Here is a code i made for you!

Code:

---

bmap the_rain = "rain.pcx";

function remove_me();
function fall();
function raindrops();



function main
{
	level_load("YOURMAP.wmb"); // load your map
	wait(3);
	raindrops(); // start the weather effect
}

function raindrops()
{
	while(1)
	{
		vec_set(temp, vector(random(1000)-500,random(1000)-500,300)); // Choose your size its in a random area between 1000 and -500 quants. it starts on a high of 300 quants
		ent_create("rain.pcx", temp, fall); // create the raindrops
		sleep(0.2); // Choose your own speed, you can also use wait
	}
	
}

function fall()
{
	my.enable_entity = on; // sensitive for entitys (player)
	my.enable_block = on; // sensitive for blocks (level)
	my.event = remove_me; // The event who starts when he hit a entity or a block
	while(my != 0) // while i´m not removed
	{
		c_move(my, vector(0,0,-8*time_step), nullvector, 0); //c_move without "glide" you can change the numbers in vector if you want ti drop it more fast and maybe a bit angular
		wait(1);		
	}
}

function remove_me() // the event
{
	if(EVENT_TYPE == EVENT_block) // if raindrops hit a wall or a buildung or whatever
	{
		wait(3); // you can take a higher value if you want your raindrops life longer. better do that if you need snow
		ent_remove(my); // remove raindrop
	}
	if(EVENT_TYPE == EVENT_entity)
	{
		wait(3);
		ent_remove(my);
	}
}

---

You can change your weathereffect if you use variables. For example you can create a variable with the name "rain_amount = 0.2" you can use that in the sleep(rain_amount) in the raindrops function. In another script you can simply change this value and enforce the rainfall or damp it.

Greetings and have fun!