Md. Nasir Uddin,
Frequencies greater than 10GHz, suffer more by rain than the frequencies less then 10Ghz.
The major factor is the wavelength.Higher the frequency smaller the wavelength.
Above 10GHz,wavelength becomes equal to the size of raindrop and as a result when it rains all the energy is being absorbed by the rain drop due to scattering and absorption.
Thats the main reason.