Week 2: Cycles

Cycles
There are many "cycles" within the Earth system. Cycles are happenings that repeat themselves. Cycles affect your life every day. In fact, a "day" is a cycle. It is usually a cycle of light and darkness. (Note: Some regions of the world experience 24-hour darkness during a portion of the year.) Below is a diagram of the cycle of a day.

Cycles can take place over different lengths of time. For example, the cycle of a day takes 24 hours to complete. Another cycle you may have experienced takes a whole year before it starts again. It is the cycle of the seasons. Seasons are regular changes in weather patterns. They involve changes in temperature (air) and precipitation (water). Below is a diagram of the cycle of seasons.

Cycles occur within each of Earth's spheres. For example, water in the hydrosphere cycles among three physical states, or forms. These forms are solid, liquid, and gas. Water moves from Earth's surface to the atmosphere and back again as it changes form. This movement continually repeats itself. It is called the water cycle, or hydrological cycle. Temperature drives the water cycle. Changes in temperature cause water to change its physical state. Extremely cold temperatures (below 0 °C) cause liquid water to freeze into a solid. Solid water is called ice. At moderate temperatures (between 0 °C and 100 °C), ice melts into a liquid. Extremely high temperatures (above 100 °C) cause liquid water to evaporate into a gas. The gas is called water vapor.

Living things also go through cycles. These cycles are called life cycles. Living things are born, live, then die. The length of a life cycle is different for all living things. Some insects live only for a few days. Meanwhile, some trees live for hundreds of years.

Many cycles on Earth are related to each other. In this way, cycles connect Earth's spheres. For example, the life cycle of a plant (living things) is connected to the cycle of seasons. The temperature (air) and amount of water (water) available change as the seasons change. Temperature and the amount of water available affect plant growth. Therefore, the life cycle of plants is connected to the cycle of seasons. Below is a diagram of this connection.

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Last updated August 04, 2000