Electrostatic field measurements taken in Bogota, Colombia, during thunderstorms in November (the rainy season due to inter-tropical confluence zone over central Colombia) were used to study the performance of an isolated electric field sensor and analyse its most importan t operational characteristics. The distances from each flash to t he studied sensor were obtained by using the Colombian lightning location system. The ΔE cf distance ratio led to defining a charge model which could be used as a reference for calibrating other electrostatic field sensors to be used as lightning warning systems.