The Highstorms

"Fleet kept running. He couldn't win, but he kept running. And when the storm caught him, it didn't matter that he’d died, because he'd run for all he had."

Manifestation

The Highstorm is a giant storm which crosses the entire continent every 1-2 weeks, and never dies. It arrives as huge thunderheads barreling across the entire horizon, black against a blue glow which emanates from the storm center itself. Airships are able to fly above the Highstorm and avoid its fury, however Roshar, the upper isles, and the rest of the world cannot escape. 
The Highstorm comes in three waves: The storm wall, the main body, the riddens. The storm wall is the most intense part of a high storm, and features the strongest winds. This part of the storm is known for hurling large tree's around easily, and can easily launch a man into the air. Eventually the stormwall passes and the wind fade to more normal speeds, still powerful enough to knock a man over and carry sizable debris around that could easily kill a man. The main body is the longest part of the high storm, and will last for hours at a time. The riddens are the safest part of the storm, and are generally the last hour or two of the Highstorm. The riddens feature non-dangerous winds, and a lot of rainfall.

Localization

Two main beliefs revolve around the Highstorm, one is the Stormfather resides in the far West, and sends the Highstorms from the Origin point somewhere out there. The other belief is the Highstorm is one continuous storm that passes across Roshar, slowly deteriorating as it makes its way over land, then it builds up speed again as it crosses the ocean. The time between storms depends on how long it takes for the Highstorm to generate enough speed again. 
The Highstorm covers the entirety of Roshar as it passes, thus everyone must weather out these storms. Because of the intensity of the storms, the ecology of Roshar has adapted to them. For example, when a Highstorm arrives the Sunwheat will retract into a protective stalk which keeps it safe from Highstorms. Furthermore, the earth as a sort of spongy topsoil which holds the drier dirt just below from being launched into the wind. This topsoil layer happens when dirt is infused with the rain and the blue light from the storm, so even in cases of freshly dug earth, the topsoil forms almost as soon as the storm hits.
The light from the storm (called Stormlight) can be infused into spheres, which are sometimes used as an alternative form of currency (see Spheres article for more info).