atmospheric pressure. Pressure is proportional to temperature and inversely proportional to humidity. And it is necessary to know both of these to compute an accurate figure. The graph was developed for a temperature of 15 °C and a relative humidity of 0%. At low altitudes above sea level, the pressure decreases by about for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure p to altitude h: . The values in these equations are: Local variation Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with an amplitude of a few hectopascals, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle, and a semi-circadian (12 h) cycle. Records The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was measured in Tosontsengel, Mongolia on 19 December 2001. The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata in Evenk Autonomous Okrug, Russia (66°53'N, 93°28'E, elevation: ) on 31 December 1968 of . The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations. The Dead Sea, the lowest place on Earth at below sea level, has a correspondingly high typical atmospheric pressure of 1065hPa. A below-sea-level surface pressure record of was set on 21 February 1961. The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (0.858 atm; 25.69 inHg), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft. Measurement based on the depth of water One atmosphere () is also the pressure caused by the weight of a column of freshwater of approximately . Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). Conversely, 10.3 m is the maximum height to which water can be raised using suction under standard atmospheric conditions. Low pressures, such as natural gas lines, are sometimes specified in inches of water, typically written as w.c. (water column) gauge or w.g. (inches water) gauge. A typical gas-using residential appliance in the US