Backing vs Veering Winds (misuse of terms?)

[Chris Harrell]

A backing wind is a wind that turns counter-clockwise with height. An example of a backing wind would be a north wind at the surface with a west wind at 700 millibars. A backing wind is associated with cold air advection and dynamic sinking.

A veering wind is a wind that turns clockwise with height. An example of a veering wind would be a southeast wind at the surface and a west wind at 700 millibars. A veering wind is associated with warm air advection and dynamic lifting.

Backing winds are indicative of cold air advection which would be sinking air. Wouldn't this be the opposite of what you would want in tornado genesis. I would assume warm air advection (veering winds) would be more ideal for proper vertical lift.

Also in forecasts I'll see people claim certain winds are veering, yet when I look at them, they are technically backing. If the 850mb is southwesterly, the 700mb a little more southwesterly, and the 500 is westerly it would be backing technically right? I've seen a lot of people refer to it as veering. Is that because it's not backing enough for their likings or what? It seems to cause some confusion for me. I would think in order for it to be veering, the lower level winds would need to be more north westerly or northerly while the upper level winds are westerly.

 

[Chad Cowan]

Chris- Those definitions are a little misleading in the context of severe weather forecasting. Veering and backing can be used in a couple different ways:

1) Change in wind direction with time- If someone says that the sfc winds are veering, that would mean the surface winds are turning more southwesterly with time. If the surface winds are backing, that would mean they're turning more southeasterly with time, which would be more conducive to tornadoes. This is how veering and backing are typically used in severe-wx forecasting.

2) Change in wind direction with height- a veering wind profile (or turning with height) is conducive to thunderstorm rotation. Example- southerly winds at the surface, southwesterly winds at 700mb and westerly winds at 500mb.

 

[Chris Harrell]

Thanks Chad, so you are indicating the definitions I acquired are not in line with severe-wx forecasting terms. I've always known what people meant in regards to veering and backing when I read their forecasts or nowcasts

One day I was reading on theweatherprediction.com I read those definitions. From that point on, it always bugged me. I wasn't sure if somewhere along the lines people weren't using it in proper context and it just came to be a norm to use it out of context.

 

[Mike Hollingshead]

Also in forecasts I'll see people claim certain winds are veering, yet when I look at them, they are technically backing. If the 850mb is southwesterly, the 700mb a little more southwesterly, and the 500 is westerly it would be backing technically right? I've seen a lot of people refer to it as veering.

It'd be veering. You even just stated it right before. Course you can obviously have that veering with height and backing with time at the same time also.

A veering wind is a wind that turns clockwise with height. An example of a veering wind would be a southeast wind at the surface and a west wind at 700 millibars. A veering wind is associated with warm air advection and dynamic lifting.

I think most cases folks just slip, like the whole "east" or "west" thing in chase accounts(so easy to screw that up, lol).

 

[Bobby Prentice]

A backing wind is a wind that turns counter-clockwise with height. An example of a backing wind would be a north wind at the surface with a west wind at 700 millibars. A backing wind is associated with cold air advection and dynamic sinking.

A veering wind is a wind that turns clockwise with height. An example of a veering wind would be a southeast wind at the surface and a west wind at 700 millibars. A veering wind is associated with warm air advection and dynamic lifting.

The above definitions are incorrect. The actual definition says nothing about a change with height. Here are the official definitions:

backing—1. According to general internationally accepted usage, a change in wind direction in a counterclockwise sense (e.g., south to southeast to east) in either hemisphere of the earth; the opposite of veering. 2. According to widespread usage among U.S. meteorologists, a change in wind direction in a counterclockwise sense in the Northern Hemisphere, clockwise in the Southern Hemisphere; the opposite of veering.

veering—1. According to general international usage, a change in wind direction in a clockwise sense (e.g., south to southwest to west) in either hemisphere of the earth; the opposite of backing. 2. According to widespread usage among U.S. meteorologists, a change in wind direction in a clockwise sense in the Northern Hemisphere, counterclockwise in the Southern Hemisphere; the opposite of backing.

Backing winds at the surface are generally favorable for severe storms because they tend to: increase the vertical wind shear, increase low-level moisture (particularly across the plains states), etc.

 

[Jason Harris]

Seems like people here could assist on a question I have (and maybe others could have as well):

I thought backing winds are what is most looked for regarding shear for tornadogenesis but I notice the NOAA glossary contains this about veering as well:

--
Directional Shear
The component of wind shear which is due to a change in wind direction with height, e.g., southeasterly winds at the surface and southwesterly winds aloft. A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development.
--

Also, could someone clearly explain exactly why there is more convergence with backing vs. veering winds. If it's all about clockwise vs. counterclockwise, why would the air diverge less moving a southeast wind to an east wind as opposed to an east wind moving south east?

 

[Bob Hartig]

Jason, I think your first question is easiest to answer with an illustration.

Let's say--and this is purely hypothetical--that winds at every level are blowing from the south. Surface, 850, 700, 500, 300 mbs...all southerly, purely unidirectional. The only shear is speed shear.

Now, let's move the 700 mb level so that it's blowing from the SSW. We have just moved it clockwise on the compass--in other words, we've veered it. Let's also move the 500 mb so that it's blowing from the SW. And let's move the 300 mb winds so that they're blowing straight from the west. All of those upper level winds have now veered clockwise with height. Moreover, each higher level has veered progressively more than the levels below it.

Let's also do one other thing. We'll keep the 850 mb wind blowing straight from the south, but let's move the direction of the surface winds a notch or two counterclockwise so that they're blowing from the southeast. We have just backed the surface winds. Now all the winds at higher levels veer, to greater or lesser degrees, relative to the surface winds. You wind up with a spiral-staircase effect and a nice, curved hodograph.

I think I've explained this adequately; if I haven't, I trust others here can improve on my illustration.

 

[Jason Harris]

Hi Bob,

And I know we've talked about this general topic before, but still trying to get a clear conception:

so based on what you just described, it's desirable to have the winds veer with height but for surface level winds to back?

So that's why the NWS explains "A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development."

The phrase "the lower part of the atmosphere" is a bit vague to me, what mb/height does that cover? Up to 300 or 200 mb? The entire troposphere?

And the glossary also mentions: "Backing of the surface wind can increase the potential for tornado development by increasing the directional shear at low levels."

So, again veering above the surface but backing at the surface? And it's that surface backing that will produce the convergence and enhanced rotation, assuming you have winds criss-crossing as directly as possible like two trains hitting to force air up and that adds to inflow and lift and convection (if heat and moisture are available)?

And what angle relationship for backing winds is ideal? You mention backing from the south to the southeast, but what about if the winds backed all the way to the East? Is more convergence and rotation a result? Or the same convergence but enhanced rotation?

I like the image of the spiral staircase as a way of remembering, and reminds me a bit of the barber pole image for a meso.

 

[Chris Nuttall]

Change in wind direction with time- If someone says that the sfc winds are veering, that would mean the surface winds are turning more southwesterly with time. If the surface winds are backing, that would mean they're turning more southeasterly with time, which would be more conducive to tornadoes. This is how veering and backing are typically used in severe-wx forecasting.

Actually, this isn't completely correct. Veering and backing refer to the direction of change in the wind components. Veering indicates a clockwise change. Backing refers to a counter-clockwise change. It all depends on what direction the winds are starting from.

Chad's situation would be correct assuming that the wind is starting with a southerly component. If there was an increase in an easterly/westerly direction, the wind would be backing/veering. If we started with a northerly component, it would be opposite (veering/backing to become easterly/westerly).

The phrase "the lower part of the atmosphere" is a bit vague to me, what mb/height does that cover? Up to 300 or 200 mb? The entire troposphere?

I've heard people refer to this several different ways. If you hear someone talk about an upper-air chart, they are usually talking about any plot above 850mb. Generally, think about it the same as cloud layers.
**Low level = 0-2km
**Mid level = 2km-6km
**Upper level = Above 6km
This is just a general reference. It really depends on the elevation of the location in question. For example, on today's 12z sounding, the first reading at OUN was below 950mb. At AMA, it was around 900mb. So, the height of the low levels are actually a bit different relative to the elevation of each station.

It also depends on the scale you are looking at. If you're focusing on an area that's of similar terrain and elevation, think of the height (in ft or km) relative to ground level and not adjusted to sea level. The Skew-T diagrams on the CoD web page plot height in meters (assuming 1013mb = sea level = 0m) along with millibars.

So, again veering above the surface but backing at the surface? And it's that surface backing that will produce the convergence, assuming you have winds criss-crossing as directly as possible like two trains hitting to force air up and that adds to inflow and lift and convection (if heat and moisture are available)?

You pretty much have the right idea. Remember, convergence will bring lift. Let's use a simple example. Assume you have a boundary of some sort (dryline, outflow boundary, cold front) oriented roughly north to south and moving from west to east. You have westerly surface winds behind the boundary, and winds that are south ahead of the boundary.

If the winds ahead of the boundary begin to back to the SSE or SE it increases convergence. Instead of having winds (behind and ahead of the boundary) being closer to parallel with each other, they oppose one another more. Also, the backing surface winds will create more directional shear, not just throughout the whole atmospheric column, but also in the lowest levels, which is key in increasing tornado potential.

 

[Paul Austin]

You pretty much have the right idea. Remember, convergence will bring lift. Let's use a simple example. Assume you have a boundary of some sort (dryline, outflow boundary, cold front) oriented roughly north to south and moving from west to east. You have westerly surface winds behind the boundary, and winds that are south ahead of the boundary.

Careful in your wording. Sorry to nitpick, but convergence at or near the surface will bring lift. Convergence at higher levels of the atmosphere can cause air to sink, though it's not as simple as that. Other factors come into play in the absence of the concrete boundary of the surface. I know what you meant. The boundary example illustrates the convergence concept well.

 

[Chris Nuttall]

Careful in your wording. Sorry to nitpick, but convergence at or near the surface will bring lift. Convergence at higher levels of the atmosphere can cause air to sink, though it's not as simple as that. Other factors come into play in the absence of the concrete boundary of the surface. I know what you meant. The boundary example illustrates the convergence concept well.

You're right. I was limiting my discussion solely to the surface, since veering/backing in upper levels had been addressed. I should have been more clear. Thanks for throwing that in there for me.

That was a lengthy post I had made. I was ready to take a nap after that.

 

[Jason Harris]

And I appreciate that lengthy post Chris! And I assume the sinking air effect you refer to Paul is subsidence yes? If you care to expand, do go ahead on the various factors, or maybe there's already an upper-level-air convergence-leading-to-subsidence thread?

 

[Mark Stephens]

Digging up this thread from ancient times as I'm still not quite clear on when backing winds create favorability for tornadoes. In the past I have certainly used "Backing" when I should have been using "Veering with Height." Understood that winds veering with height, particularly a southerly to a westerly wind, is preferred for supercell development, but is the idea that you want backing winds just at the surface and not with height to increase the likelihood of tornadogenesis?

What's tripping me up is how do winds back only at the surface? Does this just mean at the very lowest levels of the troposphere? Or is it truly backing at the same level? Does this just mean winds are turning counterclockwise over time?

Any help on this is appreciated!

 

[Bob Schafer]

Most times someone says backing winds he/she means surface winds backing over time, and I would say it would generally be in the context of predicting severe weather. I didn't go back and read the old posts, so....

 

[James K]

I had the basic idea of 'veering' down ... except that I only understood it simply as "change in direction with height" , not "clockwize change in direction with height", and 'backing' was a wind from an easterly direction (be it east/southeast/etc).
So I did find this thread helpful in that veering is clockwize change. (a previous descriptions I'd seen that stated veering as a change in direction with height, did have a diagram showing clockwize change, but didn't speciffically state 'clockwize')

But reading some of the posts (and maybe I mis-understood them) where (simplified down) they listed something to the effect of:
surface: southeast
lower: south
middel: southwest
upper: west
And listed the surface as backing, all the others as veering.
This may pretty much be the same as what Mark Stephens was asking...
But each of those is a clockwize turn as I see it? So wouldn't all be considered veering?
Or is there more to it - where yes veering=clockwize but also south is considered straight, anything east-ish is backed?

Or maybe I simply missed something in reading, and what people meant with the surface winds is like Bob Schafer said - they're talking backing over time (ie: the surface winds turn more towards the east as the day goes on)

---
Atleast where I'm at I could easily see where a surface wind out of the SE would be helpful (assuming other conditions were correct)..
Southeast here would normally tend to mean warm humid air .. and once it hits the mountains upslope flow! that'll fire off convection (and/or help break a cap if there is one).
..
Thinking about it (for here atleast) I could see why you also wouldn't want that SE surface wind to kick in too early in the day (you get one of those days that looks/feels like & is forecast to be a stormy day, then instead of afternoon *storms*, things fire up early & you get late-morning/mid-day thundershowers instead)?