First, do a rain dance like this! *Rain dance* (Or not. xD)
Next, open FH and go to "Sky Maker"
When you're in this, click the "Weather" tab, and here's what you can do to make your very own weather for a map.
Let's say you wanted to make it rain lightly, on and off, throughout a period of time.
1.) In your Weather tab, click the auto-weather (as I call them) called LightRain.
2.) Then, Load. You'll notice that it begins to rain in the map.
But! Let's say you want the rain to be even lighter, and you want the sky to look a little different when it does come to rain.
3.) Where it says "Rate," you can fiddle with the numbers a little bit. Let's drop it down to 200, and you'll note immediately that the rain becomes considerably lighter.
4.) Want to change the sky when it rains like this? Toy around with the Sky Top Colour, Sky Bottom Colour, and Cloud Colour until you're happy with the appearance of your weather.
5.) Finally, at the top of the Weather tab, re-name your weather (SprinklingRain, for example), and then hit Save.
You now have your own weather. But how do we throw this into a map? Here's how;
1.) Now that you have your SprinklingRain, tab over to "Cycle."
2.) You'll see oodles of controls and long bar stretching across your screen. Go to where it says "Weather:" and click on your new, light rainy weather.
3.) Let's say you want it to be raining lightly when you first come into the map. In the box beside Duration, put a large number (let's say 1000) into that box. Each number is a short unit of time that makes up the full cycle of weather for your map.
4.) Click "New Weather." You'll then see a block appear in the long bar.
5.) Let's say we also want the rain to fade out slowly (instead of stopping abruptly). Change the Duration box to about 500.
6.) Check the box that says "Fade Out."
7.) Click "New Weather." You'll see a shorter box shaped like a ramp appear.
Continue this process to your liking. Is a box too long or short? Click the little scroller below the weather bar, and put it under the desired bar you want to delete and click "remove."
Want to start a new weather cycle? Hit "Clear All."
Remember that the scroller is very finicky, and wherever you place the scroller is where your new weather will be placed.
And when you're happy, give it a File Name (HackersWeather), and hit Save.
So the next time you're map making, go to the World tab in Map Maker, type in HackersWeather (or whatever you named it), hit enter, and test the map. You'll have your weather in the map. :3
Hope this helped!