We developed the world lightest-smallest radiosonde iMS-100 (less than 40g including a battery) to solve the above problems. The lightest iMS-100 decreases risks when it falls on ground after the balloon bursts. It can fall-down quite safely even without a parachute because of the slow fall velocity. The lightweight also help reduce annual gas consumption by ~30% for one station. Since the maximum reachable height depends on the size of the balloon, the amount of gas, and the total weight, the lightest iMS-100 package can reach 100 hPa pressure level with even small 100 g balloon.
Downsizing of iMS-100 results in higher performance of measurements. One is to reduce thermal effect influenced by radiosonde box because of decreasing the volume of radiosonde package by 30% relative to RS-11G (our previous operational radiosonde). Second, pendulum motions of sonde package are reduced because the smallest package smoothly rides on the wind. Therefore, micro iMS-100 can better capture in-situ air in terms of temperature and winds. Besides, new relative humidity sensor mounted with iMS-100 is developed to meet an increasing requirement for accuracy of humidity measurement. The response time is within 3 and about 20 seconds at -40 degC and -60 decC, respectively, to enable more accurate measurement in the troposphere.