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 45deg .  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).   For identification on a Skew-T: the DALR lines slope up to the left with ever increasing steepness: the SALR lines slope up to the left with ever decreasing steepness.  TePhigrams present the same information in a slightly different way.


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)


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!!!


If the ELR line kinks to the right and its slope becomes less than the slope of the temperature lines then we have a temperature 'Inversion' at the height of the kink.  i.e. At this altitude the atmospheric temperature begins to rise instead of fall.  This puts a cap on any ascending air, thermals wont get much higher (they may 'punch through' a bit) and air pollution is trapped below.  Thus poor vis. below this level.

If the ELR line meets the DP line (it can never go the the left of it), stratus clound will form at these levels and it will be overcast.

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.  (On blue days the top of thermals is where the relevant DALR line meets the ELR line.) If cloud tops are above the freezing level (zero degree temperature line) there is an increased risk of showers. If there is an inversion above the freezing level and above cloudbase then any spreadout will hang about for a long time.

T-phi graphs or Tephigrams are essentially the same although they don't look it. It's just that the scales are constructed in a different way.


** as opposed to the Roch theory about humidity and air density.