jetstreams

[pantyripper]

OK I have read my MET books and they tell you all about Jetstreams and how there is a rapid change in OAT as you approach it, cirrus or cirrostratus cloud or hazy skies if no cloud, but that is about it. So a couple of questions;

1. So what is this rapid change, is it a rapid drop off of OAT or does the temperature oscillate up and down?

2. What does one do if they get this rapid OAT change, descend/climb?

3. Where do comercial aircraft travel in relation to the Jetstream? Above if they can or well below?

4. What is the significance of the Japanese Jetstream? Is it the fact that it is a sub-tropical jet that combines with the polar jet to make it so strong, and that it may affect flights flying nth/sth in that region and also on the polar route between Hong Kong and New York?

any help appreciated

Cheers P.R.:ok:

[Herc Jerk]

Seems a little quiet here so...
1&2... If you're watching the OAT like a hawk you may see it rise a few degrees- erratically. If you do see this, and don't spend time thinking about it, you may just have time to put the fasten seat belt signs on...
Decision to change alt would depend on severity and crossing angle- whether you're going to be stuck in it for a while. Climb/descent depends on where you are in your flight envelope- can you get to a higher level and still have adequate buffet margins in turb? Normally its descend.
3. Think this question is alluding to where there core of the jet is in relation to the overall jet turbulence- should be a diagram in those met books of yours.
4. <blank>

i'm guessing cx? good luck with it

HJ

[pantyripper]

Cheers HJ, yeah i'm preparing to escape g/a, so havent had any experience with jetsreams yet....
thanks for the info:ok:

[Herc Jerk]

pleasure to help a fellow cabbie;)

[Old Smokey]

Where do comercial aircraft travel in relation to the Jetstream? Above if they can or well below?
If it is a Tailwind - Right in the middle, as close as possible to the core to obtain the maximum Tailwind benefit.

If it is a Headwind - Normally below is the far better option, given that above is usually well above Optimum Altitude.

The Japanese Jetstream? - I've no idea of the mechanics of it, but when it's rough, it's very very rough, with very strong wind shear. In my experience, the rest of the world's jetstreams give little, light, or moderate turbulence. The Japanese version can be from severe to extreme. My worst encounter with the Japanese jetstream was on a par with a Cb entry.

Regards,

Old Smokey

[Fly3]

I agree with Old Smokey. The Japanese jet can be a real bitch and I have had to descend out of it quite a few times to restore semblence of order in the cabin.

[Alex Whittingham]

Jetstreams occur at height at the junction between hot and cold air masses. The temperature difference between the warm sector and the cold sector means the warm sector is a taller column of air and, consequently, the pressure in the warm sector at a particular height is greater than the pressure in a cold sector. This would initially lead to a flow of air at height from the warm sector to the cold but the coriolis effect rapidly takes over causing the air to flow parallel to the frontal surface. If there's a big temperature difference there's a big thermal wind, a jetstream.

The temperature change is an indication of passing through the front at altitude. Jetstreams tend to lie in the warm sector just below the tropopause so a sharp increase of temperature at jet cruising heights means you are near a jet.

[Old Smokey]

A nice refresher on Jetstream formation Alex, it's in one of those nice areas where "theory" is proven daily by operational observation.

Getting back to the Japanese Jetstream, does anyone have any explanation why this one is the "Mother of all Jetstreams", both in strength and ferocity?

Regards,

Old Smokey

[Craggenmore]

Alex,

I would have thought that the pressure would have been less in the warm sector, warm air being less dense thus less pressure.

Where am I going wrong/being forgetful?

Cheers

Craggs

[Old Smokey]

Craggenmore,

Consider two columns of air, of equal pressure at Sea Level, and (theoretically) zero pressure at the top where the atmosphere meets space. If they are of the same temperature, they will be of the same height. If different in temperature, the warmer column will 'stretch' upwards, as will the vertical distance between the various intermediate pressure levels. Conversely, the colder column will 'shrink', as will the vertical distance between the various intermediate pressure levels. Thus, at a given geometric elevation, the pressure in the warmer column will be greater than that in the adjacent cooler column. A Pressure gradient therefore exists from the high pressure column towards the low pressure column, and, after coriolis effect, becomes a jetstream.

Some numbers may make this more convincing. At 30,000 feet in ISA, the Temperature is -44.4°C, and the pressure is 300.89 hPa (8.885"Hg). The True Altitude is 30,000 feet.

It is well understood from Altimetry that flying at a given Pressure Height in warmer than ISA temperatures that the True Altitude is higher than indicated. Thus, in a ISA+15°C atmosphere, at 30,000 feet TRUE Altitude, the Pressure Height is 28,364 feet, where the Static Pressure (Ps) will be 323.99 hPa (9.567"Hg), and SAT = -26.2°C.

Now, if the atmospheric temperature is ISA-15°C, at 30,000 feet TRUE Altitude, the Pressure Height is 31,863 feet, where the Static Pressure is 276.24 hPa (8.157"Hg), and SAT = -63.1°C.

Now consider these two adjacent columns of air at a Geometric Altitude of 30,000 feet. The warmer column has a Pressure of 323.99 hPa, and the cooler has a Pressure of 276.24 hPa. That's a significant horizontal pressure gradient.

Now, does anyone have any idea of the mechanics of why the Japanese Jetstream is the "not so divine" wind.

Regards,

Old Smokey

[Craggenmore]

Old Smokey,

Many thanks for the explanation. It is appreciated and now much clearer..!

Have a look at this document with regard to Japanese Jet Streams...

try here (http://www.google.com/url?sa=t&ct=res&cd=1&url=http%3A//docs.lib.noaa.gov/rescue/mwr/100/mwr-100-01-0001.pdf&ei=vIMDQ6LHH6PQRbX06EY)

download the Adobe document and have a read. Its from 1969 and 1970.

Craggs

[Old Smokey]

Thank you Craggenmore, I have down-loaded it and will digest it over the next 2 days of an ultra boring layover. Even a bit of 'dry' met theory will be more interesting than where I'm going today:O

Thank You Craggs,

Old Smokey

[Ignition Override]

Pardon my skepticism regarding how such detailed theory applies to actual air transport operations.

Don't company Dispatchers "over there" keep track of where the moderate-severe turbulence should be and which cruise altitudes keep you out of it?

Who has time to watch the temperature gauge full time when you are navigating, changing altitudes, ATC frequencies while asking for ride reports, and watching for signs of airframe icing (mostly fall through spring)? Is theory to replace the latest enroute reports? Among other things, one must also anticipate how much extra fuel will be burned at lower altitudes while avoiding a rough ride, or at what actual weight the plane can climb to cruise altitude.

Do the so-called authorities (who create the training material) over there expect pilots to be weather forecasters, or pilots? Both our initial and recurrent training (19 years for me) for narrow and widebody fleets having 100-350 or so seats, has never covered such analysis of jetstreams. Updates on microbursts and mountain wave activity (our weather info includes 'red' and 'yellow' altitudes near mountains) are quite applicable.

[Craggenmore]

Pardon my skepticism regarding how such detailed theory applies to actual air transport operations. Both our initial and recurrent training (19 years for me) for narrow and widebody fleets having 100-350 or so seats, has never covered such analysis of jetstreams.
Probable because this is the Technical forum, rather that the Questions forum which is on another page, I guess!