The Storm Marines are basically a codex chapter, with the exception of their color scheme and company structure. The Storm Marine's geneseed was derived from that of the Whitescars and they share that chapters ferocity and battle- lust. They were created relatively recently, at the beginning of the first Tyrannic war. They excel at lightning raids, and the entire chapter is mobile.
The chapters arrangement of companies is slightly different from that wich the Codex Imperialis dictates, and reflects their style of fighting. The veteran company not only uses Tactical Dreadnought Armor, but also uses Jet Bikes, vehicles unique to the chapter. Veterans form either heavy Terminator squads, or highly mobile Storm Rider squads. The chapter also has one more Assault and one less Devastator company than most chapters -- they have 5 battle companies, 4 assault companies, and 1 devastator company.
The Storm Marines recruit from the planet Gallia, a cold mountainous planet. The natives of Gallia are mid-tech, with a technology level that varies wide ly from advanced firearms to crude iron spears. The planet has one main continent, with a warm ocean in the middle of it. The outer edges of the continent are very mountainous, and the home of hardy shepherds. Gallia's soil is not very rich except in a few small areas in the wet valleys between the steep mountains. The inner sea is pierced by many peninsulas, the largest of which is Latina, the capital of the planet. The planet is often assaulted by raiding Orks, so the Gallians are fierce fighters. Once the planet was divided into separate clans or tribes that often fought with each other over the precious areas of rich soil. One tribe, from Latina, began to wage war with the neighboring tribes, and instead of killing the conquered, they incorporated them into the tribe. The Latinans grew stronger and stronger, and their empire larger and larger, until the entire planet was united.
The Storm Marine's base of operations is a huge fortress carved out of a giant mountain that reaches above the clouds. This mountain is called Mount Olimpa, and lies on an island far from the main continent of Gallia. The Gallians believe that the gods live at the peak of the mountain, from where they through lightning down to the world. There is some truth to this -- the fortress of the Storm Marines bristles with defense lasers, and they often strike out into the sky, vaporizing Ork hulks that drift in from the warp.
The Storm Marines recruit from all over the planet, from the Legionaries of Latina to the wild barbarians of the mountains. All of these warriors are trained together. They are not just trained physicaly, but also trained spiritually. Thier devotion is changed from thier tribal and family gods to the chapters famous commanders and the machine spirits that guard thier weaponry. The chied deity in the Gallian pantheon of gods is a powerful and fickle storm-god, who one day brings rain to the crops, and the next stikes the farmer down with lightning. The chapter's chaplians teach the young marines about the Emperor, teaching them that he is the same as thier Storm-God.