1. a visible aggregate of minute water droplets suspended in the atmosphere near the earth's surface —note according to international definition, fog reduces visibility below 1 k (0.62 mi) —see radiation fog 2. fire a jet of fine water spray discharged by spray nozzles used to extinguish fires —note 1. fog is generally considered most efficient at nozzle pressures of about 100 psi (690 kPa) —note 2. high-pressure fog is delivered through gun-type nozzles attached to small hoses supplied by pumps discharging at pressures greater than 250 psi (1,724 kPa), which is the normal maximum pressure for standard ground tankers This definition last updated 10/24/2008