I'm not an avionics engineer but I've been an RF electronics engineer for a few years now. You are correct in saying that precipitation is too small to cause reflections (or scattering) of low frequency RF. Generally, attenuation of RF by precipitation is proportional to frequency. However, water is a poor dielectric compared to air and attenuation will occur due to absorbtion - the water absorbs the power and dissipates it as heat. Rain being denser will aborb more energy than snow.