Weather is an event that is present in every zone and duty.
Major cities and some settlements have an NPC known as a Skywatcher. These NPCs will indicate the current and upcoming weather conditions for each zone. One can also see the current weather condition of the current zone by hovering over the icon in the top-right corner of the minimap. Normal weather conditions in overworld zones and cities may change at 12:00 am, 8:00 am, and 04:00 pm Eorzean Time (every eight Eorzean hours, i.e., 23 minutes and 20 seconds real time).
Weather for a zone or a city is the same for all worlds at the same time. For example, the weather in Southern Thanalan on the World "Cactuar" at 1pm Eorzea Time will also be in Southern Thanalan on "Ultros" at 1pm Eorzea Time. Only special weather like Tension is world dependent.
Weather typically remains the same in duties, although it may change at specific points in the duty. However, dungeons in A Realm Reborn will generally reflect the time and weather conditions outside of the dungeon, a remnant of FFXIV 1.0 design when dungeons were directly linked to the open world.
Certain weather events will override the normal weather in the course of progressing the Main Scenario Quests. This condition is entirely localized to the player. A player at the beginning of Shadowbringers, for example, will see all of Norvrandt blanketed in Everlasting Light and will gradually restore normal weather in each zone. Players who have completed Shadowbringers will see normal weather in each zone.
Several activities in the game depend on the weather conditions. These include but are not limited to:
Fishing: Several fish can only be caught during certain weather conditions and times of day. Examples include the Thunderbolt Sculpin, Raincaller, and Son of Levin. Some require very specific conditions, such as one weather type following a specific previous weather type. For example, Helicoprion can only be caught between 8 AM and 8 PM during a Heat Waves that follows clouds or fog.
Sightseeing Log: Many of the sightseeing entries in A Realm Reborn areas require certain weather conditions and specific times of day. Sightseeing vistas added in later expansions do not have weather or time-of-day requirements.
World Boss FATES: Certain world bosses will herald their arrival under a certain weather condition. For example, Odin's FATE will only spawn when the weather condition in The Black Shroud is listed as Tension.
Elite Marks: Certain Elite Marks will only spawn during certain weather events. For example, Kirlirger the Abhorrent will only spawn during Fog and a New Moon.
The Diadem: Some materials in the Diadem require a weather condition to be present in order for these materials to be gathered.
Cosmic Exploration: Red Alerts are associated with random weather conditions that occur randomly every few real-life hours.
Normal weather conditions are always present in zones and will cycle at designated times in overworld zones and cities. Each zone has a set of possible weather conditions, and some weather is unique to a particular zone.
Certain weather conditions can only happen in certain zones (ie sandstorms in Thanalan, snow in Coerthas, so on.)
Special Weather Conditions
There are also several special weather conditions, usually associated with a specific event or duty. Their names, mini-map descriptions and where they appear are as follows:
Tension is world dependent.
Main Scenario Quests. This condition is entirely localized to the player. A player at the beginning of Shadowbringers, for example, will see all of Norvrandt blanketed in 
Everlasting Light and will gradually restore normal weather in each zone. Players who have completed Shadowbringers will see normal weather in each zone.
Thunderbolt Sculpin, 
Raincaller, and 
Son of Levin. Some require very specific conditions, such as one weather type following a specific previous weather type. For example, 
Helicoprion can only be caught between 8 AM and 8 PM during a Heat Waves that follows clouds or fog.
Sea Shanty: This action doubles its potency during rain, showers, and thunderstorms.
Weather: This 
Phantom Geomancer action in 
Occult Crescent turns into one of several different actions based on the current weather.
