How will I make a chance for a biome to occur?

[SpiritDeluxe (Taxes)]

I want to make a rarity system for a biome to occur, for example:

There could be a night biome:

And then a day biome:

And then whatever this is with the rarest chance of occurring:

It sounds easy in my mind, but I don’t know where to start on the rarity thing.

[Physicism (Physi)]

To make it a random chance, I would organize the names of each biome into a table.

local biomeNames = {"Normal", "Night", "Weird", "Snowy", "Lava"}

Then, by using loops and math.random:

while wait(120) do -- change biomes every 2 minutes
    local newBiome = biomeNames[math.random(1, #biomeNames)] -- random selection
    if newBiome == "Normal" then
        -- stuff here
    elseif newBiome == "Night" then
        -- stuff here
    elseif newBiome == "Weird" then
        -- stuff here
    -- same for the rest of the biomes
    end
end

[SpiritDeluxe (Taxes)]

How would I add a rarity system to this?