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Meteorologists can also represent this information about ascending parcels graphically, by means of a sounding diagram. Sounding diagrams are the simplest way of understanding lots of processes related to precipitation and convection in the atmosphere, so pay attention here. A sounding diagram is, at its core, a two-dimensional graph of temperature and pressure, with temperature on the x-axis and pressure on the y-axis. The temperature and pressure of any air parcel in the atmosphere can be represented as a point on such a diagram.
In the example below, consider an air parcel with a temperature of 20 C and a pressure of 1000 mb. Basically, a typical surface air parcel. To find that spot in the diagram, locate the place where the 1000 mb horizontal line intersects the 20 C vertical line. Well, okay, the temperature lines in this diagram are not vertical, but actually tilt toward the right. It's screwy, I know, but you'll see why in a moment.
Note that if the temperature weren't exactly 20 C and the pressure exactly 1000 mb, you'd have to interpolate to find the right spot on the diagram. For example, suppose you had a parcel whose temperature was 11.5C and whose pressure was 925 mb. Where would that point be on the sounding diagram?
Now consider what happens if this air parcel ascends. As it rises, its pressure falls. Its temperature falls too, at the rate of 9.8 C/km, and the line in the figure shows how the temperature falls as the air parcel rises. With this line on the diagram, you can determine at a glance what the air parcel's temperature would be at any pressure whatsoever. A few examples are given on the figure.
So here's why the constant temperature lines are drawn to tilt toward the right. If it wasn't done that way, the ascending air parcel, since it cools so rapidly, would rapidly move off the left edge of the diagram. This way, we fit all the information into a relatively concise diagram.
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