UPDATE: This way was designed only for 1.10.x or below, though it still works in 1.11!

The following tutorial context is only necessary for 1.10.x version of Minecraft or below. Since 1.11 snapshot 16w38a, though this design may still work, there's actually a brand-new and much simpler way to have the same purpose:

• Type the command once in your map: /gamerule doWeatherCycle false to solve the problem as described below.
  

===== THE TUTORIAL BEGINS HERE =====

Why do you need this mechanism?

If you are designing a survival/adventure map for Minecraft, you may know that building the map is a really time-consuming task, so you might not wish anything that disturb your hard work unexpectedly. By switching into creative and peaceful mode, it is very easy for you to prevent from being attacked by any hostile mobs. However, it is not that easy for you to keep the weather sunny forever, while a lightning blot in the thunderstorm weather might ruin your wooden house (or in other flammable matrrials) immediately and unexpectedly! Therefore, it is important for you to have a machine that prevents from such a scenario.

Here's a really compact design for this mechanism!

For those who desire to design a survival/adventure map for Vanilla Minecraft, here is another one-image tutorial for the simplest and the most compact mechanism for you to keep your world always sunny! As the same way in my previous tutorial for designing a simple trash can, you can also make this one if you only see this image:

How it works

• The command /weather clear 1000000 sets the weather of the whole world for 1 million seconds (instead of 1 million ticks), that is, more than one week in the real world! (not in the Minecraft time)
  • Can the argument 1000000 get even larger? Unfortunately, 1000000 is the maximum limitation, that is, it's impossible for you to ensure the weather to be sunny forever without any commands triggered afterwards.
    
  • But, don't worry, here's the very solution! In fact, this mechanism is designed for all of you that are to build your world for months or years!
    
  
  
• This design assumes that the day/night cycle is enabled and cheats are allowed in your world. The daylight sensor is triggered whenever the moment of sunrise occurs. Since there are 24,000 ticks ( = 1,200 seconds = 20 minutes) in one day in Minecraft,
  • By the use of daylight sensor, you may even decrease the time argument in the /weather command to a number that slightly more than 1,200.
    
  
  

Advanteges

(1) Place once, work anywhere.

You may only place one at any corner of your world, as long as the sunlight is reachable. Well... you might be thinking of a problem, does it still work in single player mode if the player is too far from the machine? In fact, surely it does! To verify the fact that it really works, you may teleport yourself to a few kilometers from the location you put the machine, and make sure that /gamerule commandBlockOutput true is set on. Then, you may still see the command block message in the next sunrise!

(2) Compactness

This design requires two blocks only, so you can simply put this machine inside a small room of your wooden house! (better put it next to a window, so the sunshine can get into the room)

(3) Friendly for low-grade CPUs

Some possible alternatives that repeatedly run the /weather clear command could use a redstone pulse or a repeated command block (available since Minecraft 1.9), whereas these designs run the command too frequently, which might cause some lags if your CPU is not fast enough.

(4) Supports earlier Minecraft versions

This design is applicable for Vanilla Minecraft 1.5 - 1.10+ versions, as the daylight sensor was added in the Minecraft 1.5 update.

The use in my Minecraft world "Republic of FlashTeens"

The machine is put in the second level of FlashTeens First House (which had been an abandoned bedroom since the new FlashTeens House was built), as the following image shows:


By my testing, this machine still works even if I am at the Sandyland Town area, which is at least 4 kilometers away!

If you like this design, don't forget to give a diamond and subscribe me! And, why not trying this simplest machine in your work? :D