The Crossed-Winds Rules

Thinking about replaying some of the weather posts in anticipation of unknown but surly coming weather events. This one is a good one. From April 2015. Have fun! There’s some math but don’t let that scare you. Math isn’t hard.



This is another in April’s Crappy Old Book of the Month series, Instant Weather Forecasting.

You don’t need to pay any attention to that graphic right now. It’s just impressive set up for what I take to be the lesson of the Crossed-Winds Rules– the rules that will allow us to laugh in the faces of the erroneous weather forecasts we get from our iThingies.

In reviewing my notes, it looks to me like there are three main factors: temperature, wind, pressure. And surprise, surprise! They are related.

[Note: If I sound like a moron, imbecile or idiot here– everybody knows wind speed is a factor in short-term weather predicting– you might be right. I’ve tried for years to learn the basics of weather forecasting. And even after all of these years, I cannot tell the difference between a cirrus and a circus cloud. So I’m going to start from a position of absolute ignorance, with no intension to offend readers who can read the clouds. As Missy would say, it’s not about you, it’s about me.)

Because the wind– the movement of air– is caused by changes in temperature and pressure (at the ground, in mid-atmosphere, high in the sky, and even in gullies and at lakesides), it makes sense to understand the wind. Not as causal to the story of what the sky tells, but as a way to get our hooks into the causal story.

The wind blowing around the clouds, the scatter of ice pellets as they fall to the Earth through the wind very high up, and then disperse or not somewhat lower…. the pattern of their dispersal. These tell us a lot about temperature and pressure. Weather is Brownian motion under the sun, writ large.

I’d like to see that model. I’d like to see how the model instantiates the Crossed-Winds Rules.

Here are the Crossed-Winds Rules:

(i) Stand with you back to the lower wind and if upper winds (or clouds) come from the left then the weather will normally deteriorate.

Let’s stop right here and process that. Stand with your back to the wind and look at the upper clouds.

That’s a nice way to state something we all already know, isn’t it?

       (ii) is the flip side. If the wind is blowing the other direction things will get better. All the time.

But then you have to account for the fact that you– you standing on planet Earth– don’t really know what the wind direction and speed is up there where those birds are flying around, let alone in those clouds, now do you?

Not to worry. There’s a calculation whereby you do some math and then turn clockwise 30° (10 if you are at sea) and that’s the direction the wind is blowing up there.

Which brings us to

        (iii) Stand back to the lower wind and if upper winds (or clouds) move on a parallel course the weather will normally not change very much.

“Hints on recognizing the direction and speed of the upper wind from the clouds are given in the descriptions accompanying the photographs, which have, in many cases, been chosen to help in using the rules for forecasting.

It’s like. Totalollogy or something.

The larger point is that perpendicular winds forecast precipitation. Please attend now to figure above whose caption does not do it justice