tldr; OP looking for an excuse to use his geography degrees
Some time ago, @Killaburn posted a [possible map of Runeterra](http://i.imgur.com/EAkP2la.png) based on evidence from the [Medarda Heirloom](http://na.leagueoflegends.com/en/featured/piltover/medarda-heirloom) and other sources. With his permission, I've been developing a satellite-style version of this map, like you might find on Google Earth. To do this, I need to determine what the physical geography of this Runeterra is like. This post is the first part of that process, which looks at ocean currents and weather.
(Note, I made some edits to Killaburn's original map so it would be easier for me to work with, and some changes that I thought made more sense geographically. Hope that's ok! Also note, all this is assuming Runeterra has similar shape, size, and rotation properties as Earth.)
Before I can do any climate work, I needed to figure out how big of an area I'm working with. Different climate characteristics are generally associated with certain latitudes. To determine the latitudes, I overlaid an outline of the Runeterra map onto a map of Earth, then adjusted the size until it lined up correctly. This was the result...
(Unconfirmed locations end with "?" and are based on Killaburn's logical conjectures. Other locations are based on Medarda map. Also, note the distinction between Shurima Continent and Shurima the political faction/region)
Why this size? From the Medarda Heirloom, we have a good idea of the locations for Piltover, Noxus, Shurima, Bilgewater and Ionia. From the sailing times given on the Heirloom, I decided that the length of the sea depicted there was about the size of Earth's Mediterranean Sea. So, it was just a matter of lining up that portion of the Middle Sea from Killaburn's map (about the eastern-most quarter) to our Mediterranean, and then aligning several Runeterra regions with the latitudes appropriate for the climates we know exist there. For example, Shurima _needed_ to be on the same latitude as the Sahara desert, (around 20 degrees North), and Kumungu _needed_ to be at a latitude near the equator to support tropical jungles. Freljord of course had to be in the north, and Noxus needed to be in a more temperate climate (if Noxus was atop a mountain or in its own desert, I think we would have heard something about it, so temperate climate seemed a logical choice). The end result places most of the Valoran and Ionian continents at the same latitude as Europe, Russia and Canada, while Shurima is at the same latitude as northern Africa and the southern US. From the base map of Earth, I assigned latitude lines to Killaburn's map. Freljord is a bit further north than I'd like, but everything else landed in a reasonable climate.
Cartographer's note, I'm using a mercator projection here. This means that areas near the poles are depicted as bigger than they really are. (If you want to see this for Earth, zoom out on Google Maps and compare Greenland to Africa; though Greenland looks bigger, it's only about 1/14th the size of Africa.) As a result, Freljord and the northern part of Ionia are not as big as they seem. I chose mercator because it's easier to work with for what I'm doing, and because it was easier to line up regions with their climates; several diverse areas are relatively close (Shurima, Piltover and Noxus), while Freljord is pretty far north. With mercator, I can wave off some of that distance as simple map distortion.
With latitude lines in place, it's time to determine our ocean currents. We need this to determine weather patterns and likely climate zones. On Earth, ocean currents move in giant circles called gyres. The Gulf Stream carries warm water north along the eastern US coast while the Canary Current returns cooler water down the coast of Morocco. Using Earth ocean currents as a base, I decided these are the likely currents on this part of Runeterra...
Red are warm water currents, blue are cold water, and purple are not extreme cold or warm. Warm currents are generally associated with moisture while cool currents with dryness. The warm current that circles up from near the Shadow Isles and passes between Bilgewater and the Shurima Continent is similar to Earth's Gulf Stream. This current will provide warm water for hurricanes to strike eastern Shurima Continent and Bilgewater, and help steer warmth and moisture to the southwestern shore of Ionia. On Earth, the Atacama and Kalahari deserts are both along shorelines with cold currents. In this version of Runeterra, we see cold currents along the northeast shores of Ionia and Valoran and the southwest shore of Shurima. The area around Targon would probably be pretty dry.
The Middle Sea between Valoran and Shurima Continent is somewhat problematic. The closest parallel we have on Earth is the Mediterranean Sea. While the Mediterranean does have its own currents, they don't play a role in larger ocean circulation. Because of the hot, dry areas around the Mediterranean, ocean water flows into the Mediterranean at Gibraltar to replace evaporated water in the eastern end of the sea. The Middle Sea here is different for two main reasons. One, it's much larger than the Mediterranean and has fewer obstructions (like the Italian peninsula). Two, it has openings at both ends. The Middle Sea would see extreme evaporation along the Shurima Continent shoreline, just as the Mediterranean does along Africa. But on the other hand, the northwest portion of the Middle Sea is in a temperate zone and would have more rainfall than any part of the Mediterranean. In addition, the Middle Sea would be the destination for rivers from both continents. So, we have water leaving via evaporation, and water coming via rain and runoff. Ultimately, I don't think the Middle Sea would have currents that strongly impact weather the way the bigger ocean currents do. But, this sea could serve as a source of moisture for storms to bring rain further east on the Valoran continent than they normally would (like lake affect snow, but for rain, and on a bigger scale).
And this brings us to.... THE WEATHER. On Earth, prevailing winds generally blow from east to west between the equator and 30 degrees North. This is why hurricanes move from the central Atlantic towards the Caribbean and North America. From 30 degrees to 60 degrees, winds generally blow from west to east. This is how a lot of North America gets its weather. Storms come down from Canada or the Pacific Northwest, sweep over the Midwest and exit over New England or the Maritime provinces. So, if we take what we know of prevailing winds, and how currents impact local climates, we can get a sense of what most ground covering on Runeterra would be like...
(Quick note: this assessment does not take into account mountains and the impact they can have on local climates; that will be addressed in Part 2.)
Pink lines are the main prevailing winds.
The first thing we notice is how wet the eastern end of the Shurima Continent is. This makes sense for the lower area with Kumungu Jungle (comparable to northeastern South America, on the edge of the Amazon). But the northeast corner of Shurima Continent is problematic. This area should be like the Caribbean or southeastern US (Florida especially), given its proximity to the ocean. Not really what you think of when it comes to Shurima. For comparison, look at the Sahara and then east along the Tropic of Cancer. Most of that latitude passes over land until you reach Southeast Asia, where storms from the Pacific dump a lot of rain. At that latitude, the eastern end of a big land mass should be wet and the west end should be dry.
But, there could be an explanation. The Medarda Heirloom depicts a large river system draining that exact area; the rain has to come from somewhere. It should probably be reaching further west into Shurima, but there could be an undepicted mountain range, or magical effects from the Rune Wars, keeping the moisture out. The border between wet Kumungu and dry Shurima would be grassland, similar to the Sahal south of the Sahara.
Thanks to prevailing winds and the cold current off of Targon, much of Shurima is dry. I'm not sure how far along the southern coast the jungles would extend before being replaced by grassland (comparable to the llanos tropical grassland in Venezuela and Colombia). Depending on the season and specific storm trajectory, moisture could come from the nearby coastline to extend the jungle for some distance to the west, though the grassland would not be far inland beyond that.
The northwest of Shurima Continent would be like a different world. This area extends up through the temperate zone. This is comparable to the west coast of North America. Killaburn placed Bandle City pretty far north (roughly Juneau, Alaska), but with the warm current, it wouldn't be too bad. The very northern tip would extend to taiga/boreal forests. If there are mountains, expect glaciers that drop icebergs into the Middle Sea.
The Valoran Continent is a lot like Canada and the Great Lakes/Northeastern US, though possibly wetter. In the northern hemisphere, climate bands generally go from northwest to southeast. The clearest example of this is in Canada where the border between the prairie and boreal forests and then tundra run in a line from upper Alberta and Northwest Territory down towards the Great Lakes and Hudson Bay, respectively. The northern portion of Valoran would be tundra with forest further south. In this map, Freljord ended up way way way north, about on the level of Greenland. Similarly, it should be in an icecap. I envision Freljord as more like Scandinavia though, so will probably move it a bit further south before moving forward with the project.
The southern part of Valoran would be more hospitable. This area would probably be wetter than the Canadian Prairie or the Great Plains in the US, thanks to the Middle Sea. While the Middle Sea isn't large enough to establish regional climate changes the way the ocean currents do, it would still serve as a source of moisture for passing storms. Winters along the southern shore could be very snowy as a result. Summer would be mild. There would probably be more forest than grasslands along the shore.
Any large prairie in Valoran would have to be on the plain between Noxus and Piltover. This is the only place that could be dry enough and warm enough for such grasslands. This area is also the most likely to form tornadoes, given the warm, moist air coming up from the Middle Sea and the ocean off Bilgewater. Eastern Valoran shore would be similar to the northeast US, but more mild during winter.
The northern half of Ionia would be forest or tundra. Great place for new things to appear. The southern half would be temperate. Southwest Ionia would see significant rain, comparable to England. That large bay would be especially comfortable, and likely to form the core of a local trade network.
The Shadow Isles should have a climate similar to Cuba. Bilgewater is in the perfect place to be regularly lashed by storms. During the winter, storms coming off Valoran to the west would strike the islands. During the summer, tropical storms would batter the islands from the south and southeast.
The main thing missing here are mountains. A few mountain ranges can change things significantly. Much of eastern Washington state is dry thanks to Seattle and the Cascades catching the moisture from incoming Pacific storms. Here, a mountain range south of Piltover could extend Shurima's dryness further east. A coastal mountain range along the southern edge of the Shurima Continent could limit how far the Kumungu Jungle spreads. Mountains in the middle of south-central Valoran would lead to a dryer Noxus and more grassland further east. Similarly, mountains in southwestern Ionia would result in a dryer interior to that continent. These are the things I hope to explore in the next part of this project.