Can anyone explain the legend on the LLWS GFA Chart?

Aye Effaar

Pre-Flight
I like the aviationweather.gov weather details and it's my first stop when looking for pre-flight. The one chart I can't understand is the LLWS chart as part of the GFA tool. I get it that the further to the right in the color scale is "bad", but can anyone explain what these numbers mean?

That explanation hurts my head. But thanks for digging up the reference (somehow I missed it on the Info page).

I've wondered this in the past and had never noticed (or maybe taken the time) to read that definition. Makes sense when you put real numbers to it, e.g. a 25 mph wind speed delta at 200 ft thick would be 0.183/s.

I've wondered this in the past and had never noticed (or maybe taken the time) to read that definition. Makes sense when you put real numbers to it, e.g. a 25 mph wind speed delta at 200 ft thick would be 0.183/s.

I'm not sure I follow the math. Would you mind walking me through it (for the "calculations challenged flyers").

I forget what we used to call a number like that...but it's not a normal thing.
the numerator is dimensionless...and yeah that does make the noggin hurt.
It's really just a dimensionless relative number I suppose.
zero is good, infinity is bad

It doesn't represent anything "real"/tangible. It is a way of quantifying the combination of wind speed differential and the distance across which that differential exists, and it's intended to give you an idea of how concerned you should be about the wind shear. 0 = no worries. As the number gets bigger/colors get warmer, it should get increasingly more of your attention/concern.

OPS in baseball stats is another number with no tangible "realness", derived from the combo of two other stats. There are lots of such measurements in the world.

I forget what we used to call a number like that...but it's not a normal thing.
the numerator is dimensionless...and yeah that does make the noggin hurt.
It's really just a dimensionless relative number I suppose.
zero is good, infinity is bad
It’s not dimensionless, it has dimension of 1/time.

It’s just a measure of shear strength. It’s saying if you had a shear layer of 1m thick, how different would the wind velocity be above and below the shear layer. The math is quite simple. Imagine traveling vertically a distance of X meters. Where you start the wind velocity is Y1 m/s, when you stop the wind velocity is now Y2 m/s. Then the shear strength is simply (Y2 -Y1)/X

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I'm not sure I follow the math. Would you mind walking me through it (for the "calculations challenged flyers").
25 mph = 11.176 m/s
200 ft = 60.96 m

Numbers:
11.176/60.96 = 0.1833

units:
(m/s)/m = 1/s

Life gets a lot easier in SI.

That explanation hurts my head.
It got even worse when my eye automatically wanted to apply the legend colors to Mark’s state map avatar.

25 mph = 11.176 m/s
200 ft = 60.96 m

Numbers:
11.176/60.96 = 0.1833

units:
(m/s)/m = 1/s

Life gets a lot easier in SI.

Yeah, thanks for showing my work for me.

My only reasoning for doing that was taking something I'd seen/experienced and expressing it using their equation. It's _slightly_ less abstract with actual, real-world numbers of something one's experienced.

And I agree, the units seems a bit weird, but @AKiss20 explained above better than I could've.

Don’t need meters. Doesn’t have to be (m/s) /m to get /s.

Example in familiar units:

You descend through a 200’ thick layer where the wind speed changes by 27 mph. Convert to feet per sec, using google: 27 mph = 40 ft/sec.

(40 ft/sec) /(200 ft) = 0.2 /s
That would be red color, in the chart.

25 mph = 11.176 m/s
200 ft = 60.96 m

Numbers:
11.176/60.96 = 0.1833

units:
(m/s)/m = 1/s

Life gets a lot easier in SI.
25mph = 36.667 f/s
36.667 / 200 = 0.1833

The SI system is no easier than the FPS system.

Don’t need meters. Doesn’t have to be (m/s) /m to get /s.

Example in familiar units:

You descend through a 200’ thick layer where the wind speed changes by 27 mph. Convert to feet per sec, using google: 27 mph = 40 ft/sec.

(40 ft/sec) /(200 ft) = 0.2 /s
That would be red color, in the chart.
25mph = 36.667 f/s
36.667 / 200 = 0.1833

The SI system is no easier than the FPS system.

But but but km/h ==> m/s is way easier because in SI you use decimal seconds instead, which doesn't use 3600s in an h, but only 1000, so you can do everything in your head by only moving decimal points!!!! OMG it's so easy!!!!!

Oh wait, no you can't, you still have to divide by 3600. If you're busting out the calculator anyway SI or imperial doesn't mean **** for easiness. I don't know anyone that can multiply by .277777777 in their head. OK, yeah 100km/h is easy, and you can ball park from there but I also just remember that 60mph is 88fps and I can also ball park from there.

Try doing thermal analyses and any real engineering calculation in imperial and doing unit analyses compared to SI.

Imperial has BTU for thermal energy, foot-lbf for mechanical energy, watts or horsepower electric for electrical power, horsepower for mechanical or shaft power…the list goes on. Not to mention the whole lbm/slug debacle with factors of gravitational constant always floating around/getting missed.

SI: Joules and watts which is just joules per second. That’s it. Prefixes for orders of magnitude to save zeros.

In all my analysis codes I immediately put everything into base SI units, makes the whole thing so much easier. You can argue otherwise but my entire working experience has taught me that engineering in SI makes so much more sense. All that being said, non-dimensionalization is really the way to go and how I prefer to work when possible for research work.

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Try doing thermal analyses and any real engineering calculation in imperial and doing unit analyses compared to SI.

Imperial has BTU for thermal energy, foot-lbf for mechanical energy, watts or horsepower electric for electrical power, horsepower for mechanical or shaft power…the list goes on. Not to mention the whole lbm/slug debacle with factors of gravitational constant always floating around/getting missed.

SI: Joules and watts which is just joules per second. That’s it. Prefixes for orders of magnitude to save zeros.

In all my analysis codes I immediately put everything into base SI units, makes the whole thing so much easier. You can argue otherwise but my entire working experience has taught me that engineering in SI makes so much more sense. All that being said, non-dimensionalization is really the way to go and how I prefer to work when possible for research work.

You do all that in your head with no calculators. Impressive!

You convert everything from knots and nm to km/h and km when flight planning too? Change the units on your ground vehicle to km/h so you can try and convert the mileage signs to know how long it's going to take to cover 47 miles?

You do all that in your head with no calculators. Impressive!

You convert everything from knots and nm to km/h and km when flight planning too? Change the units on your ground vehicle to km/h so you can try and convert the mileage signs to know how long it's going to take to cover 47 miles?

When I flew in Australia, altitudes were in feet based on metric pressures, distances in a mixture of metres (runway lengths), kms, and nautical miles, airspeeds in knots, fuel capacity in litres, passenger weights in kg, etc., etc. (often depending on the specific GA aircraft) — quite a challenge even for this engineer who grew up bilingual Imperial / metric. But I still tend to think in metric and have to convert to Imperial when I'm left to my own devices…

You do all that in your head with no calculators. Impressive!

You convert everything from knots and nm to km/h and km when flight planning too? Change the units on your ground vehicle to km/h so you can try and convert the mileage signs to know how long it's going to take to cover 47 miles?
That’s not what I said and you are well aware. Not going to feed the troll any further.

You said life gets easier in SI.

Explain how, in this example where we are already using feet, that it is easier when you have to convert mph ==> m/s and then convert f ==> m when you could just convert mph => fps.

All you did was make more work. How is more work making life easier?

Yes, when dealing with things already in SI, absolutely. I'm not going to try and work with pounds and mols. But how is it easier in this case?

Answer: It doesn't, and you got caught.

We have a SIGMET for Severe Turbulence today, so, I thought I'd check the LLWS chart.

But if you check the LLWS Chart, there's no material coloration...just some white, which is the lowest level (and it almost always shows white).

So then I thought, OK, a SIGMET is a forecast, whereas the LLWS is actual conditions. I went to check the PIREPS, and found several instances of pilots within the SIGMET area were reporting LLWS on final into Newark and LaGaurdia.

Seems like the LLWS chart may not be that useful.

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Generally speaking, turbulence is in the vertical direction, and wind shear in the horizontal. I wouldn't necessarily expect them to be coexistent. And forecasts get botched all the time.

Try doing thermal analyses and any real engineering calculation in imperial and doing unit analyses compared to SI.

Imperial has BTU for thermal energy, foot-lbf for mechanical energy, watts or horsepower electric for electrical power, horsepower for mechanical or shaft power…the list goes on. Not to mention the whole lbm/slug debacle with factors of gravitational constant always floating around/getting missed.

SI: Joules and watts which is just joules per second. That’s it. Prefixes for orders of magnitude to save zeros.

In all my analysis codes I immediately put everything into base SI units, makes the whole thing so much easier. You can argue otherwise but my entire working experience has taught me that engineering in SI makes so much more sense. All that being said, non-dimensionalization is really the way to go and how I prefer to work when possible for research work.
I do the exact opposite of what you do. I convert everything into FPS units and perform my analysis from there. In the FPS system complex concepts are defined in layman's terms, so it is easier to understand what they relate to. I am in the Standby Diesel Electric power business. Rules of thumb allow for quick relations to nearly everything. For an engine with a belt driven radiator It takes 1.5 Hp x kW to turn a generator of any size over 15 kW. It takes about 1 kW per shaft HP to turn an electric motor, A boiler Rated 100 Boiler HP will run a 100 HP steam engine. The list goes on and on. I guess it is all in how you learned.

Unless you're west of the Mississippi river, in which case.....