Skew-T Log-P diagrams

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Skew-T Log-P diagrams

Postby Lert » Mon Dec 04, 2006 10:28 am

Hi all

For those who have difficulty with Skew-T Log-P diagrams (and I'm not leaving myself out) this site looks good.. The exam is next week :lol:

http://www.personal.psu.edu/smd293/meteo482/
2006 - 543.5mm, 2007 - 701.5mm, 2008 - 833.5mm, 2009 - 579mm, 2010 - 631.5mm, 2011 - 872.5mm, 2012 - 770mm

2013 Jan 5mm YTD 5mm

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Re: Skew-T Log-P diagrams

Postby Pete » Wed Jul 20, 2011 9:42 pm

Here's my own interpretation. :)

This is a sounding taken from today.
Image

A sounding 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. The temperature readings are indicated at the base of the sounding. 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 25C at 1000mb and its bang on -60C 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 14C at 1000mb and again -60C at 200mb. 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.) This means that the portion of the atmosphere where this occurs is extremely moist.

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 temperature line crosses to the right of the grey line then then that part of the atmosphere is stable. If you follow the temperature line up on the sounding you'll see it crosses back to the right of the grey line between 200mb and 300mb. The area where the line crosses is called neautral stability and once the temperature line is back to the right of the rising air parcel temperature the atmosphere is stable once more.

A cap or inversion can be identified if the temperature line is left of the grey line but crosses right 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.

There are a few other acronyms which show up on a sounding but usually don't have values next to them so I didn't put them in. :)

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Re: Skew-T Log-P diagrams

Postby Lert » Thu Jul 21, 2011 7:37 pm

Mate, that is seriously good work.. thanks for doing that and should go a long way for others to understand those charts. :D
2006 - 543.5mm, 2007 - 701.5mm, 2008 - 833.5mm, 2009 - 579mm, 2010 - 631.5mm, 2011 - 872.5mm, 2012 - 770mm

2013 Jan 5mm YTD 5mm

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Re: Skew-T Log-P diagrams

Postby Pete » Thu Nov 03, 2011 7:54 am

I just want to know why the saturated adiabatic lapse rate is not a constant? And under what conditions would it change? I understand that this lapse rate changes with surface temperature but by how much per 1000 metres when the surface temperature changes by 1 degree?

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Re: Skew-T Log-P diagrams

Postby Lert » Thu Nov 03, 2011 9:03 am

2006 - 543.5mm, 2007 - 701.5mm, 2008 - 833.5mm, 2009 - 579mm, 2010 - 631.5mm, 2011 - 872.5mm, 2012 - 770mm

2013 Jan 5mm YTD 5mm

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Posts: 3443
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Re: Skew-T Log-P diagrams

Postby Pete » Sat Nov 05, 2011 7:10 pm

Thanks Lert that's great. :) @-)

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Re: Skew-T Log-P diagrams

Postby Pete » Thu Apr 19, 2012 5:04 pm

Another thing to note when reading soundings: You can have a positive CAPE value without a negative LI value, but you can't have a negative LI value without a positive CAPE value.

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