The assumption of considering the Earth's surface as a plane perfect conductor is examined. The electric field produced due to a point charge or a model thundercloud in the atmosphere and the field changes produced due to a leader in a lightning flash have been calculated if the Earth's surface, instead of a perfect conductor, is considered as a dielectric of finite dielectric constant and zero conductivity. It is shown that when the dielectric constant of the surface is <10, the horizontal and vertical components of electrical field may be comparable to each other at comparatively large distances from the charge. The net electric field in such a case may be inclined from the vertical by as much as 45¿ or even more. At different distances, the field changes caused due to the leader in a lightning flash change not only its magnitude but direction as well. Certain igneous and metamorphic rocks with low moisture contents, fine-grained sand when it is dry or has a moisture content of <1% or is at low temperatures, and ice at very low temperatures of <-50 ¿C or oriented such that its c axis is perpendicular to the direction of electric field have dielectric constant values <10 and conductivity <10-9 S/m.

Therefore, for some regions , such as polar regions, high-latitude regions covered with sea ice or dry sand, hot and dry regions covered with sand in the midst of vast deserts, and regions covered with hot and dry igneous and metamorphic rocks, the average representative values of dielectric constant and conductivity of the surface may be such that the Earth's surface at those places cannot be assumed as a perfect conductor. Consequently, the assumption of treating the Earth's surface as an insulating surface generally taken for fast field changes during a stepped leader in a lightning flash, may be extended for much slower field changes occurring even in fair weather. It needs to be mentioned, however, that thunderstorm activity is generally minimal in most of the regions mentioned above. It is proposed that instead of measuring only the vertical component of electric field, as is normally done, a measurement of the electric field vector will give a full description of the intensity of the electric field at some places. ¿ American Geophysical Union 1993