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So far, I've been giving you all these numbers and diagrams, and it's been helping you see how air parcels behave. But there are a couple of things missing. First, you don't know how to apply this knowledge to weather forecasting. Second, you don't have any tools yet to allow you to determine for yourself exactly what an air parcel will do, unless it's a completely dry air parcel.
Let's tackle the second problem first. Below is a complete sounding diagram:
On this diagram, in addition to the temperature and pressure lines we've had all along, are three other sets of lines:
- The first set, in yellowish-brown, shows the paths on the diagram that air parcels will take if they are ascending or descending while unsaturated. These lines slope at the rate of 9.8 C/km, the dry adiabatic lapse rate, and are called dry adiabats. An air parcel ascending in the atmosphere will cool at a rate parallel to the dry adiabats. Our 20 C, 1000 mb dry parcel starts right on a dry adiabat and stays there, dropping to 6 C at 850 mb, -9 C at 700 mb, etc.
You try one. Suppose an air parcel starts at -27 C at 700 mb. What temperature would it reach if it sank to 1000 mb?
2. The second set, in blue, shows the paths on the diagram that air parcels will take if they are ascending or descending while saturated. These lines are called moist adiabats, and are more curved than the dry adiabats because the moist adiabatic lapse rate is not constant. Consider again the 20 C, 1000 mb parcel if it's saturated: it will ascend parallel to the moist adiabats, cooling to around 13 C at 850 mb and 5 C at 700 mb.
Now your turn. Suppose an air parcel starts at 0 C and 1000 mb, and is saturated. What temperature would it reach if it rose to 700 mb?
3. The third set, in green, are the lines of constant saturation mixing ratio. Air parcels will ascend parallel to the dry adiabats until their mixing ratio reaches saturation, and will ascend parallel to the moist adiabats after that point.
But how do you use the saturation mixing ratio lines? How do you know when an air parcel will reach saturation? Funny you asked that...
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