I made it lieke this:

BMAP test_map,<test.bmp>;

function snowfall()
{
rain_mode = 1;
while(1)
{
rain_pos.x=camera.x;
rain_pos.y=camera.y;
rain_pos.z=camera.z; emit(25,rain_pos,particle_rain);
wait(1);
}
}

function particle_rain()
{
if(MY_AGE == 0)
{
my_pos.z =
camera.Z + 50 + RANDOM(50);
my_pos.x =
camera.X + RANDOM(1000) - 500;
my_pos.Y =
camera.Y + RANDOM(1000) - 500;
MY_SPEED.X = 5-RANDOM(10);
MY_SPEED.Y = 5-RANDOM(10);
MY_SPEED.Z = -10 - RANDOM(5);
my_size = 1000;
my_map = test_map;
my_flare = on;
my_alpha = 90;
end;
}
if (MY_POS.Z < camera.Z - 1000) | | (MY_POS.Z<0)
{
MY_ACTION = NULL;
}
}