In atmospheric electricity, the electrical resistance of a column of air 1 m square, extending from the earth's surface to some specified altitude. Measurements extending to an altitude of 18 km indicate that the atmospheric columnar resistance to that height amounts to about 1017 ohm m−2. Probably, this is only slightly less than the total columnar resistance from earth to ionosphere. In fact, roughly half of the total columnar resistance from the earth to 18 km is contributed by the lowest 3 km of the column where, in addition to the greater density of the air, the high concentration of atmospheric particulates leads to a relatively high population of poorly conducting large ions rather than the more mobile small ions. Total columnar resistance does not vary greatly with either time or locality. By contrast, the columnar resistance of the lowest fraction of a kilometer varies greatly, causing fluctuations in the atmospheric electric field at sea level, especially in industrial areas of highly variable atmospheric pollution. See ion mobility.