On a clear day, when the atmosphere is clear of storm clouds, the primary source of electric charge creating an electric field on the surface of the earth is the ionosphere. This can be thought of as a large dome-shaped electrode high above the earth, which produces positive charges which contrast to the relatively negatively charged earth. This scenario creates what is termed a "fair weather" electric field due to the positive charge overhead. When this "fair" field is measured by the EFS 1001 field mill, it can be seen to produce an output of from 50 to about 200 Volts per meter ("V/m"). This value varies, depending upon conditions in the atmosphere, and is also altered by "local effects". Such effects are caused by anything which can carry electrical charge, including but not limited to atmospheric space charge, dust, smoke, litter, etc. Usually, though, the field stays between -50 and -200 V/m during fair or non-stormy weather.