What is Rain Scatter Propagation in Amateur Radio?

I have heard of Meteor Scatter and Aircraft Scatter propagation earlier. It was only yesterday that I came across Rain Scatter in a post by F5LEN on X. He has posted a map of his 10 GHz Rain Scatter QSOs with best distance 722 km. He has kindly directed me to a good article on Rain Scatter propagation by G0MJW. Radio waves can be scattered by irregularities within the troposphere. An effect which is important above 3 GHz, that is in the microwave range, is Rain Scatter. A generic name would be Hydrometeor scatter which will include scatter by different types of water particles within the atmosphere. Water in liquid form can be there as rain, mist and fog. Water in the form of ice can be there in clouds, hail and snow.

We know that frequencies in the microwave range typically have line of sight propagation. Rain Scatter can extend the propagation beyond line of sight. If the rain is 2-4 km above ground level, Rain Scatter propagation can be several hundred kilometers, as described by F5LEN in his post on X. When the particle size is much smaller than the wavelength, the type of scattering is known as Rayleigh scattering, with radiation pattern like that of a dipole. That will be more to the front and back rather than to the sides. Particles of size of the order of the wavelength produce Mie scattering, in which there is more of forward scattering with a forward radiation lobe. Dependence on the particle size is the reason why Rain Scatter does not occur at VHF and UHF range, only above that, in the SHF or Super High Frequency range which has been defined as 3 to 30 GHz by the International Telecommunication Union. Most of the scattering of microwaves is Rayleigh scattering.

There is a web application which helps radio amateurs in making Rain Scatter contacts at rainscatter.com. The application automatically downloads storm data from the National Weather Service in the United States and identifies possible propagation paths between stations. The map shown there is that of the United States of America, Mexico and up to part of South America. Though I tried reducing the size of the map to see if it covers rest of the world, I could not see. There is also a clickable icon labelled ‘DX’ which asks you to input grid. I tried entering my grid, but nothing happened, may be because my region is far away from the range of data which they have in the database! Of course, I have not heard of any radio amateur communicating in the microwave range in this region except on QO 100 Geostationary Amateur Radio Satellite. If you sign up on rainscatter.com and post your location, others who wish to have contacts will be able to see it.

It has been mentioned that 5.7 GHz and 10 GHz are the optimum bands for Rain Scatter propagation. In fact, 10 GHz has been called as the “Rainy Day Band”. That is because of expected rain particle sizes favour these frequencies. While a tropical downpour which we experience in this region can have a rain particle size of 3mm, drizzle has a particle size of around 0.5mm. Fog or mist particles can be from 10 microns to 1 nanometer. From this description, I would expect lower frequency to be useful in my region, though I am yet to see information on that, as mentioned earlier. For actual operations, both stations should be in range for the rain cell which is the scatter point and antennas should be pointed at it. CW works better, while SSB can have distortion of audio as in Auroral Propagation due to Doppler spreading of signals. May be some of our friends in this region could turn their 10 GHz QO 100 antennas towards rain cells and try Rain Scatter QSOs! Wishing them all success as I hear heavy rain outside now.