Pure water freezes at 0 degrees Celsius at 1 atmosphere of pressure. The freezing point of salt water is lower and depends on the concentration of salt.
At sea level, water with no inconsistencies freezes at 0 degrees Celsius. At altitude, where the air pressure is lower than at sea level, the freezing point of water increases slightly.
Salt lowers the freezing point of water so that it won't freeze until the temperature drops below 0 degrees Celsius. The exact temperature needed depends on how much salt is in the water. The higher the concentration of salt, the lower the water's freezing point.
