**
SOUNDINGS:**

**
SKEW T/LOG P DIAGRAMS & TEPHIGRAMS**

The following relates to
Skew T/Log P diagrams. Tephigrams are very similar.

**Think of it like this……**

The
underlying grid is Pressure v. Temperature but warped (skewed?) so that although
P is the vertical scale, the T scale is at approx 45^{deg }. Neither scales
are linear or, necessarily, the lines straight.

The P scale can be interpreted as altitude in the Earth’s
Atmosphere and so is also labelled as such (for the Standard Atmosphere).

Superimposed on this grid are the dry adiabatic and saturated adiabatic lapse rate
lines (DALR, SALR). i.e. the lines along
which parcels of dry and saturated air, respectively, rise. These are not labelled with any units (although
I guess they could be). All you need
to know is that if a parcel of air starts anywhere on a line (i.e. at a particular
T & P) then it will stay on that line – if dry until it becomes saturated
- when it will swap to the saturated line.
This all depends on the principle that there is essentially no transfer
of heat (energy) across the boundaries of a ‘parcel’ of air which effectively
remains insulated from the surrounding atmosphere as it rises (or falls).

Also
superimposed on the grid are the ‘__saturated__ mixing ratio’ lines (isohumes). These are
labelled in grams of water vapour per kilogram of air. i.e. indicating the
‘amount’ of water vapour in a certain parcel of air.
A line joins all the
possible pressure/temperature pairs of
a parcel of air
with the indicated amount of
water vapour
__when it is saturated__.
^{*} The
fact there are only certain allowed value pairs is to do with latent heat,
vapourisation etc.

These
three sets of lines superimposed on the P/T grid (i.e. already plotted for
you) are
the result of Physics and the Gas Laws and are, for
all practical purposes,
invariable in the Earth’s Atmosphere.
Therefore the whole lot can be printed as ‘graph paper’ for plotting
purposes.

So
what do we
plot?

On
the graph paper are plotted the following
zig-zag lines, obtained from
the ascent of a weather
balloon ('sounding') for a particular date/time/place – which gives the characteristics
of that particular airmass:-

a) the Environmental Lapse Rate line (ELR) (to the right and normally plotted in red) = the actual temperatures at various altitudes (pressures)

and

b) the Dew Point line (DP) (to the left and normally plotted in blue) = the dew point temperature at various altitudes (pressures).

The rest is interpretation!!!

**Thermals:** The classic
theory ** says a thermal
rises
because the temperature of that parcel of air is higher than the surrounding
environment . As it rises
its temperature falls
(by the Gas Laws)
until it reaches the surrounding environmental temperature when
its ascent stops.
Start at (guestimated) surface temperature and follow DALR line (^{}then SALR line*) until it hits the ELR line. This is the top
of thermals. The **area**
between the D/SALR lines and the ELR line is an indication of energy i.e.
thermal strength.

* Dew Point at the surface effectively indicates moisture content of the
airmass at surface level and the appropriate isohume for the day. (You
might have to guestimate how this has changed since
the sounding.) Follow the isohume up from the surface dew point until it intersects the
DALR line concerned. This is cloudbase.
When a parcel of air
(ascending along a dry adiabat from the surface),
starting at the actual/predicted surface temperature, reaches the associated
isohume (representing the water content of the ascending
air), cloud will form, because it will have reached a temp/pressure pair for
saturation: hence cloudbase for cumulus cloud. **From here on the parcel
ascends along the saturated ** **adiabat**. If
the parcel never hits the isohume, the day will be blue.

Cloud
top (cumulus) is indicated by where the relevant SALR line hits the ELR line. i.e. the top of thermals.