A quantitative term denoting the electric field strength of the atmosphere at any specified point in space and time. In areas of fair weather, the atmospheric electric field near the earth's surface typically is about 100 volts (V) m−1 and is directed vertically in such a sense as to drive positive charges downward to the earth. In areas of fair weather this field decreases in magnitude with increasing altitude, falling, for example, to only about 5 V m−1 at an altitude of about 10 km. Near thunderstorms, and under clouds of vertical development, the surface electric field (the electric field measured at the surface of the earth) varies widely in magnitude and direction, usually reversing its direction immediately beneath active thunderstorms. In areas of minimal local disturbance, a characteristic diurnal variation of electric field strength is observed. This variation is characterized by a maximum that occurs at about 1900 UTC for all points on the earth and is now believed to be produced by thunderstorms that, for geographic regions, are more numerous for the world as a whole at that universal time than at any other. It is now believed that thunderstorms, by replenishing the negative charge to the earth's surface, provide the supply current to maintain the fair- weather electric field in spite of the continued flow of the air–earth current that tends to neutralize that field. The range of the electric field in fair weather varies considerably with geographical area, from one part of the globe to another. If, however, there are no local sources of pollution, the surface electric field has its maximum amplitude around 1900 UTC.