It not a paticular wave length( though some waves are a bit better) most of the radars are in the same range. It is matter of setting the controls. In the case of our Raymarine
unit you can set the unit on automatic and it will adjust various gains to cut through rain and background clutter, but it usually sets it to eliminate the most clouds and background clutter.
We set the gains manually it was some trial and error to find the setting the provided the best combonation to see heavier clouds that could produce rain and making it too sensitive where all clouds were picked up.
We then programed this setting in. This way we could change settings should conditons warrent but be able to always go back. It did put more returns on the screen, but we rather see more and figure out what it is than screen it away and be suprised later. In heavy traffic areas we would detune so as not to be overwhelmed by traffic clutter and then at the press of a button we could switch back.
The other advantage is the setting didn't preclude ship hunting. in most cases the more sensitive control settings allowed us to pick up thing that might have been tuned out at longer distances or more questionable returns.
If I wasn't clear let me know and I will try to reword.