Answers: Wind direction

By Jack Williams ©2015

Q: In your article in the January 2010 issue of AOPA Flight Training, you say: “If the lines on a weather map showing air pressures of 500 millibars and 496 millibars were straight and parallel, the PGF and Coriolis forces would soon balance and the wind would flow parallel to the lines of equal atmospheric force.” I thought that air masses would flow from high to low pressure. Could you help me understand where I am going wrong. Thank you.

A: Answering this question requires an illustration and fortunately I found one. The image  below is a National Weather Service (NWS) surface analysis chart showing observed air pressure and winds over the north-central U.S. on May 6, 1999 (the time isn’t shown, but the NWS produces such maps each three hours.)

National Weather Service surface chart showing isobars and wind directions. NWS chart

National Weather Service surface chart showing isobars and wind directions. NWS chart

The red “L” is the center of an area of low atmospheric pressure. The solid black line around it is the isobar–a line of equal pressure–for 992 millibars (mb), as seen in the figures to the lower right of the “L”. The next circle is 996 mb., the third one 1000 mb, and so on for each 4 millibars.

A center of high pressure is off this map to the west (the left). A line directly from the high to the low pressure would cross the five isobars over South Dakota at roughly right angles. Such a line from the high to low pressure shows the direction of the Pressure Gradient Force (PGF), which begins pushing air, causing the wind.

But, you have to keep in mind that as you move out in any direction from the L, the air pressure increases, at least for the area seen on this map.

The black dots on the map are weather stations and the thin lines extending out from the dots are wind arrows. The wind is blowing along each line to the station it’s attached to. For example, at the station just to the left of the “L” the wind is  blowing approximately from the northwest.

By the way to learn how to read weather maps like the one above, you best bet is to buy a copy of Aviation Weather Services: Advisory Circular AC00-45F (FAA Handbooks). This book is the source of questions and weather maps and reports on FAA written tests.

If  you look at the other wind arrows you will see that that winds are blowing roughly parallel, but slightly across the isobars toward the low-pressure center. So far, this illustrates your question.

Now, we’re ready to look at the answer.

In my latest book, The AMS Weather Book: The Ultimate Guide to America’s Weather, I explain all of this on pages 53 to 57 with the help of five illustrations.

Instead of telling you only to buy the book and read these pages, I’ll give you a quick answer:

As wind, ocean currents, or other objects move across the earth’s surface without being attached to the earth. it turns under them, which causes them to follow a curved path across the earth’s surface. The reason for this curving motion is called the Coriolis Force. The faster the wind, the stronger the Coriolis force. For more on this, see Explorations: The Coriolis Force on the supplemental Web site for my book.

One way to think about why the wind flows roughly at a right angle to a line between high and low pressure is to image what happens to a “parcel” of air, a small blob of the air, as the PGF starts pushing it from the west toward the “L” on the map.

As the air starts moving toward the L, the Coriolis force turns it to the right (in the Northern Hemisphere). If the air continued turning toward the right, however, it would be pushing against the pressure gradient force.

You can think of what happens as a component (like a cross wind component when you’re landing) of the PGF pushing against the air. This would slow it down, which would weaken the Coriolis force, allowing the PDG to turn the air back toward the left.

Also, don’t forget, that as an air parcel goes around the L, higher pressure is always pushing it toward the L.

To see how this works, you could print out the chart above and then draw arrows showing what the PGF and Coriolis forces are doing step by step as an air parcel moves.  (This would give you images somewhat like those in my book.

Being near the ground adds another complication. The friction air moving over the ground slows it, which weakens the Coriolis force. This in turn, allows the PDF to push the air more toward the low pressure center than it would high above the earth where friction doesn’t affect wind speed. This is why the wind arrows show that the air is crossing the isobars as it spirals in toward the low center, where it rises.

I hope this helps. This stuff can become pretty complex, but taking some time drawing arrows on the chart will help make it clear. I think, of course, that my book, will speed this up.

By the way, the surface weather chart image above is from the Rapid City, S.D., NWS office Web site. It’s used to illustrate why strong winds hit western South Dakota and northeastern Wyoming on May 6, 1999.

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