When a layer of cloud rolls or clumps extends in long lines that stretch off to the horizon, the effect of perspective makes these lines converge, like railway tracks, towards a point. Such a formation is a variety known as radiatus, and it can be found in low-, mid- and high-level clouds.

The parallel cloud lines form parallel to the wind direction up at the cloud level. (When they form perpendicular to the wind, the cloud is of the variety known as undulatus.) If this variety forms within the high-level Cirrus cloud, as shown here, it is sometimes known as ‘jet-stream Cirrus’. Such radiatus formations occur in clouds at the top of the troposphere, and are caused by the 180mph winds of the jet streams, which encircle the globe in the mid-latitudes. As the Cirrus is teased out by the high winds, it can occasionally seem to extend all the way from one horizon right overhead to the opposite one. While converging at each horizon, the perspective causes the cloud rows to bulge dramatically in between.