A low-cost Unmanned Aircraft System (UAS) has been developed based on commercial off-the-shelf components. The UAS includes a foam flying wing of expanded polypropylene with a 1.8 m wing-span and a total mass of 4 kg. It includes lithium polymer batteries which power a single propeller, resulting in a minimum airborne autonomy of 1 h. Experiments have been performed with a pitot tube to measure wind speed and wind direction on a vertical profile from 100 m a.s.l. to 3500 m a.s.l. A spiral pattern with a 100 m radius was followed on the ascent and descent. Results have been compared with radiosonde data obtained at Santo Domingo station located 80 km to the south. Wind data show a precision better than 1 m/s during the slower ascent stage and results are consistent with radiosonde data obtained 3 hours later at Santo Domingo station.