Pool owners who live in Los Angeles or Miami can splash around all year. But for those who live where winter is on the way, closing is a necessity. The process involves several steps, Mr. White said, some of which may be better left to a professional. In-ground pools generally require more work and technical skill to close; above-ground pools are simpler. Leave the water in the pool to protect it. When empty, an above-ground pool may collapse and an in-ground pool may be damaged by freezes and thaws in the surrounding soil.

Be sure that the water is clean and free of bacteria and algae and that the chemistry is correct. Terms to know (a glossary can be found at poolcenter.com) include pH, the scale of relative acidity or alkalinity, expressed in a range from 0 to 14.0, with 7.0 being neutral; alkalinity, the condition of having pH above 7.0; acidity, when pH is below 7.0; and calcium hardness, the level of dissolved calcium.

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