Thursday, 4 August 2011

The Rasp skew T explained

I have seen on a few occasions puzzled faces surrounding a computer screen, with lots of head scratching and discussions going on and most occurrences these are down to people trying to interpret a sounding for the following day.

I will do my best to explain as simply as possible, so here is a sample sounding.
So first we look at the scales, the left and right vertical axes are height shown in pressure hPa and 1000's of feet, with most soundings showing the boundary layer up to the tropopause. The horizontal axis shows us temperature in degrees Celsius.

The red line shows us the ELR or Environmental Lapse Rate which is the rate at which the atmosphere cools without any added energy. Our blue line shows us the dew point which is the temp the air needs to be in order for it to be fully saturated. Now, the key thing to note is a skew T is not like a standard graph with the point axis (temp) (moisture) travelling straight up the chart, you will notice the orange lines from the temps are slanted up to the right, you will also notice another set of orange diagonal lines sloping from bottom right to top left, these are our air parcel DALR or Dry Adiabatic Lapse Rate which is the rate at which the parcel cools while it is not saturated. The reason they slant is because if the were vertical the chart would be very large and wouldn't make much sense which is why they are skewed, hence the skew T (tephigram)

There are also two green lines, one dotted, which is the water content at a given temp or how saturated it is.we then have the solid green lines which are more vertical, these show the SALR or Saturated Adiabatic Lapse Rate which shows the cooling of the Parcel one it has become saturated.

Also on the right is the wind direction and speed with altitude.

So lets take a closer look,
So on this skew T we can see that the Environmental surface temp for the given time is going to be around 25dc we can also see that the dew point is around 15dc at the surface, Tracing both lines up the graph shows us the general state of the atmosphere in terms of temp and moisture content. Now we can also see a handy dotted line, this shows us the forcasted max temp for the day and what happens to a parcel of air once it reaches this temp. so lets look at its journey.

So our parcel of air (thermal bubble) reaches critical temp of say 26dc (can be higher if it remains on the ground longer, warming up more before it is triggered) it is then triggered into the air. As it begins to rise it expands and cools (I wont get into the intricacies of pressure etc...) at the DLAR or ~3dc per 1000ft which roughly follows the dotted line. As you follow our parcel up draw a line from the dew point Parallel to the nearest moisture content dotted line up until it meets our parcel (something like this)

What happens here is that our parcel has cooled to the dew point, this means it can no longer hold onto its water vapour so it starts to condense, this gives us the LCL or Lifted condensation level or Cloud base for glider pilots and if you look at the scales and apply the dew point difference of 10dc x 400 = 4000ft its almost bang on. from there on the parcel follows the green (more vertical) SALR of around 1.5dc per 1000ft the SALR is less than the DALR because condensation releases latent heat stored during the evapouration process.

A little further up the ELR (red line) you can see a bump indicating a slight inversion (where the air warms slightly or doesn't cool as quick) Now our parcel has saturated before reaching this hump meaning we will have clouds, if as in the example below the parcel does not saturate before the inversion then we have a blue day.

If it looks like this then we will have Cu's
You can see the LCL and then cloud formation begins until it bumps into the inversion, in this example the dew point drops away quickly afterwards which should produce classic Cu's with little spread out  or over development.

Our first example showed the parcel saturating and then following the SALR, it misses (more to the point punches through) the inversion because at that altitude the parcel is still warmer than the surrounding air so continues to rise, now we get into the classic over development situation that causes large CuNim's. The number of degrees between our parcel and the ELR at 500hPa or ~18kft gives us our lifted index in this case a -2, any minus number here indicates instability and the greater it is the more instability there is. The total area between the parcel dotted line and out ELR give us our CAPE or Convective Available Potential Energy which shows how much energy is potentially available, the larger the CAPE value the more powerful any resulting storm/rain shower would be.

The RASP skew T's are not bad, there are some inaccuracies, I usually compare 2 different tephigrams and also a couple of forecasts which give good boundary layer coverage. the only thing to bear in mind is that most models used to forecast diverge after around 48 hours becoming less accurate, they can show you a trend but I always say "oh this weekend looks OK" on Monday or Tuesday and then pick the day to fly on Friday as you have a much better idea.

Hope this helps all those that have been baffled by skew T's and if anyone has any other MET related questions please feel free to ask.


grokist said...

Doesn't the red line show you the environmental temperature, from which you may infer the ELR?

lhitchins said...

Yes, you could probably say that. if you track the red line it gives you environmental lapse rate but at any given point it gives you the temperature.

jtletsfly said...

I know you posted this a couple of years ago but it is a very useful explanation.. thanks.

lhitchins said...

No problem, I am going to try to contribute more this year.

onepixelone said...

Thanks Lee

Nice to find your notes, finally got it!


J said...

I believe that by LCL you are actually refering to CCL (Convective condensation level) according to this
LCL is used when air is mechanically lifted for example by a slope.

Unknown said...

Can somebody explain the little red dotted line in layman's terms?