The Feltrite Tempests (as well as the Feltrite Storm) are the events in RAGE that depend on the main storyline progress made. There are two Feltrite Tempest events in the game that will take place in Northern Wasteland, and one Feltrite Storm event.
Roughly, every event is a sort of minigame in which you have an opportunity to acquire some Feltrite Crystals in a given period of time. Once the Feltrite rain starts, speed up to collect the falling Meteors marked by distinctive black smoke trails.
After you accept the quest "Deadly Delivery", leave Wellspring and head to the Kvasir's Lab through the Northern Canyon. The Feltrite Tempest event will be announced once you leave the Wellspring Entrance territory. The event will take place between a short way from Wellspring and a small distance beyond the Shrouded Bunker. The task is simple: you have 20 seconds to "ram" every Meteor to pick it up. You will be able to collect up to 7 crystals.
After you accept the quest "Liberate Captain Marshall", leave Wellspring and head to the Authority Prison through the Northern Canyon. The Feltrite Tempest event will start once you leave the Shrouded Clan territory and head up the ramp at the Scorcher territory. The event will take place between the beginning of the highway and the Southern Valley where the Authority Prison is. The task is essentially the same: "ram" every fallen Feltrite Meteor to pick it up in 45 seconds. And once again, you will be able to collect up to 7 crystals.
The Feltrite Storm differs from the tempest events. It takes place in a limited area, and you have to drive back and forth to collect all the Meteors.
When you accept the quest "Recover ID Drive", leave to the Dam Facility area and take the drive. On the way back you'll be passing a valley with a small crater (the "In the Basket" Vehicle Jump place) and a few rocks just past the destroyed barricade. A Minigun nest has been seen there, and bandit vehicles are known to spawn there from time to time.
The Feltrite Storm event will be announced once you pass the barricade, and now you have a minute to collect over 20 Feltrite Crystals. The bandits will keep spawning constantly during the event, and beside shooting and ramming you they will try to pick up some Meteors as well. Use hard break, you'll need to turn back often.
- Those events are a great opportunity to advance towards collecting enough Feltrite Crystals to complete the side quest "Finding Feltrite" given by Jacob at "Second Chance".