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.

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