Select the Set Weather Effects command (it's on the second page of event commands, at the bottom of the first group in the right- hand column), and you'll get presented with a box. This box is pretty simple to operate, but I'll go through it anyway. First of all, the weather type. None is - well, it cancels any other weather effects that you have going at that time. It's the default, no effect weather. Rain is rain streaking down from top-right corner of the screen. Storm is like rain, only with slightly thicker drops, and they fall at a slightly sharper angle. It doesn't have thunder and lightning built in, that's a do-it-yourself addition. And finally, Snow is white whirling flakes that fall in the same direction as rain.
The power can be set for Rain, Storm or Snow, from 1 to 9, five being average. When with rain, the strength makes more raindrops fall from the sky. It's difficult to show in images, so have a go and see what I mean. With Storm, the effect is much the same, and finally, with Snow, a higher power makes more flakes fall.
Lastly, the time value tells you how many frames the weather effect will take to appear. I usually go for something around 30, but it is completely up to you.