When upper and lower winds are moving at right-angles, a major weather change is imminent.
Look at the low and high clouds in the time lapse video above. We can see that there is almost a 90 degree difference in the direction they are travelling. Rain followed shortly afterwards.
The Cross-Winds Rule is a simple method that works even if you don’t know why it works, but if you’d like to learn more then this section, from the book, The Secret World of Weather, explains the science background:
Thomas Hardy wrote in Far From the Madding Crowd,
The night had a sinister aspect. A heated breeze from the south slowly fanned the summits of lofty objects, and in the sky dashes of buoyant clouds were sailing in a course at right angles to that of another stratum, neither of them in the direction of the breeze below. . . .
Thunder was imminent, and, taking some secondary appearances into consideration, it was likely to be followed by one of the lengthened rains which mark the close of the dry weather for the season.
Thomas Hardy, Far From the Madding Crowd
Hardy, like Homer and many other greats since, sets his tales in a natural world that rings true. We’re more likely to believe the intricate inventions of love, hatred, and revenge if they are set against a world we recognize, even subliminally. In this case, bad weather follows soon after clouds were seen moving “at right angles” to those at another level. This is probably the best literary demonstration of an important sign known as the crosswinds rule.
The practical part is quite simple. Stand with your back to the low main wind (the wind above tree level, moving the lowest clouds) and note the direction that the highest clouds, probably the cirrus, are traveling. If it is from left to right, a frontal system and bad weather are probably on the way. If the lower wind and upper wind are aligned, no change is imminent. If the upper clouds are moving from right to left, improvement is more likely than deterioration.
How does this work? The crosswinds rule is an odd beast. The theory doesn’t fit neatly into any one chapter, as it contains low winds, high winds, fronts, clouds, and pressure systems. But it’s mostly about sensing winds, so we’ll meet it here. It’s easy to use and hard to explain. Fasten your seatbelts, as they say.
When a frontal system is approaching, there will always be a mismatch between the upper and lower winds. We know that winds circle counterclockwise around a low-pressure system. With our back to the wind, the low-pressure system is on our left and any high-pressure system is on our right. It is a relationship known as Buys Ballot’s law(named for a nineteenth-century Dutch meteorologist). The upper clouds allow us to gauge the wind direction at a much higher altitude.
Here is a weird analogy: You are an ant sitting between tall blades of grass on a lawn. In the distance you can see a gardener with a lawnmower moving toward you. You gauge the low breeze and it’s coming from the same direction as the gardener is walking. No problem. Then the lawnmower passes over you and the wind you feel changes suddenly: now it’s coming from a different direction than the gardener. Trouble ahead! In this strange analogy, the gardener is the jet stream and the lawnmower is the low-pressure system; the rotating blades are the fronts. The blades pass over you and you live to tell the tale, but they cause a lot of wind changes and “bad weather.”
When used in conjunction with the cloud clues, especially the progression of cirrus, then cirrostratus, the crosswinds rule is robust.
Rain followed not long after the video above…
