Optional: Activate the Records of Eminence 15th Vana'versary IV objective Red Rover (W) while hunting for the Carbuncle's Ruby, for sparks and experience.
Alternatively, Bloodsuckers in Toraimarai Canal may have the highest drop rate. However, the area is difficult to get to for newer players, and there are only two spawns - at H-9 and J-9 on Map 1.
The rate listed by FFXIDB is about 1% without TH, and 3%+ with TH3+.
The drop rate is low, so if you're specifically looking for the ruby instead of having acquired it while doing something else, consider using THF for Treasure Hunter as the regular drop rate is very low (and you might get other potentially useful drops like coffer keys in the process), or if you haven't learned MP Drainkiss yet as BLU consider using that job to make the leech fights more useful.
Go to Windurst Walls (G-3) and examine the door of the House of the Hero for a cutscene.
You will be instructed to experience 7 types of weather in outdoor areas. It seems only zones available in or before the Rise of Zilart Expansion will work.
Note: The Carbuncle's Ruby must be in your inventory to receive the weather cutscene. You also can't simply idle in a zone - you must zone into an area while the weather is occuring to get the scene.
Color
Zones
Red (Heat Waves)
Western / Eastern Altepa Desert (Rare, available all year)
Cape Teriggan (Rare, available from March to early Nov.)
To experience the weather of one of the seven colors, you must enter the area while the weather is active, you will receive a brief cutscene confirming the weather effect.
To know the weather of the area you are in, check the top left corner of where your compass (above the day/time/pos in the bottom left of your screen - assuming you have it toggled-on by using /clock )
The most difficult color to obtain is Red. Hot spells and heat waves are more frequent in the game's summer months, although random hot spells can pop up any time of the game year.
You can check the in-game month by using the /clock command (identical to Menu -> Current Time). Note that the command is a toggle that shows/hides the compass, day, time, and pos in the lower left of the screen, so you'll want to use it twice to get back into the same state your client was in.
Weather is the same across the zone, and sometimes identical in other zones nearby.
While you can't get the scene in Rabao due to it being a non-Field area, you can use Home Point #1 there as your warp destination to see it periodically first-hand. From here, you can walk outside the entrance or use the nearby survival guide to jump into one of the zones.
Weather is not necessarily the same across the region as a whole, see Zulkheim having 3 distinct weathers. Also, weather patterns aren't necessarily bound by region lines either - see Elshimo Uplands & Lowlands sharing their weather trends for Yhoator & Yuhtunga Jungles as well as Kazham.
It is worth noting that Red weather also tends to be more prevalent around midday (12:00 noon) in the relevant zone. So if you are having difficulty finding this weather, try in the middle of the day rather than checking during nighttime.
The pre-Zilart field areas where fire weather & Fire Elementals spawn were mainly used by Japanese players who didn't have the expansion pack, as SMN was released as a one-off job but not actually part of the Zilart expansion. In the modern game, almost all players will have the expansion (English players have had it since launch), but these areas might be still useful for any ambitious Free Trial players who can only visit Original areas from the base game.
Only expansive outdoor areas count as Field areas, places like Dangruf Wadi, Davoi, & Ifrit's Cauldron that feel a bit outdoorsy all count as dungeons. As a general rule of thumb, if you can't use a mount there, it won't count.
You can see reminder as to which zones are "Original Areas" or "Zilart" by looking for region-related quests in your Records of Eminence quest log, or by setting your Home Point & Survival Guide lists to "Select by expansion" instead of "Select by region".
Once you've experienced all 7 weather effects, you'll receive a cutscene with Carbuncle who requests you go to La Theine Plateau.
Optional: look to the sky around the Crag of Holla for a rainbow which appears when someone unlocks the Summoner job. If it's raining, then the rainbow will appear when the rain stops.
. Hot spells and heat waves are more frequent in the game's summer months, although random hot spells can pop up any time of the game year.
