The experiment as discussed in section 2 is set up by mounting the holding mechanism near the wind mast with a distance of m. The data of measured variables: rotating angle of the holding mechanism, swing angle, and tension of a pulling cord, are acquired from motions of the helium balloon in one day. The data of rotating angle is used to determine the wind direction. Eqs. (15)-(16) are applied by substituting data of swing angle and tension so as to determine the wind speed at the balloon height. A linear regression of Eq. (17) is fitted with data of the wind speeds and the