When it rains in Middle Earth, I'm reminded of the lyrics of an old Benton Brook song:
"A rainy night in Georgia, such a rainy night in Georgia
Lord, I believe it's rainin' all over the world
I feel like it's rainin' all over the world
How many times I wondered
It still comes out the same
No matter how you look at it or think of it
It's life and you just got to play the game...."
Because when it rains in Middle Earth, it does seem to rain all over the world.
Recently a study was published by a climatologist at the University of Bristol who signs himself as "Radagast the Brown". Said scientist applied a climate model first used to model long-term global weather patterns in the present day and during the late Cretaceous period to Middle Earth itself - using data found in Tolkien's maps and in climatological and geological descriptions from LotR, The Hobbit, and other sources.
You can find the study here. (You can also find the same study in "fake" elvish and dwarfish script from the same site, but since the languages are direct translations and grammatically dissimilar to Tolkien's own language inventions, I wouldn't bother.)
I realize that creating an accurate and comprehensive pattern of weather all over Middle Earth has no profitable value to Turbine or LotRO. Even so, the study is of value to anyone interested in the finer points of Tolkien's world-building.