When air masses meet. When a moving cold air mass meets a warm air mass, that is lighter, it tends to wedge below the latter, thus giving origin to a cold front. The warm air is forced upwards and its ascent causes the formation of clouds.
Strategies and More Information. Tyranitar is a big Rock/Dark PokÃ©mon, but it is sometimes considered to have Dragon traits, even though it has no Dragon type. Tyranitar comes with the ability Sand Stream which summons a Sandstorm. It generously helps Rock, Ground, and Steel type PokÃ©mon by causing a little damage to other types every turn. It will also boost a Rock type PokÃ©mon's Special Defense one stage (50%).
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.