How do I make rain?

Never thought about this! Good idea! I guess you can make rain by first creating 2 models:

1) The rain model (just a very very tiny and straight blue brick with cylinder mesh, like you said):

2) A big transparent brick in the sky covering the entire zone where it rains, let's call it CLOUD_SPAWN.

After that you can get the Vec3 angles of the CLOUD_SPAWN like this:

A########
#########
########B

We can define the X and Z in this way:

 --Remember to set CanCollide to false and Transparency to 1!
local CLOUD_SPAWN = game.Workspace.CloudSpawn
local xCoord_START = CLOUD_SPAWN.Position.X
local xCoord_FINISH = xCoord_START + CLOUD_SPAWN.Size.X
local zCoord_START = CLOUD_SPAWN.Position.Z
local zCoord_FINISH = zCoord_START + CLOUD_SPAWN.Size.Z

Now we need to do the spawning code:

function spawnRainBrick()
    local randomXCoord = math.random(xCoord_START, xCoord_FINISH)
    local randomZCoord = math.random(zCoord_START, zCoord_FINISH)
    local rainBrick = game.Lighting.RainBrick:Clone()
    rainBrick.Parent = game.Workspace
    rainBrick.Position = Vector3.new(randomXCoord, CLOUD_SPAWN.Position.Y, randomZCoord)
end

After this we need to make a loop that check if your rain event is active, let's call it eventRain.

-- This function will check if in the game is raining each second
function rainUpdate()
    --I suppose to have a model called "Events" wich contains every event value as a BoolValue
    while game.Workspace.Events.eventRain.Value == true do
        spawnRainBrick()
        wait(0.3)
    end
end

This will spawn a rain brick each 300ms (3 ds (deciseconds)) But what if the game lags? If the game lags it's because there are a lot of rain bricks, we need to destroy them when they touch something =), this is the code:

function destroyRainBrick(part)
    part:Destroy()
end

And then we need to call it in another function or just outside everything, I recommend to do outside everything (so inside the script's body)

for _, workspacePart in pairs(game.Workspace:GetChildren()) do
    -- The rain brick model need to be named as "RainBrick"
    if workspacePart.Name == "RainBrick" then
        workspacePart.Touched:Connect(destroyRainBrick)
    end
end

FINAL CODE:

 --Remember to set CanCollide to false and Transparency to 1!
local CLOUD_SPAWN = game.Workspace.CloudSpawn
local xCoord_START = CLOUD_SPAWN.Position.X
local xCoord_FINISH = xCoord_START + CLOUD_SPAWN.Size.X
local zCoord_START = CLOUD_SPAWN.Position.Z
local zCoord_FINISH = zCoord_START + CLOUD_SPAWN.Size.Z

function spawnRainBrick()
    local randomXCoord = math.random(xCoord_START, xCoord_FINISH)
    local randomZCoord = math.random(zCoord_START, zCoord_FINISH)
    local rainBrick = game.Lighting.RainBrick:Clone()
    rainBrick.Parent = game.Workspace
    rainBrick.Position = Vector3.new(randomXCoord, CLOUD_SPAWN.Position.Y, randomZCoord)
end

-- This function will check if in the game is raining each second
function rainUpdate()
    --I suppose to have a model called "Events" wich contains every event value as a BoolValue
    while game.Workspace.Events.eventRain.Value == true do
        spawnRainBrick()
        wait(0.3)
    end
end

function destroyRainBrick(part)
    part:Destroy()
end

for _, workspacePart in pairs(game.Workspace:GetChildren()) do
    -- The rain brick model need to be named as "RainBrick"
    if workspacePart.Name == "RainBrick" then
        workspacePart.Touched:Connect(destroyRainBrick)
    end
end

This is a SIMILIAR code, I hope it works because... WARNING: The code is not tested and may not works, if it doesn't then send me a PM with the problems given in the output! Bye!

Before we begin, Insert a brick into the workspace and color it and size it, Etc. (Name it: Drop)

while true do
    wait()
    local rain = game.Workspace.Drop:Clone()
    rain.Parent = game.Workspace
    rain.Position = Vector3.new(math.random(-500, 500), 100, math.random(-500, 500))
    rain.Anchored = false 
    wait(2)
    rain:remove()
end