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First, let's review the various lines on a sounding diagram, also known as a thermodynamic diagram. The horizontal lines are pressure: they provide an indication of the height of the air. The lines sloping most steeply to the right are temperatures. The lines sloping most steeply to the left are dry adiabats, or lines of constant potential temperature: they indicate how the temperature of an unsaturated air parcel would change if raised or lowered. The lines which slope not so steeply to the left are moist adiabats, or lines of constant equivalent potential temperature: they indicate how the temperature of a saturated air parcel would change if raised (or lowered, if it somehow remains saturated). Finally, the lines which slope not so steeply to the right are lines of constant saturation mixing ratio, and indicate the amount of water vapor an air parcel would have to include to be saturated at various temperatures and pressures.
To plot the thermodynamic characteristics of an air parcel required two points: one to indicate the parcel's pressure and temperature, and one to indicate the parcel's pressure and dew point. Now take this one step farther: suppose you have temperature and dew point data for every level in the atmosphere. How would that be represented on a sounding diagram? By two lines! Go to the next page for an example.
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