Ascent of a Typical Air Parcel

So far, we've considered two extreme cases: an air parcel with no water vapor whatsoever, and an air parcel that starts out completely saturated with water vapor. Now let's consider a more typical case. Take the air parcel at 20 C and 1000 mb, but at 30% relative humidity.

Such an air parcel would have 4 grams of water vapor for every 1 kg of dry air. We refer to this as a mixing ratio of 4 g/kg.

What will happen to the air parcel? It will ascend for a while, cooling at the dry adiabatic lapse rate. Eventually, it will cool so much that it will become saturated. Above that level, the parcel will cool more slowly, at the moist adiabatic lapse rate.

First issue: when will the parcel become saturated? We can draw a line on the sounding diagram which indicates the temperature and pressure of all air parcels which are at 100% relative humidity if the mixing ratio is 4 g/kg. Here's the line:

Any air parcel that starts off with 4 g/kg of water vapor to the right of that line is unsaturated. But if that parcel rises, eventually it reaches the 4 g/kg saturated mixing ratio line. Any higher or cooler, and water vapor would start condensing.

Our actual air parcel, starting at 20 C and 1000 mb, ascends dry adiabatically at first, and reaches the saturation line at a pressure of 760 mb and a temperature of -1 C. Above that level, condensation takes place as the parcel continues to ascend, and the rate of cooling decreases.'

If you're keeping score, the dry air parcel cools the most and the saturated air parcel cools the least. Parcels that are partially saturated cool at the dry rate until they reach saturation; after that they stop cooling so rapidly.

Previous Page | Next Page