coldwater

Cold water refers to water at relatively low temperatures compared with surrounding environments or with warm-water contexts. The phrase is context-dependent: domestic cold water refers to water supplied at low temperature (often near 4–15 °C in refrigeration contexts), while in swimming and aquatics, temperatures below about 20 °C are typically described as cold, with temperatures near or below 10 °C characterized as very cold or extreme.

Physically, water behaves in characteristic ways as it warms or cools. Up to 4 °C, water becomes denser as it cools; at 4 °C it reaches maximum density, and above and below that temperature density decreases. Water also holds more dissolved gases, including oxygen, at lower temperatures, which affects aquatic life. In natural bodies, cold surface or near-bottom layers can persist in spring and winter or in deep oceans; thermal stratification in deeper lakes and some seas creates colder, denser layers that may underlie warmer, lighter ones during warmer seasons.

Ecologically, cold-water habitats are home to many specialized species such as salmon, trout, and other fishes that require low temperatures and high dissolved oxygen. These habitats are often found in mountain streams, high-latitude rivers, and deep lakes. Anthropogenic uses include cooling in industrial processes and drinking-water distribution, where cold water is usually stored and transported separately from hot water systems.

Safety and health: sudden immersion in cold water can cause cold shock and rapid breathing; prolonged exposure risks hypothermia. Protective gear, gradual acclimation, and awareness of environmental conditions are advised for activities like cold-water swimming.