Welcome New Members! We want to hear from you. Register, stop lurking and start posting!

How to read Sounding charts

Archived Nerdy Forum (Weather Q&A).
Locked
User avatar
daviescr
Supercell
Reactions:
Posts: 1394
Joined: Wed Dec 02, 2009 5:40 pm
Location: Warranwood, Vic

How to read Sounding charts

Post by daviescr »

Thought it might be good to copy Pete's excellent post on how to read Soundings. Will also post his example sounding chart below.

Chris
Pete » Wed Jul 20, 2011 11:33 pm wrote:
Pete wrote:Sure. :)

This is a sounding, which is a vertical profile of the atmosphere obtained from a radiosonde. The red line on the right is the temperature line. The brown/dark yellow lines going diagonally across the chart are the temperature readings. As the temperature goes further up into the atmosphere its gets colder. The values to the left of the sounding (1000, 900, 800 ect,) indicate height and a measured in hPa. Though you can see they are converted to height in metres as well as in feet. You can see that the temperature gets colder the further up you go into the atmosphere. On the sounding that I have posted the temperature is roughly 14C at 1000mb and its bang on -50C at 200mb.

The red line on the left is the dew point line. Again it tends to get colder as it goes up. On the sounding above it's roughly 9C at 1000mb and almost -40C at 400mb. Try not to get the temperature and dew point line mixed up as sometimes they get quite close to each other (meaning that there's almost no temperature difference between them.)

Alright now the grey line is the Theoretical Air Parcel Plot line or (TAPP). Basically this line tells you how the air will rise. If the grey line lies to the right of the temperature line then that part of the atmosphere is unstable. The further to the right the grey line is the more unstable the atmosphere is. You can see in the sounding above that the atmosphere is relatively unstable.

However if the grey line lies to the left of the temperature line then then that part of the atmosphere is stable. If you follow the grey line up on the sounding you'll see it crosses back to the left of the temperature line between 300mb and 400mb. That part of the atmosphere is stable but it didn't affect the storms forming today because most winter storms go above 25,000 feet.

A cap or inversion can be identified if the temperature line is right of the grey line but crosses left sharply at a certain height.

At first the grey line is two grey lines that meet and form a triangle shape. Where they meet at the point of the atmosphere is where convection will occur.

Sometimes on a sounding the temperature line and dew point line will quite far apart. This is generally indicative of a dry atmosphere. If they meet or even cross this means the atmosphere is quite moist.

Now for the green dashed lines. These are quite complicated and I generally get a headache just trying to interpret what they mean. The dashed green lines that start at the bottom, and rise diagonally to the left, is the Dry Adiabatic Lapse Rate (DALR). This is the path that a parcel of unsaturated air will rise in the atmosphere.
The dashed green lines that rise diagonally to the right are the saturated mixing lines. These lines represent the amount of water vapour in the atmosphere measured in grams.
Then finally there's the dashed green lines which go straight up and then 'curve'. This is the Saturated Adiabatic Lapse Rate (SALR). This indicates the path that a saturated parcel of air will take as it goes up into the atmosphere.

Now for the abbreviations on the sounding.

PS = surface pressure
TS = surface temperature
DS = surface dew point

LCL = (Lifted Condensation Level) is the pressure level a parcel of air reaches saturation by lifting the parcel from a particular pressure level. As a rising parcel of air cools, the relative humidity increases inside a rising unsaturated parcel. Once the RH first reaches 100% in the parcel, the LCL occurs there.

LFC = (Level of Free Convection) is the lower boundary of the most significant region of CAPE in the troposphere. It is the point at which a lifted parcel of air will become equal in temperature to that of the environmental temperature. Once a parcel of air is lifted to the LFC it will rise buoyantly on its own all the way to the top of the CAPE region.

CAP = The cap. The stable region in the atmosphere. Positive values mean there's a cap and it prevent storm development until broken. Negative values mean an unstable atmosphere and thunderstorms will form if other conditions are favourable.

CAPE = (Convective Available Potential Energy) is the integration of the positive area on a Skew-T sounding. The positive area is that region where the theoretical parcel temperature is warmer than the actual temperature at each pressure level in the troposphere. Basically the higher the CAPE the bigger the chances of severe thunderstorms, large hail, tornadoes, ect. It is measured at Joules per kilogram (j/kg). CAPE of more than 2500 j/kgs is indicative of extremely severe thunderstorms. Generally CAPE of 100 j/kgs+ means storms are likely.

CIN = (Convective Inhibition) measures the cap. 0-50 = weak cap. 51-99 = moderate cap. 199+ = strong cap.

LI = (Lifted Index) measures instability. Positive values mean generally stable weather. Negative values mean generally unstable weather (unless there's an unbreakable cap present)

TT or TTI = (Total Totals Index) measures the likelihood of convection. <44 = convection not likely. 44-50 = thunderstorms likely. 51-56 = severe thunderstorms.

SWEAT = (SWEAT index). Values between 150-300 means severe thunderstorms are slightly possible. 300-400 and above and severe thunderstorms are likely.

These are the values I generally look at. I hope I haven't made it too complicated (though I'm sure I have) and hope you'll understand it. Oh and someone with more knowledge on these things, feel free to correct me on any mistakes I may have made as I'm still learning to full interpret these things properly.
User avatar
daviescr
Supercell
Reactions:
Posts: 1394
Joined: Wed Dec 02, 2009 5:40 pm
Location: Warranwood, Vic

Re: How to read Sounding charts

Post by daviescr »

Here's the sounding from yesterday's storm in WA:
Pete » Wed Jul 20, 2011 9:31 pm wrote:
Pete wrote:Had a thunderstorm over my place this arvo! :D Looking at the sounding I can understand why. I had to re-plot the sounding because the surface temperature and dew point were two degrees off but this is what it looked like after I'd finished.
Image
J
Jake Smethurst
Supercell
Reactions:
Posts: 3583
Joined: Mon Nov 23, 2009 8:49 pm
Location: Cheltenham

Re: How to read Sounding charts

Post by Jake Smethurst »

A fantastic explanation Pete!! I am absolutley horrible at explaining soundings to other people, particularly over a forum, however you have done this very well mate! I'm sure many people will read this and better understand them. :)
Jake - Senior AWF Forecaster
Feel free to send me a private message if you have any questions.
P
Pete
Cumulonimbus
Reactions:
Posts: 470
Joined: Mon May 30, 2011 9:02 pm
Location: Albany, Western Australia

Re: How to read Sounding charts

Post by Pete »

Thanks Power Storm. I just realized I got something mixed up. I wrote that a cap or inversion can be identified if the temperature line is right of the grey line but crosses left sharply at a certain height. But actually its the other way round. A cap or inversion can be identified if the temperature is to the left of the grey line and crosses sharply to the right of the grey line at a certain point.
Still learning myself. :cheers:
P
Pete
Cumulonimbus
Reactions:
Posts: 470
Joined: Mon May 30, 2011 9:02 pm
Location: Albany, Western Australia

Re: How to read Sounding charts

Post by Pete »

Just a question:
Image

On a sounding like this wouldn't all that lovely CAPE be useless because the LCL and LFC are at 746mb and 745mb respectively?
P
Pete
Cumulonimbus
Reactions:
Posts: 470
Joined: Mon May 30, 2011 9:02 pm
Location: Albany, Western Australia

Re: How to read Sounding charts

Post by Pete »

lol when I just re-read my earlier post, I see I made a few mistakes there. :oops:

Anyway, what's with the Rockhampton sounding today?
Image

Why is the dew point line brown after 700mb and what does it mean?
Locked