Thunderstorms over tropical continents have been shown recently to have a large impact on the thermal structure, the chemical composition, the humidity and the aerosol concentration of the lower stratosphere well above the tropopause up to 20-21 km altitude. The reason for the ignorance of this mechanism until now is the difficulty of performing relevant observations from satellites and high altitude research aircraft, in the presence of strong overshoots at vertical velocity up to 60m/s, permanent lightning and cold temperature associated to those systems, as well as performing large balloons flights next to heavy rainfall and wind gusts. An alternative is to probe the upper troposphere and the lower stratosphere using small balloons flown next to the thunderstorms or long duration balloons passing above, carrying lightweight and high performance sensors now feasible using modern technics. In this objective, several lightweight instruments have been developed which will be flown in a first step before long duration balloons are made available, on a number of small balloons during a TROPICO balloon campaign planned in Southern Brazil during the convective season in February-March 2012. The list includes tuneable diode laser and Lyman alpha hygrometers, aerosol optical counter and backscatter diode laser, UV and visible spectrometers, electric and magnetic probes, lightning and blue-jet optical sensors, combined with a lightweight Iridium system for controlling the flight and transmitting the data. Shown in the presentation will be a description of operation procedures of such small balloons and sensors near thunderstorms, and a presentation of most relevant results.