Stratospheric water vapor measurement in the tropical zone by means of a frost point hygrometer on board long‐duration balloons

Two long‐duration balloon flights were performed during austral summer in the southern hemisphere tropical zone, between 23°S and 10°S, for stratospheric water vapor measurement by means of a frost point hygrometer. The observations show a mixing ratio minimum of about 3.5 ppmv near 50 hPa that corroborates the existence of the hygropause in low‐latitude areas. Also observed was the approach of saturation conditions near a particularly cold tropopause in the highly convective area around New Hebrides, where cloud top temperatures as low as −84°C were seen with GMS satellite data.