The saturation adiabats are the curved green lines that intersect the 1,000-millibar isobar at 2C intervals. These lines curve upward toward the left and diverge as they get closer to the top of the diagram. They terminate at the 200-millibar level and are labeled at that point in green numbers. The values can also be read at the 1,000-millibar level, where they are the same as the value of the intersecting isotherms. These lines represent the rate at which a saturated parcel of air will cool as it moves upward in the atmosphere. Saturated air will not warm at this rate as it moves downward in the atmosphere, because it will become nonsaturated the moment it begins to descend and the pressure increases. The difference in the dry adiabatic and saturation adiabatic lapse rates is the heat of condensation gained by the air when water vapor condenses out of saturated air as it is lifted.

The dashed green lines extending from the bottom of the diagram upward diagonally toward the right are the saturation mixing ratio lines. They are labeled near the bottom of the diagram in green numbers representing grams of water per kilogram of air. We use the mixing ratio lines later to find how much moisture is in the air (actual mixing ratio) and how much moisture the air can hold (saturation mixing ratio).

Ten horizontal scales printed in black extend across the central portion of the diagram parallel to the isobars. These are the thickness scales. Each is labeled on the left side by two numbers separated by a solidus, such as the 1000/700 we find on the bottommost scale near the 840-millibar isobar. They are graduated, with increments being in feet above the horizontal line and in meters below the horizontal line. The graduations are labeled in hundreds of feet and in hundreds of meters. These scales are used to calculate the thickness of the layer of the atmosphere between the two values. We would use the bottom scale to calculate the thickness of the 1,000-millibar to 700-millibar layer, for instance.

At the 500-millibar isobar between the 36 degree and 47 degree isotherms are four fine black lines extending upward to the right all the way to the top of the diagram. At the 400-millibar isobar (which is also the 100-millibar isobar) are four dashed black lines extending upward to the right, ending at the 40-millibar level. These are the contrail formation scales for the 500-millibar to 100-millibar and 100-millibar to 40-millibar levels. They are labeled from right to left as 100, 90, 60, and 0. The lines indicate the temperature and relative humidity necessary at any pressure above 500 millibars for saturation to occur by the addition of water vapor from a jet aircraft engine. We will use these scales to analyze contrail formation levels.

The U.S. Standard Atmosphere is a represen-tation of an ideal atmosphere based on the thermodynamic equation and the defined stan-dards for sea level pressure (29.921 inches of mercury) and sea level temperature (59.0F). It is not a climatological average for the continental United States. The temperatures of the standard atmosphere are plotted as a single brown line extending from the bottom of the chart at the 17C isotherm upward to the left to 256 millibars, then along the 56.5C isotherm to the 100-millibar level. Standard heights are printed in both meters and feet on the left side of the diagram under the standard pressure levels, as well as on a scale to the right of the diagram.

Learning Objective: Describe the computa-tion procedure to find derived measure-ments from plotted data on the Skew T, Log P Diagram.

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