CLOUDS, TEMPERATURE, AND AIR PRESSURE

Objectives:

Clouds are an ever-present feature in Earth’s atmosphere. A cloud is a visible suspension of tiny water droplets and/or ice crystals formed when water vapor condenses or deposits within the atmosphere. Air temperature changes resulting from air pressure changes play major roles in determining where clouds do and do not occur. Clouds develop where air ascends and dissipate where air descends.

After completing this investigation, you should be able to:

• Describe how air temperature changes as air pressure changes.
• Make clouds appear and disappear in a hypothetical bottle.
• Describe the role condensation nuclei play in enhancing cloud formation.
• Explain how most clouds form in the atmosphere.

Introduction:

Cloud formation and dissipation are closely related to temperature and pressure changes in the atmosphere. Vertical motions play a primary role as air rising or sinking in the atmosphere experiences pressure changes. These pressure changes, in turn, bring about temperature changes that can result in condensation and cloud formation, or evaporation and cloud dissipation.

The relationship between air pressure and temperature can be explored in the following thought demonstration. [It is recommended this experiment actually be done if possible.] Place a thermometer, like a thin liquid crystal temperature strip, in a clean and dry empty (air-filled!) plastic 2-liter or larger beverage bottle. A liquid crystal temperature strip works well because it is very sensitive to the temperature changes of its immediate environment, in this case, the surrounding air in the bottle. Secure the temperature strip with a piece of tape to hang at the center of the bottle. Screw the cap on tightly. Figure 1 shows this bottle arrangement.

Figure 1. Temperature strip in bottle.

[Temperature strips are available where aquarium supplies are sold, or at http://www.ametsoc.org/amsedu/AERA/ed_mats.html ().]

After sealing the bottle and letting it rest for a minute or two, note the temperature. Then, exert pressure on the bottle for at least 30 seconds so that its volume decreases. [A good way to squeeze the bottle to increase pressure on the air inside is to place the capped bottle so that about half of its length extends beyond the edge of a desk or table. Standing and with one hand on each end, push down on both ends of the sealed bottle so that it bends in the middle and partially collapses.] Reading the temperature strip after a few seconds have elapsed will show that the temperature of the air in the bottle rises.

Release the pressure so that the bottle expands again. It will be seen that as the bottle returns to its original shape, the air temperature in the bottle falls. [Be sure to hold the bottle in a squeezed position for at least a half-minute or so until the temperature stabilizes, then stop pushing down on the ends of the bottle.] Try the bottle squeeze-and-release sequence several times while continuing to carefully observe temperature changes of the air in the bottle. Repeated trials confirm that a predictable relationship exists between air temperature and changes in air pressure.

Air pressure and temperature relationships

1. Compressing the air by squeezing the bottle was accompanied by a(n) [(decrease)(increase)] in the temperature of air inside the bottle.

2. The expansion of air that occurred when the bottle was allowed to return to its original shape and volume was accompanied by a(n) [(decrease)(increase)] in the temperature of air inside the bottle.

3. These observations indicate that when air is compressed, its temperature increases, and when air expands, its temperature [(decreases)(increases)].

4. Air pressure in the open atmosphere always decreases with an increase in altitude. This happens because air pressure is determined by the weight of the overlying air. Air rising through the atmosphere expands as the pressure acting on it lowers and, in turn, its temperature [(decreases)(increases)].

5. Air sinking in the atmosphere is compressed as the air pressure acting on it increases, and its temperature [(increases)(decreases)].

Making clouds appear and disappear

Now imagine removing the bottle cap and pouring a few milliliters of water into the container. Twist and turn the bottle to wet the inner surface before pouring out the extra water. Then, reseal with the cap. After a couple of minutes enough water will evaporate to saturate the air in the bottle.

Next reopen the bottle to introduce some smoke to the air inside. The smoke is being added because atmospheric water vapor requires particles (nuclei) on which to condense. In the atmosphere, particles acting in the same way are called cloud condensation nuclei. [To do this, place the bottle on its side, open the bottle, and push down to flatten it to about half its normal diameter. Have another person light a match, blow it out, and insert the smoking end into the open bottle. Quickly release your pressure on the bottle so it returns to its original shape and the smoke from the extinguished match is drawn inside. Quickly cap the bottle tightly.]

Now apply and release pressure on the bottle as before, noting the temperature changes. When the bottle is allowed to spring back to its original shape, the temperature lowers—and a cloud appears in the bottle! The cloud is evidenced by a change in air visibility. Repeating the process of applying and releasing pressure will cause the cloud to appear and disappear.

6. The cloud forms when the pressure acting on the saturated air lowered and the temperature [(increased)(decreased)].

7. Most clouds in the atmosphere form in a similar way as the cloud in the bottle. With the temperature change due to expansion, some of the water vapor in the saturated air must [(sublime)(evaporate)(condense)], thereby forming cloud droplets.

8. Once you have a cloud in the bottle, squeeze the bottle to make the cloud disappear. The cloud disappears when the air temperature is raised by [(compression)(expansion)]. The change in temperature results in evaporation of the cloud droplets.

9. It can be inferred from this investigation that in the open atmosphere where it is cloudy, air is generally [(rising)(sinking)] and cooling. Where the atmosphere is clear, the air is generally moving in the opposite direction.

10. Generally, high pressure areas in the atmosphere tend to be clear because air in them experiences [(upward)(downward)] motion. Low pressure areas tend to have clouds because air in them experiences motion in the reverse direction.

Vertical motion, pressure change, and temperature change are of major importance in the formation and dissipation of most clouds. However, another major factor is at work. At any given temperature, there is a maximum concentration of water vapor that can ordinarily occur. This condition, called saturation, occurs when the temperature and dewpoint are equal. (The dewpoint is the temperature to which the air must be cooled at constant pressure to reach a relative humidity of 100%.) Air always contains some water vapor, but usually less than the maximum possible for its temperature. Cloud formation requires saturation be achieved so that, with further cooling, excess water vapor can change to the liquid (or solid) state. Thus, the atmosphere must undergo some process whereby saturation is achieved and further cooling takes place if clouds are to form.

The cloud-in-a-bottle investigation shows how atmospheric processes can produce saturation by changing air pressure. Lowering air pressure leads to lower air temperatures and, if enough water vapor is available, saturation is achieved.

Clouds are aggregations of tiny water droplets (and/or ice particles) that form from condensation (or deposition) of water vapor. This requires air to have a relative humidity of 100%, meaning saturation. Therefore, atmospheric processes that lead to saturation above Earth’s surface form clouds. The first part of this investigation demonstrated how an air parcel containing water vapor, rising through the atmosphere would expand and could eventually cool to saturation. This occurs where the temperature equals the dewpoint. Continued lifting and condensation can lead to precipitation.

Figure 2 is the surface weather map for 00Z 14 OCT 2013. At map time cloudy conditions affected several areas of the country. Clouds and precipitation were occurring along a stationary front that curved from western New York State to western Texas. Also, a combination warm, stationary and cold front crossed the western states from South Dakota to California. And many locations in the mountainous areas of the east and west had cloudy conditions brought on by orographic flows up elevated terrain.

Figure 2. Surface weather map for 00Z 14 OCT 2013.

11. The patterns of wind directions about the high-pressure centers in eastern Maine and Wisconsin were generally [(counterclockwise and inward)(clockwise and outward)].

12. Wind flows from central Texas to North Dakota were generally westward or northwestward, [(toward)(away from)] rising terrain and mountainous areas. These areas experienced lifting of air by orographic effects.

13. In the eastern U.S., the front that had slowly been moving eastward was positioned near Buffalo, New York. The temperature and dewpoint at Buffalo at map time were 62 °F and 61 °F, respectively. Because the temperature and dewpoint at the surface were not equal, it indicated the air in Buffalo [(was)(was not)] saturated.

14. The Buffalo station model showed the cloud cover at map time as [(clear)(partly cloudy)(mostly cloudy)(overcast)].

15. This sky cover condition [(did)(did not)] confirm that saturated air existed aloft over Buffalo.

16. The station model symbol for present weather conditions (obscured by radar echoes!) at Buffalo was two dots, a report that [(rain)(snow)(fog)] was occurring at map time.

17. Furthermore, the shadings of radar returns at Buffalo indicated that precipitation [(was)(was not)] occurring over this area. Precipitation is a result of cloud formation and droplet growth processes.

18. Therefore, we [(can)(cannot)] conclude that conditions associated with the passing frontal system were generally forcing air to rise, producing clouds in the Buffalo area.

Figure 3 is the Buffalo, NY (BUF) Stüve diagram for 00Z 14 OCT 2013, the same time as the Figure 2 surface map.

Figure 3. Stüve diagram of Buffalo, NY (BUF), sounding for 00Z 14 OCT 2013.

19. On the Stüve diagram, 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. The separation of the temperature and dewpoint values at and near the surface indicates that the surface air [(was)(was not)] saturated. (From the radiosonde text data, not shown, there is a 1.7 C° difference between the temperature and dewpoint at the surface.)

20. Air, rising from the surface at Buffalo, would [(expand and cool)(be compressed and warm)]. The sloping frontal surface separating cooler air below and warmer air above was the primary agent lifting the humid air.

21. The rising air above Buffalo cools, and at about 975 mb, its temperature and dewpoint [(do)(do not)] become equal.

22. The air over Buffalo at 975 mb [(was)(was not)] saturated.

23. The temperatures were equal to the dewpoints from 975 mb up to about 600 mb. These equal temperature-dewpoint conditions [(do)(do not)] suggest there was an extensive, thick layer of clouds over Buffalo.

It may also be noted that, from about 600 mb up to about 555 mb, there was a layer where the temperature and dewpoint separation indicated that clouds were not present. Then the temperature and dewpoint were equal again at about 555 mb, suggesting another thin layer of clouds existed. Investigation 6B will consider further details of rising air motions.

Suggestions for further activities: You might compare Stüve diagrams for the station nearest you with surface cloud reports or with satellite observations to see ways whereby the existence of clouds can be determined. If you have periods of fog or predictions for it, you might check meteograms to follow the temperature and dewpoint values over time. (Fog is a cloud in contact with Earth’s surface.)

Another process achieving saturation is the mixing of hot, humid air with cold, dry air in the formation of contrails (condensation trails - cloud-like streamers frequently observed to form behind aircraft flying in clear, cold air). This process is described at: http://cimss.ssec.wisc.edu/wxwise/class/contrail.html ().