The climate is one of the main factors in sabkha development. Rainfall in this arid region usually occurs as thunderstorms and averages 4 cm/year. Temperatures can range in excess of 50 °C to as low as 0 °C. Humidity is linked to the wind direction, with humidity as low as 20% in the mornings from off the dry interior and building in the afternoon as a strong, onshore wind prevail. At night, relative humidity of 100% can lead to dense fogs. Water temperatures vary by depth with shallow water as much as 10 °C warmer. These high temperatures drive high rates of evaporation in the Persian Gulf, as much as 124 cm/year leading salinity to increase in the shallow lagoons to as much as 70 ppt. The net rate of evaporation from the sabkha can be as much as an order of magnitude less and has averaged 6 cm for the last 4,000 to 5,000 years. The reasons for this are that the sabkha surface is not a free-water surface, the high humidity during the night, and vertical stratification of the air column. Despite the loss of water due to evaporation, the groundwater, never deeper than 1.5 m, flows seawards and is recharged by continental waters, rainstorms, and the northwest "shamal” gale-force winds that create waves of greater height than the intertidal height and drive water as much as 5 km inland over the sabkha to a depth of a few centimeters.