<P> In prehistoric times, great amounts of sediment collected on the floor of Lake Amora . The sediment was heavier than the salt deposits and squeezed the salt deposits upwards into what are now the Lisan Peninsula and Mount Sodom (on the southwest side of the lake). Geologists explain the effect in terms of a bucket of mud into which a large flat stone is placed, forcing the mud to creep up the sides of the bucket . When the floor of the Dead Sea dropped further due to tectonic forces, the salt mounts of Lisan and Mount Sodom stayed in place as high cliffs (see salt dome). </P> <P> From 70,000 to 12,000 years ago, the lake's level was 100 m (330 ft) to 250 m (820 ft) higher than its current level . This lake, Lake Lisan, fluctuated dramatically, rising to its highest level around 26,000 years ago, indicating a very wet climate in the Near East . Around 10,000 years ago, the lake's level dropped dramatically, probably to even lower than today's . During the last several thousand years, the lake has fluctuated approximately 400 m (1,300 ft), with some significant drops and rises . Current theories as to the cause of this dramatic drop in levels rule out volcanic activity; therefore, it may have been a seismic event . </P> <P> The Dead Sea's climate offers year - round sunny skies and dry air . It has less than 50 millimetres (2 in) mean annual rainfall and a summer average temperature between 32 and 39 ° C (90 and 102 ° F). Winter average temperatures range between 20 and 23 ° C (68 and 73 ° F). The region has weaker ultraviolet radiation, particularly the UVB (erythrogenic rays). Given the higher atmospheric pressure, the air has a slightly higher oxygen content (3.3% in summer to 4.8% in winter) as compared to oxygen concentration at sea level . Barometric pressures at the Dead Sea were measured between 1061 and 1065 hPa and clinically compared with health effects at higher altitude . (This barometric measure is about 5% higher than sea level standard atmospheric pressure of 1013.25 hPa, which is the global ocean mean or ATM .) The Dead Sea affects temperatures nearby because of the moderating effect a large body of water has on climate . During the winter, sea temperatures tend to be higher than land temperatures, and vice versa during the summer months . This is the result of the water's mass and specific heat capacity . On average, there are 192 days above 30C (86F) annually . </P> <Table> <Tr> <Th_colspan="14"> (hide) Climate data for Dead Sea, Sedom (390 m below sea level) </Th> </Tr> <Tr> <Th> Month </Th> <Th> Jan </Th> <Th> Feb </Th> <Th> Mar </Th> <Th> Apr </Th> <Th> May </Th> <Th> Jun </Th> <Th> Jul </Th> <Th> Aug </Th> <Th> Sep </Th> <Th> Oct </Th> <Th> Nov </Th> <Th> Dec </Th> <Th> Year </Th> </Tr> <Tr> <Th> Record high ° C (° F) </Th> <Td> 26.4 (79.5) </Td> <Td> 30.4 (86.7) </Td> <Td> 33.8 (92.8) </Td> <Td> 42.5 (108.5) </Td> <Td> 45.0 (113) </Td> <Td> 46.4 (115.5) </Td> <Td> 47.0 (116.6) </Td> <Td> 44.5 (112.1) </Td> <Td> 43.6 (110.5) </Td> <Td> 40.0 (104) </Td> <Td> 35.0 (95) </Td> <Td> 28.5 (83.3) </Td> <Td> 47.0 (116.6) </Td> </Tr> <Tr> <Th> Average high ° C (° F) </Th> <Td> 20.5 (68.9) </Td> <Td> 21.7 (71.1) </Td> <Td> 24.8 (76.6) </Td> <Td> 29.9 (85.8) </Td> <Td> 34.1 (93.4) </Td> <Td> 37.6 (99.7) </Td> <Td> 39.7 (103.5) </Td> <Td> 39.0 (102.2) </Td> <Td> 36.5 (97.7) </Td> <Td> 32.4 (90.3) </Td> <Td> 26.9 (80.4) </Td> <Td> 21.7 (71.1) </Td> <Td> 30.4 (86.7) </Td> </Tr> <Tr> <Th> Daily mean ° C (° F) </Th> <Td> 16.6 (61.9) </Td> <Td> 17.7 (63.9) </Td> <Td> 20.8 (69.4) </Td> <Td> 25.4 (77.7) </Td> <Td> 29.4 (84.9) </Td> <Td> 32.6 (90.7) </Td> <Td> 34.7 (94.5) </Td> <Td> 34.5 (94.1) </Td> <Td> 32.4 (90.3) </Td> <Td> 28.6 (83.5) </Td> <Td> 23.1 (73.6) </Td> <Td> 17.9 (64.2) </Td> <Td> 26.1 (79) </Td> </Tr> <Tr> <Th> Average low ° C (° F) </Th> <Td> 12.7 (54.9) </Td> <Td> 13.7 (56.7) </Td> <Td> 16.7 (62.1) </Td> <Td> 20.9 (69.6) </Td> <Td> 24.7 (76.5) </Td> <Td> 27.6 (81.7) </Td> <Td> 29.6 (85.3) </Td> <Td> 29.9 (85.8) </Td> <Td> 28.3 (82.9) </Td> <Td> 24.7 (76.5) </Td> <Td> 19.3 (66.7) </Td> <Td> 14.1 (57.4) </Td> <Td> 21.9 (71.4) </Td> </Tr> <Tr> <Th> Record low ° C (° F) </Th> <Td> 5.4 (41.7) </Td> <Td> 6.0 (42.8) </Td> <Td> 8.0 (46.4) </Td> <Td> 11.5 (52.7) </Td> <Td> 19.0 (66.2) </Td> <Td> 23.0 (73.4) </Td> <Td> 26.0 (78.8) </Td> <Td> 26.8 (80.2) </Td> <Td> 24.2 (75.6) </Td> <Td> 17.0 (62.6) </Td> <Td> 9.8 (49.6) </Td> <Td> 6.0 (42.8) </Td> <Td> 5.4 (41.7) </Td> </Tr> <Tr> <Th> Average precipitation mm (inches) </Th> <Td> 7.8 (0.307) </Td> <Td> 9.0 (0.354) </Td> <Td> 7.6 (0.299) </Td> <Td> 4.3 (0.169) </Td> <Td> 0.2 (0.008) </Td> <Td> 0.0 (0) </Td> <Td> 0.0 (0) </Td> <Td> 0.0 (0) </Td> <Td> 0.0 (0) </Td> <Td> (0.047) </Td> <Td> 3.5 (0.138) </Td> <Td> 8.3 (0.327) </Td> <Td> 41.9 (1.65) </Td> </Tr> <Tr> <Th> Average precipitation days </Th> <Td> 3.3 </Td> <Td> 3.5 </Td> <Td> 2.5 </Td> <Td> 1.3 </Td> <Td> 0.2 </Td> <Td> 0.0 </Td> <Td> 0.0 </Td> <Td> 0.0 </Td> <Td> 0.0 </Td> <Td> 0.4 </Td> <Td> 1.6 </Td> <Td> 2.8 </Td> <Td> 15.6 </Td> </Tr> <Tr> <Th> Average relative humidity (%) </Th> <Td> 41 </Td> <Td> 38 </Td> <Td> 33 </Td> <Td> 27 </Td> <Td> 24 </Td> <Td> 23 </Td> <Td> 24 </Td> <Td> 27 </Td> <Td> 31 </Td> <Td> 33 </Td> <Td> 36 </Td> <Td> 41 </Td> <Td> 31.5 </Td> </Tr> <Tr> <Td_colspan="14"> Source: Israel Meteorological Service </Td> </Tr> </Table>

Where does the salt in the dead sea come from