Skew-t diagrams

[hugh flung_dung]

In another thread someone posted this skew-t diagram:
http://meteocentre.com/archive/tephi/2007/02/10/2007021012_07481.gif
I recall learning about them at some stage but I confess that I've never used one and have forgotten (if I ever really knew) how to do so.
Am I missing-out on a potential treasure-trove of useful info, or is this the preserve of dedicated met "persons" who kindly translate it into the more easily understood info that's presented in the usual aviation met forecasts?

HFD

[IO540]

Assuming you actually fly from from A to B, it is indeed a treasure trove you are missing on :)

You can find out more background if you google on e.g.

tephigram explanation

here here (http://www.metlink.org/resources/ews/activities/act21.htm)etc. A tephigram and a skew-t are a similar thing. The former is a British term; the latter is more American.

I am no expert on this but they are invaluable for getting a grip on cloud tops, for IFR flight. Somebody who understand them can also work out the stability of the atmosphere by comparing the actual lapse rate against the adiabatic one, or whatever. I just use them for cloud tops, and use the SigWx for other stuff.

The temp / dewpoint spread tells you the probability of cloud formation at that level. I have the following from a UKMO forecaster:

< 1C means 7-8 octas cloud
1-2C means 6-7 octas cloud
2-3C means 4-5 octas cloud
3-5C means 2-4 octas cloud
>5C means clear

Forecast ones can be found here (http://pages.unibas.ch/geo/mcr/3d/meteo/)(look under Animated Soundings). Actual (past) ones can be found here (http://weather.uwyo.edu/upperair/sounding.html)(select Europe / GIF-skew-t...). The former site is the only one I know of that does forecast ones; most of the time it is reasonably good. It gets its data mostly from GFS. The UK Met Office does not publish this stuff.

[hugh flung_dung]

Unfortunately I'm not old enough for the first link :)
The wind, DALR and SALR info is pretty obvious, I assume the brown line is the T sounding and the red one is the DP sounding(?) and that cloud top info comes from comparing ELR (T slope) with SALR. Can you talk me through the rest, especially the green curves and the red/blue shading?
Apologies if I'm being thick, it's been a long week!

The met site you referenced (http://pages.unibas.ch/geo/mcr/3d/meteo/) is one I hadn't seen before; very interesting.
Maybe we need a thread somewhere to gather pointers to useful resources, feel like starting one?

HFD

[john_tullamarine]

Maybe we need a thread somewhere to gather pointers to useful resources...[

.. already in this Forum as a sticky .. and the above URL is now there ...

[fantom]

How times change. When I was a lad you had to use this in the CPL Met exams!
Tell that to the kids today and they wouldn't believe you.

[SNS3Guppy]

I don't know...right at the moment my primary assignment is to fly into weather, and we get several skew-t charts as part of the briefing every morning. I find the information completely useless.

It's relevant for the met scientists who are gathering data on our flights and who are administering the program, but from a pilot perspective (excepting glider flights where a skew-t and lifted k index has some relevance)...it's meaningless to me. It doesn't affect what I do or how I do it.

Basically what you have there is a radiosonde graphic representation showing temperature and dewpoint from the surface to whatever altitude the sonde quit providing data, or went out of range. It does provide wind data at altitude, which is useful, but doen't give the location; the balloon and radiosonde are drifting and it's not point data. It's the same data, however, that's refined into your winds aloft reports.

The forecast models are surprisingly accurate, often nearly identical, to the actual data obtained by sonde or by aircraft (we do both; skew-t plots are obtained from each of my flights, and compared to the balloon data), to the point where we occasionaly substitute the model data (data obtained from the computer using a global weather model...the same thing that provides all the weather predictions you use on a daily basis) for the real thing when something is missing or a balloon can't be sent up.

What the skew-t can do for you is reasonably predict cloud bases and clouds at altitude, as well as the liklihood for icing, and the potential for convective activity. You can also predict the minimum surface temperature necessary for convection, and the height to which the convection may be expected to rise, and may even calculate the values of rising and falling air based on the chart data, in and out of convective activity. This is good and well for the scientist, but of little application to us as airplane drivers. I am interested in METARs and TAF's, winds aloft data, area forecasts and other data pertinent to my flight, but it's entirely possible to overthink the planning with things such as the skew-T.

[IO540]

SNS3

Myself and plenty of other instrument rated pilots would definitely pay for a 1-day course on how to interpret these (and other) diagrams. Do drop me a line if you know somebody who can do one, in southern UK somewhere.

I fly a nonpressurised non-turbo plane with a ceiling of about 20k feet i.e. about 500mb, and not really de-iced either.

European airway MEAs are high enough to place you into icing temps most of the year, so flying below cloud is usually not an option - unless you go VFR and then you have the usual scud running scenario, etc.

So, the strategy in IFR flight planning is to find VMC on top, and find it fast, and stay up there for the whole flight.

It's actually not much different for a fully de-iced plane; not many of them can sit in mod ice for 5 hours, and not many passengers want to sit in IMC for hours, puking up.

The particular use of a skew-t (or tephigram) in this scenario is to find how high the cloud tops are likely to be along the route. One then files the flight plan well above that level. During the climb to the FP level, say FL160, if you find tops at FL100, you ask for a stop climb at FL110 and that gives you a better speed (unless you have a tailwind in which case you may want to go higher) as well as conserving oxygen.

This data was not available a few years ago. Pilots would climb and hope for the best. In N Europe, cloud tops are rarely above FL160 unless there is some "vertical development" going on, and then all bets are off; FL250 is common.

That Unibas site uses (I believe) mostly GFS data, and somebody clever can work out cloud tops from GFS directly, e.g. this (http://www.arl.noaa.gov/ready/cmet.html).

Sorry if a couple of URLs got merged in my previous post above...

[dontpickit]

This may be useful, even I can understand some of it!

http://homepage.ntlworld.com/booty.weather/metinfo/thdia_hd.htm

Note also links to Jack Harrison's excellent site + Unisys

[SNS3Guppy]

It's not that simple. My whole function presently is finding weather, flying into it, disecting it, recording it, findign as much ice as I can, and exploring it with all kinds of bizarre little sensors hanging off collecting ice, getting struck by lightening, and all the other things that come from being in weather. I'm backed by an international team of scientists who do understand this data, and brief me on it every morning. Further, I collect the data that becomes the SkewT for the following day, in person.

When I get up there, the weather doesn't look anything like the chart. You can't really look at it and say that you're going to get clouds between 12,000 and 15,000, and so forth. Were often surprised by the weather we see, even when looking at soundings taken a few hours before.

You can, however, look at winds and temperatures aloft and note the temps for your altitude. Some of this is on the Skew-T, but once more, it's a theoretical plot of actual data, spread out downwind of a point in space, which may or may not represent what you'll really find when you get there. The gist of the chart is that it's showing you the temperature and dewpoint spread t any given altitude.

You'll find much more meaningful information in your winds and temperatures aloft forecasts, which appy over a given area to your specific flight level. It's really an amalgamation of multiple soundings put into a mosaic, which you can't get from a single skew-t chart.

You shouldn't necessarily try to plan the flight to be betwen layers...you don't really have that option, nor have that data available. You can pick a temperature to fly...you're going to find some of your maximum icing at the minus ten C level...especially where convective activity exists. Where the supercooed water is most abundant...and just above the cloud base, especially in freezing temperatures. Plan on knowing where warm weather is, and plan your trip for the winds. Avoid the levels between plus five and minus fifteen, and you'll miss lot of the ice. Otherwise, look at radar plots, look at your TAF, and plan accordingly.

The skew-t isn't made for you; it's made for meteorologists. You can use it to come degree, but it's more useful for lookiing at convection and stability than for specifics you might use to plan a flight.

[hugh flung_dung]

Thanks for the replies, I'm now slightly wiser than I was.

I hadn't spotted the sticky but there's some great stuff linked from there - many happy hours of reading. Thanks for maintaining it.

HFD

[Pugilistic Animus]

one excellent explaination on use and interpretation of such charts in soaring is.

Peter Lester's FAA Ac-0045E Aviation weather and inclusive references
very easy to understand and apply

I don't fly gliders, but understand them--- laziness

[bflyer]

Hi guys

This sure looks complicated..and to think it was required to pass a CPL exam gives me shivers
what i would like to know is this..what type of informations would these charts offer for an airline pilot going from A to B that the other more commonly used charts would not give?

[Loose rivets]

Having totally mastered the teffy-gram, I was told that I was saying it incorrectly...it was the teff-io-gram. Instantly, all that I had learned was erased.:ugh:

[Pugilistic Animus]

Sorry, I posted the wrong AC reference for the pseudo-adiabatic chart. I meant AC 00-6A the author's also incorrect, must have been a Freudian slip but AC 00-45E is also very important and one leads to the other anyways

[fantom]

Tee- fye - gram.

Must do better.

[grob103]

Myself and plenty of other instrument rated pilots would definitely pay for a 1-day course on how to interpret these (and other) diagrams. Do drop me a line if you know somebody who can do one, in southern UK somewhere.
Take a look at http://www.lasham.org.uk/learning/met.asp. I went on this course a few months ago, and I now know more about soundings than I ever thought I'd need.

I think all the planned courses are full, but Lasham might be able to organise some more, and Matt might be able to cut down his material to a one day's worth, more suitable for power pilots.

Cheers,

David