Weather, like Entry Hazards, may periodically occur throughout the game. Sometimes it may be triggered by your location, or it may be by a certain move. Either way, the weather is another game mechanic which varies in effect. Depending on the Pokemon with the weather-based Ability, a certain event will trigger. One more thing to factor in is that all moves that summon the weather lasts for five turns. Natural weather (i.e. the occasional rain in some routes) will not go away unless you have a Pokemon with the Ability Cloud Nine, which eliminates all weather effects.
The first kind of weather is strong sunlight, which occurs through the move Sunny Day or the Ability Drought. This move is primarily used to power up Fire-type moves to 150% and weaken Water-type moves to 50% power. Some Grass-types use it because certain Abilities like Chlorophyll have beneficial effects. The two-turn move SolarBeam, because of the sun, is now converted into a one-turn move, making it more efficient.
The second form of weather is Rain, which occurs more often and naturally in the Generation V games. The move Rain Dance summons rain, while the Ability Drizzle can also trigger rain. Rain is the sunlight’s counterpart, so Water-type moves gain the advantage, while Fire-type moves lose half of their power. This form of weather’s secret is that it increases the accuracy of Thunder and Hurricane to 100%. Pokemon with the Ability Swift Swim will have their Speed doubled, giving them the advantage. Places like Misaltron City in Unova have Rain most of the time.
The third form of weather is Hail, which is less common. This is summoned by the move Hail, or the Ability Snow Warning (Snover/Abomasnow). There’s not too much in this weather; it has two main effects. The commonly known one is that it damages all Pokemon that are not Ice-types. The lesser known one is that it increases Blizzard’s accuracy to 100%. In Winter, Icirrus City has Hail as a weather condition if you run around the patches of grass near that place.
Sandstorm is the fourth kind of weather. Like sun or rain, this is more known, and has many abilities that depend on it. Sandstorm is a Rock-type move, and can also be summoned by having the Sand Stream Ability. The effects are similar to Hail: it damages Pokemon that are not Rock, Ground, or Steel-types. The three Abilities that benefit off of sandstorms are Sand Rush, Sand Veil, and Sand Force. A common misconception is that the accuracy of all moves is decreased. This is not the case; but Sand Veil will make that Pokemon harder to hit.
The final weather effect is Fog. There is no way to summon Fog except through the Generation IV HM Defog. It decreases the accuracy of all moves. Weather proves very useful, and having it on your side can help in gaining the advantage in a battle.