What happens when unsaturated air is cooled?

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RISING AND SINKING AIR

As described in Investigation 6A, when air moves vertically in the atmosphere, it experiences changes in surrounding atmospheric pressure. These changes in pressure allow a rising parcel (a term used in meteorology to imply an idealized small volume or body of air) to expand as surrounding pressures decrease, and cause a sinking parcel to be compressed as surrounding pressures increase. Rising, unsaturated air (relative humidity less than 100%) expands and cools at a rate of 9.8 C° per 1000 m (5.5 F° per 1000 ft). This is called the dry adiabatic lapse rate. Sinking unsaturated air compresses and warms at the same rate.

A rising unsaturated air parcel expands and cools, and may eventually become saturated. Upon saturation, further ascent will continue the expansional cooling. But, some heating will also take place within the parcel because of condensation (or deposition at low temperatures). This warming occurs as latent heat is released to the air in the parcel when water vapor condenses into droplets or deposits as ice crystals. The simultaneous cooling by expansion and warming by condensation or deposition results in a net (observable) cooling rate, called the saturated (or moist) adiabatic lapse rate, that is less than the cooling rate for unsaturated air. The saturated adiabatic lapse rate varies with temperature, but can be considered to average about 6 C° per 1000 meters (3.3 F° per 1000 ft).

After completing this investigation, you should be able to:

• Describe how to use a Stüve diagram to follow atmospheric temperatures and pressures.
• Determine the temperature of air that rises or sinks in the atmosphere.
• Describe how the water vapor saturation of air can affect atmospheric temperatures.

The Figure 1 Stüve diagram in this investigation includes lines representing the adiabatic processes of dry (unsaturated) and saturated air. The solid, straight green lines sloping from lower right to upper left in the body of the chart graphically represent the dry adiabatic lapse rate, showing visually the temperature change of an unsaturated air parcel that is undergoing vertical motion in the atmosphere. The dashed, curved blue lines sloping from lower right to upper left represent the temperature change of saturated air undergoing vertical motion, the saturated adiabatic lapse rate. Locate an air parcel with a temperature of 17 °C and a pressure of 1000 mb by placing a dot on the chart on the 1000 mb horizontal line where 17 °C would occur.

Figure 1. Vertical Atmospheric (Stüve) Chart with adiabats.

1. If this air rises as unsaturated (dry) air from 1000 mb, determine its temperature at 500 mb by following the solid, straight green dry adiabatic lapse rate line passing through the starting point, up to 500 mb. At 500 mb, the temperature of the unsaturated air parcel is about [(–5)(–35)(–45)] °C.

2. If this air rises as saturated air from 1000 mb, determine its temperature at 500 mb by following the dashed, curved blue saturated adiabatic lapse rate line passing through the starting point, up to 500 mb. At 500 mb, the saturated air parcel’s temperature is approximately [(–15)(–25)(–35)] °C.

3. At 500 mb, the temperature of the unsaturated air parcel is [(lower than)(the same as)(higher than)] the temperature of the saturated air parcel.

4. This comparison demonstrates that rising unsaturated, clear air cools [(more)(less)] than rising saturated, cloudy air over the same pressure change.

5. Begin once again with unsaturated air at 17 °C at 1000 mb. Because it is unsaturated, its relative humidity initially is [(greater than)(equal to)(less than)] 100%.

6. As this air rises, assume it becomes saturated at 800 mb. Being unsaturated from
1000 mb to 800 mb, it will follow a [(dry)(saturated)] adiabatic lapse rate line.

7. Becoming saturated at 800 mb, its relative humidity is now [(greater than)(equal to)(less than)] 100%.

8. As the air continues to rise, it will follow a [(dry)(saturated)] adiabatic lapse rate line.

9. Continue the ascent to 500 mb. The air parcel temperature is now approximately [(–18)(–27)(–34)] °C.

10. This temperature is [(higher than)(equal to)(lower than)] the temperature achieved by the unsaturated parcel that ascended dry adiabatically the entire way to 500 mb in item 1.

11. If condensation was occurring during the ascent from 800 mb to 500 mb, the air parcel would have [(gained)(lost)(had no change in)] water vapor during the ascent.

12. Throughout this saturated portion of the ascent, the relative humidity of the air parcel is [(greater than 100%)(100%)(less than 100%)].

13. Assume that all the water that condensed (or deposited) during the ascent was immediately lost as precipitation from the parcel. Therefore, if the air parcel at 500 mb begins to descend, it will experience warming by compression and immediately become an unsaturated parcel. As the parcel sinks back to the 1000-mb level, it will warm at the dry adiabatic lapse rate, as shown by following the dry adiabatic lapse rate line down from the point at 500 mb. When it arrives back at 1000 mb, its temperature is [(17)(27)(37)] °C.

14. This parcel’s final temperature is [(higher than)(the same as)(lower than)] its beginning temperature when it was initially at 1000 mb.

15. The relative humidity of this air parcel is now [(greater than)(equal to)(less than)] what it was when it began its journey at 1000 mb.

16. The change from the initial parcel temperature to the final parcel temperature at the 1000-mb level was caused by condensation (or deposition) which [(releases)(absorbs)] latent heat.

As directed by your course instructor, complete this investigation by either:

1. Going to the Current Weather Studies link on the course website, or
2. Continuing the Applications section for this investigation that immediately follows.

Investigation 6B: Applications

The Applications section of Investigation 6A examined the formation of clouds and precipitation associated with a frontal system that curved across the country. There we examined the identification of clouds on a Stüve diagram from the Buffalo (BUF) rawinsonde observation at 0000Z 14 OCT 2013. Here we will consider that sounding in more detail and also look at another sounding from Green Bay, Wisconsin acquired at the same time.

17. The Investigation 6A, Figure 2’s surface weather map for 00Z 14 OCT 2013 showed that Buffalo, NY [(did)(did not)] have overcast sky conditions at map time.

18. Green Bay, WI was near the center of the high-pressure system that was dominating the weather in the upper Mid-West. Green Bay’s temperature was 50 °F (obscured) and dewpoint was 40 °F; the sky cover on the surface map was reported as [(clear)(partly cloudy)(mostly cloudy)(overcast)].

19. Based on the reported sky conditions, Buffalo had saturated atmospheric conditions above the ground and Green Bay [(did also)(did not)].

20. Recall that on Stüve diagrams, the bold irregular curve to the right is the temperature profile while the bold curve to the left is the dewpoint profile. Where the curves are superimposed, the temperatures and dewpoints are equal. Buffalo’s Stüve diagram (Investigation 6A, Figure 2) showed by the separation of the temperature and dewpoint values at and near the surface that the surface air [(was)(was not)] saturated.

21. Air, rising from the surface at Buffalo, would [(expand and cool)(be compressed and warm)].

22. The temperature profile from the surface up to about 970 mb over Buffalo was along the [(straight, solid, green dry)(curved, dashed, blue saturated)] adiabatic lapse rate line printed on the diagram. This was evidence of surface air moving upward and cooling by expansion at the unsaturated adiabatic lapse rate.

23. The air rising from near the surface above Buffalo cooled, and at about 970 mb, its temperature and dewpoint [(did)(did not)] become equal.

24. The air over Buffalo at 970 mb [(was)(was not)] saturated.

The following were values from the Buffalo rawinsonde text data (not shown): 998 mb (surface) occurred at 215 m where the temperature was 17.2 °C and 970 mb was located at 460 m where the temperature was 14.8 °C. Therefore, the temperature difference between those two levels was 2.4 C° over an altitude change of 245 m. The temperature lapse rate was therefore equivalent to 9.8 C° per kilometer.

25. The lapse rate of unsaturated rising air in the atmosphere is theoretically 9.8 C° per km. This calculated lapse rate value in the actual atmosphere in this case was [(equal to)(several degrees different from)] the unsaturated adiabatic lapse rate. Unsaturated rising air really does follow an adiabatic process!

26. The temperature profile from 970 mb up to about 600 mb over Buffalo was approximately parallel to a nearby [(straight, solid, green dry)(curved, dashed, blue saturated)] adiabatic lapse rate line printed on the diagram.

27. The Buffalo rawinsonde text data also provided the following: 601 mb occurred at 4400 m with a temperature of ‒5.1 °C. The temperature difference between 970 and 601 mb over the altitude difference was therefore an equivalent 5.1 C° per kilometer. The average lapse rate of saturated rising air in the atmosphere is approximately 6 C° per km. This calculated lapse rate value is [(within a degree of)(five degrees from)] the average saturated adiabatic lapse rate.

Figure 2 of this investigation is the Stüve diagram from the Green Bay rawinsonde observation at 0000Z 14 OCT 2013, the same time as the surface map and Buffalo sounding from Investigation 6A.

Figure 2. Stüve diagram of Green Bay, WI (GRB), sounding for 00Z 14 OCT 2013.

28. The separation of the temperature and dewpoint profiles over Green Bay show that the air [(was)(was not)] saturated. Note the increase of temperature (an inversion) from the surface to about 995 mb over Green Bay. This was likely due to radiational cooling of the ground following sunset, which in turn, cooled the air immediately above the surface.

29. Air, rising from above that inversion layer over Green Bay, would [(expand and cool)(be compressed and warm)].

30. The temperature profile from about 995 mb up to about 825 mb over Green Bay is nearly parallel to the [(straight, solid, green dry)(curved, dashed, blue saturated)] adiabatic lapse rate line printed on the diagram.

The following values come from the Green Bay rawinsonde text data for Figure 2. Over Green Bay, 997 mb occurred at 258 m where the temperature was 13.2 °C and 826 mb was located at 1802 m where the temperature was –0.9 °C. Therefore, the temperature difference between those levels (14.1 C°) over an altitude change 1544 m) gave a temperature lapse rate of 9.1 C° per kilometer.

31. The 9.1 C°/km lapse rate value [(was)(was not)] within a degree of the theoretical 9.8 C° per kilometer value of the dry adiabatic lapse rate.

While rising, unsaturated air as seen above the surface over Green Bay was cooling only by expansion, the rising saturated air over Buffalo was simultaneously cooling by expansion and warming from the release of latent heat due to condensation of water vapor (which formed the extensive cloud layer).

Stüve diagrams of actual observations confirm that vertical atmospheric motions do follow the theory! Call up Stüve diagrams as dramatic weather changes affect your area. Weather systems (Highs, Lows, fronts) force air to move vertically causing accompanying temperature changes. Orographic effects in mountainous areas also drive tropospheric temperature patterns.

Suggestions for further activities: You might print out the text data of rawinsonde observations and plot them on a blank Stüve diagram (available on the website) when Highs, Lows, and fronts pass nearby. Then compare local cloud and sky conditions with the temperature and dewpoint profiles you have plotted.