Occluded fronts. An occluded front forms when a cold front reaches a warm front, forcing all the warm air to rise to higher altitudes and the cold air is stratified near the ground. At our latitudes, over the Atlantic, cyclonic areas form continuously, these are fed by the Azores anticyclone and the Polar anticyclone.
Pikachu - Raichu. Eevee - Jolteon. As of the IV generation, only two Pokemon will evolve if exposed to a Thunder Stone: Pikachu into Raichu and Eevee into Jolteon. Despite the number of Electric-type Pokemon, only these two require a Thunder Stone to evolve. pikachu- evolves into raichu. eevee- evolves into jolteon.
On the flipside, this means that when you're up against a powerful Gyarados, bring in a fast Electric type like Jolteon or Voltorb. Always remember that Gyarados is a very versatile Pokemon and is able to learn Ice, Poison, Water, Thunder and Fire attacks. When in doubt (and you're out of Electric types), use a powerful Normal type to confront it. By the way, although Gyarados is typically blue, a rare Red Gyarados exist in Pokemon Gold/Silver.