Thunderstorm Technology electric field mills are unsurpassed for their ability to make reliable, accurate, and repeatable electric field measurements under the extremes of long-term environmental exposure.
When active thunderstorms are nearby or are approaching, the existence of hazardous conditions is obvious from the proximity and frequency of the lightning activity. But how can you tell if the clouds overhead or those approaching are becoming electrified and pose a risk of the first lightning strike? Or when thunderstorm activity is subsiding, which lightning strike will be the last? While an electric field mill can detect the signals associated with lightning, it really shines in its ability to detect the presence of developing, decaying, or residual electric charge in clouds that can occur before, during pauses, and after thunderstorm lightning activity.
An Electric Field Mill detects the presence of electrified clouds by sensing the slowly varying electric fields produced by the electric charge in these clouds. Typically, a "shutter" or rotating-vane type mill uses a moving shield electrode to alternately expose and cover a detection electrode to produce a signal proportional to the electric field. Special electronic circuitry amplifies this signal to report the amplitude and polarity of the electric field.
Airborne Shutter Mill from 1980's
Used on NASA U-2 and ER-2 high-altitude thunderstorm overflight missions.