why do you need a tutorial for it ? All you have to do, is to do ctrl + alt + 2, and search "weather", it will find you this opcode :
set_weather 1
Use it in your cleo script, and put if condition before that line, and done.
:test
wait 0
if
0AB0: key_pressed 123 //F12 key
else_jump @test
01B6: set_weather 11 //a clear sky
jump @test
Change 11 to a random number to get a different sky, and that's all >.<
