Weather will be given in enough detail to explain how it works, what it does and anything else notable.
How it may work
Everyone is looking for random weather, and from my excessive search, I got a good idea on how it might work:
The day will be broken up into 7 parts: Midnight, sunrise, mid-morning, noon, mid-afternoon, evening, sunset. The game would determine weather at each part of the day. It would have 2 stages to go through to pick weather:
1: Change or not?
There would be something like a 75% chance that the weather wont change, a 20% chance to change and 5% chance to reset it to sunny.
2: What Weather?
If it decides to change weather, then it would pick the weather from the list. "Safe" weather would have the highest percent chance while "harmful" weather has a lesser percent chance. When biomes are added, certain weather would be disallowed (like snow for a desert), and thus not be able to be picked.
With this system, it would be best to keep the game running global weather rather then regional, otherwise large amounts of lag will occur at these times during a day. The system would also run when you start a world.
Rain, snow and fog type stuff. Visuals and speed are the most that are effected.
Causes it to rain.
The current classic rain graphic will appear. Additionally a forced fog will be placed will a radius slightly less then the "normal" setting. If you have your fog setting smaller then this, it will not change.
It rains harder, and it starts to puddle.
This will only happen if it is raining already. The rain graphic will be much denser, think slightly less then leaves, and the rain fog will still occur. If possible, low areas will start to gather puddles (the game would create a source block and generate water according to its location, but it will not be running water). Puddles will dissipate shortly after it stops raining.
Causes it to snow.
It will start to snow. Fog levels will be brought to the "normal" setting. Snow will not start to accumulate. If weather is at "light snow" for more then 2 parts of a day, it will automatically switch to snow.
Causes snow to start piling up on the ground.
This will happen if its already snowing, or the game could skip to this. Fog levels will be brought to "normal", and snow will pile up in 8th blocks (like it is right now).
Causes a blizzard.
This can only happen if its lightly snowing or snowing. Snow will pile up in 1/2 blocks and spread under covered objects (like into caves, under trees in a similar fashion as water/lava). Fog will be set to rain fog (just under normal). When a blizzard stops, the snow levels quickly revert to the normal 8th snow block (not instantly, but quickly).
New block - 1/2 snow block
The 1/2 snow block is not a solid block, and you can/have to walk in it, though there will be some difficulty (like a lesser extent as walking in water). When destroyed with a shovel, you will get 4 snowballs.
Causes everything directly under the sky to be slippery.
This can only happen if it is lightly snowing or harder. Everything directly under the sky (no blocks above it) will become a little slippery (think the ice block). Once the weather stops, everything will stop being slippery. Fog is not affected.
Causes early morning fog where applicable.
Fog levels are brought to small from sunrise to mid-morning. Can only happen in high places (say 80 BASL [Blocks Above Sea level]) but will almost always happen (80%+ chance). Blocks 65 BASL and higher will have fog that builds up to the fog at 80 BASL.
The normal weather option. Fog levels will be brought down to slightly larger then "small".
Causes heavy fog to sweep over the land.
Fog levels are dropped right down to tiny. Can only happen if it is Dense Fog.
Causes a heavy breeze.
If wind is added to the game, this is not it... this is a stronger version. Wind will slow movement down 75% when you are not shielded by a wall or object 2+ blocks high. If you are standing still, you will be moved in the direction the wind is blowing (the way the clouds move, which are sped up 180% [100% being their normal speed]). Items that are loose on the ground (the object created when you press Q) will be slowly moved in the direction the wind is moving. You are not effected if you are underwater (totally underwater).
Notes: A wall will stop wind only for X blocks depending on how high it is. X is determined by how high the wall is (every 2 blocks high is 1 block free of wind behind that wall).
Causes clouds to expand, covering the sunlight.
Days become 3 shades darker. That's it.
This weather is somehow harmful to the player. They are less likely to happen, but require more care when they do.
Causes thunder, lightning and rain.
it will start to rain. Additionally, every few minutes in-game (so 30-45 seconds IRL), lightning will be generated somewhere within your render distance. Thunder will be played louder the closer you are to that strike. Lightning will hit the highest object within 14 block of its generated location, and will damage any mob/player 7 hearts if they are hit. Fog levels are dropped to "normal".
Causes acid rain to fall.
When acid rain occurs, a green tinted rain graphic will appear. All mobs under the sky will be dealt half a heart per 4 second. Blocks are not destroyed by this because that would completely destroy the map until everything is rock or water (a world without trees, dirt, sand...etc). Plus newly generated terrain would look new and untouched. Fog levels are dropped to normal rain levels (just below normal).
Causes hail to fall.
Hail would be a larger version of the snow graphic, and would damage mobs/players 1/2 heart of damage every 2 seconds. Fog would be dropped to normal.
Causes painful hail to fall.
Any mob/player outside would be hurt 1/2 hearts every second. Fog would be dropped to half-way-between-normal-and-small. Can only occur if its already hail weather.
Causes a painful windy dust/sand storm.
This would act similarly to windy (75% slower, wind...etc), but everything that is affected by the wind is also dealt 1/2 heart of damage per 5 seconds.
Causes a funnel cloud.
When generated, a large gray translucent funnel will appear somewhere in you render distance. This will not deal damage, but rather send you and other mobs flying sky high, only to feel drop damage.
Creates a whirlpool in a large ocean.
Less of weather, and more of an event, this can occur during weather events. A spinning vortex will suck you and other mobs down into the pit (probably 10-20 blocks below sea level. If you have little oxygen, look out.
At night, it will cause something similar to the northern lights.
This only happens at night. It will have an awesome graphic played in the starry night sky, and light up the night. How is this harmful? Mobs will be able to follow you from a greater distance, and Creepers seem more plentiful during these rare nights (SSSsssSSSsss).
Day is replaced with night.
Happens very rarely, but the sun is blocked by the moon. Night is extended until the next day. TIP: Don't remove that iron armor you are wearing.
There is the weather that has been publicly denied for destructive reasons, but I feel that they should be an option in-game. Of course those with amazing castles would turn it off, while survivors would probably turn it on.
Causes the ground to tremor and destroys blocks to create a ridge.
It would simply cause the screen to shake, and would create a huge ridge somewhere within the generated world (maybe in your render distance, maybe not). Would not harm the player, but would make mining take twice as long (as if you were mining under water.
Meteors fly in from space producing a small supply of diamond (or other crystalline ore if one is added).
Imagine you are hunting a cow for leather when you spot your prey. You approach and BAM, a meteor kills it, and leaves a nice crater. You grab your iron pickax and get to work. Of course it would be less funny if it hit your home instead.
Causes trees to randomly catch fire.
I do think an explanation is useless. Trees randomly catch fire in places that are hotter then the norm (or during a heat wave). Snowy maps are immune to this, naturally.
100% useless and/or repetitive. Most would say "No. Just no."
Too much. Wind is already painful enough, but any more would not only be repetitive, but way to annoying to deal with.
A huge meteor hits the world and all but rock is destroyed. Effects newly generated terrain.
I don't believe it has been suggested yet, but something this big would be past a devastating event. Respawning should be disabled if this happens because the world is just too... dead.
Things I found that are interesting:
During my 12 page search, I came across some threads that held some interesting ideas dealing with weather. In no way am I linking ideas in this thread to them, but rather showing them here. You are responsible if you are going to bump any of these. AKA: DON'T BUMP THEM!
Weather by day time (inspiration for the weather system stated above)
Cloud Mechanics (Interesting aspect of clouds)
Isobars and Difficulty (Interesting take on weather systems and difficulty)
Also, some interesting things posted here:
It's symbol will be similar to the water bubble, but with snowflakes.
When it snows outside, and you're not wearing the right equipment (uses for wool!), the cold bar will accumulate (instead of decreasing). Once it is filled, you start taking periodic damage. Being near a fire (magma or actual fire) will reduce the cold bar quickly. Being in-door (no direct LOS to the sky) stops the bar from accumulating (if it was full, drop to 9.5 filled).
Can also add a new weather "Heat Wave", can tie into "Forest Fire". On player it is similar to cold, except now its heat bar. Accumulates when you are out-door (sky above you) and take damage when full. Standing in water reduce it quickly. Being in-door reduces it slowly. During heat-wave, leaves may spontaneous combust, triggering a Forest Fire (so if you're taking under the shade of tree, beware that said tree may catch on fire.
Fire and lava should also increase heat bar (albeit much more slowly).