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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.

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

Altitude in Millibarsis 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 MSL500MB

18750ft600MB

14100ft700MB 10100ft800MB 6500ft850MB 4900ft900MB 3350ft950MB 1900ft1000MB 500ft

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

Moist Adiabatic lapse rate - Dashedcurved 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).GreenLines

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

Actual Dew Point Sounding - Single Bold Green Line

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

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 Liftis 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 Indexat 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 Threeis 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 marginindicate the direction and velocity at different levels.

Clouds will formwhere the bold red and green lines come together.