A wind that under certain conditions blows over the escarpment of the Pennines, near Cross Fell from the eastward, when a helm (helmet) cloud covers the summit. The helm bar is a roll of cloud that forms in front of it, to leeward.
See "Report on the Helm Wind Inquiry," by W. Marriott, Quart. Journ. Roy. Met. Soc. xv. 103.