Temperature refers to how cold or hot water is. It is an important physical parameter to examine and assess the water quality. The variation of water temperature is mainly affected by the climate. However, waste water discharge also influences temperature. Temperature changes the physical characteristics of the water such as density, viscosity, vapor pressure and surface tension. It also influences the chemical characteristics including chemical reaction rates, gas solubility and so on. Biologically it affects the activity of microbes and the metabolic rate as well. Usually the water temperature could be measured by a calibrated thermometer.