Park and Dejnakarintra [1973> presented a formulation to calculate high-altitude electric fields due to electrified clouds, taking into account the presence of the ionosphere. Calculation of these fields is necessary to test the hypothesis that whistler ducts might be caused by plasma interchange of geomagnetic flux tubes driven by thundercloud electrostatic fields. To test the accuracy of the formulation, the calculated dc electric fields are compared with those measured during a thunderstorm campaign reported by Holzworth et al. [1985>. Using a conductivity profile measured during their campaign, it is shown that the formulation of Park and Dejnakarintra produces electric field predictions which are consistent with the measurements of Holzworth et al. This indicates the formulation developed by Park and Dejnakarintra is valid. The comparisons agree with measurements which show that large negative screening charges commonly exist around the top of thunderclouds, reducing the high altitude fields. These calculations indicate that the effective thunderstorm charge used in such calculations needs to be significantly reduced to account for screening charges. Such reductions lead to serious increases (up to a factor of 10) in the time required for high-altitude thunderstorm electric fields to produce whistler ducts. ¿ 1998 American Geophysical Union