Resource: Observe Water in Winter and Summer

Atmospheric conditions on Earth change daily. Think about this: Weather broadcasts cite different expected high and low temperatures for each day of the week. In fact, conditions can change over the course of a single day, so you may hear a meteorologist say, "Clouds are expected to give way to fair skies." When you compare conditions from season to season, it becomes even clearer that variable conditions can completely transform the environment around you.

Physical factors such as temperature and pressure can prompt substances to undergo a phase change. During a phase change, the substance changes from one state to another, but does not alter its chemical composition. The primary phase changes are as follows: A solid may turn into a liquid or, less commonly, directly into a gas. A liquid may turn into a gas or a solid. And a gas may turn into a liquid or, less commonly, directly into a solid. Still, even though a substance may move from one phase to another, it remains the same substance.

Temperature is a key determinant of phase. Generally, an increase in temperature adds energy that causes a substance's particles to become more active. This, in turn, breaks the bonds between the particles. By contrast, a decrease in temperature slows particle movement and allows the bonds to reform. Each substance has a critical temperature at which bonds break or reform and thus cause it to change from one phase to another. At standard pressure, water changes from a liquid to a solid at 0°C (32°F), and from a liquid to a gas at 100°C (212°F).

Water's phase changes play a fundamental role in determining weather on a global scale. Each phase change either absorbs or releases energy in the form of latent heat. This heat provides the energy that drives water and wind circulation, which in turn determines weather.

To learn more about how water changes state, check out Water Phases.

To learn more about the processes involved in the water cycle, check out Water Cycle Animation and The Hydrologic Cycle.

• How many clues can you find that tell you whether it is winter or summer?
• What makes water so different in summer and winter?
• If you compared the sunshine in the winter and summer, what differences do you think you would notice?
• Compare the winter and summer views of a river near where you live. What are four or five differences you notice? What other changes do you think you would notice if were standing beside the river?
• Most liquids get heavier when they turn into a solid. Water is different — it gets lighter (less dense) and floats. When you see lakes in winter, where is the ice? Why is this important to the fish and other creatures living in the lake?