I agree with everything Belkira said. Here is some more info: Spell Accuracy and Damage
The character under the effect of a weather spell will have a 33-50% chance of receiving a 10% bonus to elemental spell accuracy and potency for spells of the same element. You can see this in damage numbers for elemental spells and HP for curing spells directly, but the accuracy of elemental and enfeebling magic is also believed to be increased.
For example, if I cast Firestorm on a BLM, his Flare spell will have a 33-50% chance of a 10% bonus to accuracy and damage.
SCH weather seems to overwrite zone weather if the zone weather is different. I don't think anyone has proved this conclusively except on equipment (see below). Equipment Activation
Weather spells do activate equipment that is based on weather. For example, they activate the elemental sea Obis and Dessert Boots. Despite some ambiguous descriptions on these items, only the single weather effect is needed to cause the latent to occur, and SCH spells offer this single weather effect.
A nice list of equipment and spells can be found here: http://ffxi.allakhazam.com/db/jobs.html?fjob=20;mid=1194122629278032256;num=45;page=1
Weather spells do NOT activate equipment that is based on DAY. This seems obvious but many people get excited about it working on avatar rings, but it does not.
You cannot receive the latent effect from two different pieces that are activated by different weather. For example, if one is activated by rain and another by sand, and the zone is rainy and you have sandstorm cast on you, only the sand equipment's latent will activate. Helix Spells
If the SCH casts a helix that matches their current weather, the 10% damage increase occurs 100% of the time, not 33-50%. Miscellaneous
Weather spells do NOT fill the Carbuncle Ruby for the SMN flag quest. Speculation
Does SCH weather completely overwrite zone weather?
It's been shown that this is true for equipment activation only, when zone weather differs from the weather spell. What is not known is if spell casting is only affected by the SCH weather, or if SCH weather stacks to create double or triple weather when it matches the zone weather. Some people have reported that this stacking of same weather does happen.
Does casting a weather spell on the mob's current target affect accuracy of spells cast on the mob from other players?
Does weather have any effect on crafting?
Do weather spells have any barspell-type effect giving you resistence to, say, bomb-toss, when Rainstorm is up? Evaluation (mine)
For waiting 41 levels and not being able to sub these, their confirmed effects seem underwhelming. Good in certain situations (e.g. sandstorm on a kiter wearing Dessert Boots), but for 60 levels they're the only unique spells SCH gets and their single weather potency seems like too little, in addition to their long recast time and rather short duration. They look cool and are a very cool concept but in practice are underpowered. People on this and other forums have had fantastic ideas on how to enhance them. Edited, Jan 28th 2008 1:12pm by MordruMidgard