I just discovered this little detail in White 2. Specifically, the woman in the Weather Institute who tells you everything about weather, given a Pokémon knowing the move. And yes, I do mean everything.
"Oh, my! Your Cherrim knows the move Sunny Day!
For a Trainer like you, here comes a Pokémon weather forecast!
Ahem!
Using the move Sunny Day or sending out a Pokémon with the Ability Drought will cause the sun to shine brightly.
When it's sunny, the power of Fire-type moves will be boosted by 50 percent!
The power of Water-type moves will be reduced by 50 percent!
Weather Ball will become a Fire-type move, and its power will be boosted by 100 percent!
The stat increase of the move Growth is boosted by 200 percent!
When it's sunny, it's been reported that Pokémon can't be frozen, and frozen Pokémon are defrosted!
By the way, the Pokémon called Castform is known to change its form when it's sunny and become a Fire type.
The clear sky increases the amount of HP recovered from the moves Morning Sun, Synthesis, and Moonlight.
This is a weather advisory!
When it's sunny, the move SolarBeam can be used every turn. It doesn't have to charge up first!
Please be aware that the moves Thunder and Hurricane will be more likely to miss!
By the way, there are some Abilities that take effect when it's sunny, such as Chlorophyll, Harvest, Solar Power, and Harvest. (Edit: I probably messed this part up lol)
Please check your Pokémon's status!
The lucky item on a sunny day is the Heat Rock!
By the way, the sunshine may feel nice, but remember to take care of your skin and try not to get a sunburn."
I wish I came across this when I was a new player :o