they can happen at any time. Tornados occur when warm air at the surface of the earth rises quickly, and for a long distance. Air that is actively warmed at the surface, as occurs during the spring, is more buoyant and more likely to form a tornado.

Although hurricanes can form year-round, June 1 through November 30 is officially the Hurricane Season on the Atlantic coast of the United States. Hurricanes generally arise in the Caribbean and travel westward to the coast. Typically during these months, the most frequent type of storm is a tropical storm. Hurricanes are more frequent in the months of August, September, and October; however, severe hurricanes are usually limited to September, the midpoint of hurricane season.