PandA Povenmire & Associates Aviation Safety Consultants Training Design, Evaluation and Instruction

How to Read the Skew T/Log P*

*Skewed Temperature, Logarithmic Pressure Chart. Drawing it this way lets them squeeze a hole bunch of information into this little space.

There is a full explanation in the FAA Glider Flying Handbook on page 9-8 The diagram there uses different colors. 

Temperature in Celcius is a diaglonal blue line skewed from the bottom up the right edge with the index along the right edge.

 

Altitude in Millibars is a horizontal blue line from left to right with the index along the left edge. These lines are spaced logarithmically for some meteorological reason. 1013.2 is standard sea level. Here is an approximation table for altitudes from Unisys. You have to interpolate:

 

MILLIBARS	Appoximate Altitude MSL
500MB	18750ft
600MB	14100ft
700MB	10100ft
800MB	6500ft
850MB	4900ft
900MB	3350ft
950MB	1900ft
1000MB	500ft

 

Dry Adiabatic lapse rate - Dashed Orange Lines Slanted Up to the Left at approximate right angles to the temperature. (The Standard is -3-degrees C per thousand).

 

Moist Adiabatic lapse rate - Dashed Green Lines curved from nearly vertical at the bottom to the left near the top. (Standard starts at -1-degree C per thousnad, and slopes to near the Dry Adiabatic rate as condensation decreases). 

 

Actual Temperature Sounding - Single Bold Red Line is the from RAWINSONDE balloons. (so much for the standard -2-degrees per thousand)

 

Actual Dew Point Sounding - Single Bold Green Line

 

Mixing Ratio or Dew Point Adiabatic rate - Faint Dashed Yellow lines slanting slightly to the right. (about -0.5-degree C.)

 

In simple terms, if the bold red sounding line cools off as fast or faster than the dry adiabat, glider pilots get excited.

 

First Usable Lift is the surface temperature that must be reached to find a dry adiabatic line where the lifted air will stay warmer than the sounding to a reasonable level.

 

The Thermal Index at 5000 ft is the predicted temperature of surface air lifted to 5000 along the dry adiabat minus the actual temperature of the sounding air at 5000.

 

Height of the Minus Three is where the Thermal Index is -3-degrees C.  That is about where the lift just keeps up with the glider's sink rate.

 

Wind Arrows along the right margin indicate the direction and velocity at different levels.

 

Clouds will form where the bold red and green lines come together.