Weather is calculated individually for each battle. 3d20 are rolled to determine the weather makeup. The final weather is posted along with battle announcements each turn. This indicates the 'average' weather for the week and is more like a prediction than a certainty. It is possible to wait for weather to change, but don't forget the opponent might not be willing to let one!
The rolls can be found on Invisible Castle and a link is posted with battle announcements. What follows is what the rolls mean, and the basic forms of each plane in weather and construction. But beware! There is a very small chance (3/200) that a battle will have a disaster instead of or in addition to the rolled weather! This is entirely up to whims and what looks fun at the time.
17-19: Heavy Rain
17-19: Strong Wind
Dominaria is very similar to Earth, just a lot bigger. So much bigger that not all of it is represented on the given map. Terrain and weather are the same as one would expect however. There is even a wide variety of climates and terrain features, so anything that is likely to be found on Earth will be easy to find on Dominaria as well.
For completeness, bodies of water are made of water. There is a clear day/night cycle with 24 hour days, just like Earth.
17-19: Light Rain
20: Acid Rain
17-19: Strong Wind
Phyrexia is a designed plane, and thus very different to the standard planetary system. It instead consists of nine nested spheres; it thus lacks both celestial bodies and a standard atmosphere. Weather (in the form of wind and rain) is imposed by machinery operated elsewhere to keep things running smoothly. While if one flies high enough, one will hit a ceiling before running out of air - this isn't always the best idea though, as there are constant thick (and unhealthy) clouds in the way.
The first sphere is where the war takes place. This sphere is a artificial mockery of life. Plants and animals exist and act like living creatures, but are closer to machines than organic creatures. Plant nourishment is provided from deeper spheres eliminating the need for rain and preventing droughts from occurring. For game purposes however, it is easiest to treat the terrain as ultimately similar to Earth. What one find and do with materials on Earth also applies for Phyrexia for all intents and purposes. It must be noted that the whole plane is kept at a constant temperate temperate however, so there is not the full range of potential terrain forms present.
Being a mechanical plane where acid falls from the skies, the bodies of liquid aren't exactly water. To be precise they are various acidic and oily compounds. Not safe for non-Phyrexian creatures to drink! There are no significant ill effects from immersion over the course of a battle however (in the short term anyway, the ultimate results of Phyrexian oil is well known). For the sake of convenience, a day/night cycle similar to Earth also exists.
Number of Moons
20: Heavy Rain
17-19: Strong Wind
The plane of Mirrodin is a designed pocket plane, consisting of the planet itself and five orbiting moons (although also called suns). Everything native to the plane is made of metal, and introduced lifeforms will tend to develop metallic segments from long (generations) exposure. As such, the many creatures, while truly organic are still part metal. Even the plants are metal; the trees of The Tangle are corroded copper, while the grass of the Razorgrass Plains are actually razors. Ultimately, each area is relatively uniform in what is present, so finding mixtures of terrains is difficult and time consuming. What these lands appear as can be directly seen in the MTG Arcana.
As this plane is a lot smaller than either Dominaria or Phyrexia, it is possible to move over the edges of the map. Since the ground is also metal, digging isn't the easiest of tasks. Even then, the plane is actually hollow. The best way to access the center is through lacuna, located at The Radix, the Seat of the Synod, Taj-Nar, Kuldotha, and Ish-Sah.
The five moons of Mirrodin (White, Blue, Red, Black, and Green) have different orbital paths and speeds. Yet there is always at least one in the sky; which ensures the usual concept of day/night simply does not exist. Throughout a battle week, the number and which moons in particular are present will change unless any are specified as dominant. However, the more moons present, the brighter and hotter it is - especially with all that metal around! The darkest and coldest period is when only the Black moon is present, while the hottest and brightest occurs when all are.
While the atmosphere is similar to that of Earth with the usual clouds, wind, and rain generated through normal means, not all liquid is water. The large body is instead known a liquid known as 'quicksilver' (not actually mercury, just flows like mercury). This is not drinkable, and in its current form tainted with Phyrexian oil. Immersion does not impact units differently to water however. The rain though, that is usual water.
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