They are one of the most important physical properties of the atmosphere but the most difficult to parameterise. The forecasting of short-term climatological excursions weather requires numerical integration of complex dynamical models. However, over longer time periods it may be possible to include cloud cover without resorting to explicit atmospheric dynamics. Here we suggest that over evolutionary time periods 10 8 —10 9 yr the Earth's percentage cloud cover has remained approximately constant.
This is in general agreement with present ideas about the stability of the Earth's evolution 1—3. Over medium-term climatological periods 10 4 —10 7 yr we have found that the position of large cloud masses may be directly related to the changing surface configuration, caused, for example, by continental drift. Global cloud cover fluctuates about a mean, which is near the present-day value and reinforces albedo changes caused by surface configuration; this could be highly significant for theories of climatic change.
Owen, T. Nature , Margulis, L. Icarus 21 , Henderson-Sellers, A. Google Scholar. Schneider, S. Ohring, G. Work Pap. Cess, R. Article Google Scholar. Tellus 31 , Roads, J. Sellers, W. Space Phys. Planet Space Sci. Goody, R. Atmospheres Prentice Hall, New York, When discussing stability in atmospheric sciences, we typically think about air parcels, or imaginary blobs of air that can expand and contract freely, but do not mix with the air around them or break apart.
The key piece of information is that movement of air parcels in the atmosphere can be estimated as an adiabatic process. Adiabatic processes do not exchange heat and they are reversible. The air parcel has the same temperature and pressure as the surrounding air, which we will call the environment.
If you were to lift the air parcel, it would find itself in a place where the surrounding environmental air pressure is lower, because we know that pressure decreases with height. Because the environmental air pressure outside the parcel is lower than the pressure inside the parcel, the air molecules inside the parcel will effectively push outward on the walls of the parcel and expand adiabatically.
To summarize, rising air parcels expand and cool adiabatically without exchanging heat with the environment. The air parcel is moving into an environment with higher air pressure. The higher environmental pressure will push inward on the parcel walls, causing them to compress, and raise the inside temperature. The process is adiabatic, so again, no heat is exchanged with the environment. However, temperature changes in the air parcel can still occur, but it is not due to mixing, it is due to changes in the internal energy of the air parcel.
A decrease in temperature with height is called a lapse rate and while the temperature decreases with altitude, it is defined as positive because it is a lapse rate. This drop in temperature is due to adiabatic expansion and a decrease in internal energy. Stability in the atmosphere refers to a condition of equilibrium. As discussed with the example of the boulder on a hill or valley, some initial movement resulted in either more unstable , less stable , or no change neutral.
Given some initial change in the elevation of an air parcel, if the air is in stable equilibrium, the parcel will tend to return back to its original position after it is forced to rise or sink. In an unstable equilibrium, an air parcel will accelerate away from its initial position after being pushed. The motion could be upward or downward, but generally unstable atmospheres favors vertical motions. Finally, in a neutral equilibrium, some initial change in the elevation of an air parcel will not result in any additional movement.
How do you know if an air parcel will be stable after some initial displacement? Stability is determined by comparing the temperature of a rising or sinking air parcel to the environmental air temperature.
Imagine the following: at some initial time, an air parcel has the same temperature and pressure as its environment. If you lift the air parcel some distance, its temperature drops by 9.
If the air parcel is colder than the environment in its new position, it will have higher density and tend to sink back to its original position. In this case, the air is stable because vertical motion is resisted. If the rising air is warmer and less dense than the surrounding air, it will continue to rise until it reaches some new equilibrium where its temperature matches the environmental temperature. In this case, because an initial change is amplified, the air parcel is unstable.
In order to figure out if the air parcel is unstable or not we must know the temperature of both the rising air and the environment at different altitudes. One way this is done in practice is with a weather balloon. We can get a vertical profile of the environmental lapse rate by releasing a radiosonde attached to a weather balloon. A radiosonde sends back data on temperature, humidity, wind, and position, which are plotted on a thermodynamic diagram. This vertical plot of temperature and other variables is known as a sounding.
If an air parcel is dry, meaning unsaturated, stability is relatively straightforward. An atmosphere where the environmental lapse rate is the same as the dry adiabatic lapse rate, meaning that the temperature in the environment also drops by 9.
After some initial vertical displacement, the temperature of the air parcel will always be the same as the environment so no further change in position is expected. If the environmental lapse rate is less than the dry adiabatic lapse rate, some initial vertical displacement of the air parcel will result in the air parcel either being colder than the environment if lifted , or warmer than the environment if pushed downward.
This is because if lifted, the temperature of the air parcel would drop more than the temperature of the environment. This is a stable situation for a dry air parcel and a typical scenario in the atmosphere. The global average tropospheric lapse rate is 6. Finally, if the environmental lapse rate is greater than the dry adiabatic lapse rate, some initial vertical displacement of the air parcel will result in the air parcel either being warmer than the environment if lifted , or colder than the environment if pushed downward.
This is because if lifted, the temperature of the air parcel would drop less than the temperature of the environment. This is an unstable situation for a dry air parcel. When moisture is added, everything gets more complicated. In Chapter 4 we learned that whether or not an air parcel is saturated depends primarily on its temperature and, of course, its moisture content.
The graph of the Clausius-Clapeyron relationship shows us that given the same amount of moisture, air is more likely to be saturated at a lower temperature. We know that as an air parcel is lifted, its temperature drops according to the dry adiabatic lapse rate.
So what happens when the air parcel is cold enough that the air becomes saturated with respect to water vapor? The short answer is that if it continues to cool, water vapor will condense to liquid water to form a cloud. When water vapor condenses, it goes from a higher energy state to a lower energy state. Energy is never created nor destroyed, especially in phase changes, so what happens to all that excess energy?
The energy gets released in the form of latent heat. The latent heat of condensation is approximately equal to 2. This has large consequences for the lapse rate of an air parcel and distinguishes the dry adiabatic lapse rate from the moist adiabatic lapse rate. As latent heat is added from the process of condensation, it offsets some of the adiabatic cooling from expansion.
Because of this, the air parcel will no longer cool at the dry adiabatic lapse rate, but will cool as a slower rate, known as the moist adiabatic lapse rate. The effects of moisture change the lapse rate of the air parcel and, therefore, affects stability. However, the concepts are still the same and we still compare the air parcel temperature to the environmental temperature. This can also be easily shown using the gas law. Recall that parcels in the atmosphere adjust their size so that the air pressure inside the parcel equals the air pressure outside the parcel This is always the case.
If the air temperature inside a parcel is warmer than the air temperature of the air surrounding the parcel, the number density inside the parcel is lower than the number density outside the parcel. Thus, the air parcel weighs less than an equal volume of air outside the parcel and it will rise upward see Figure R. In the atmosphere, beside the mechanism of surface heating and free convection, the only other way in which parcels become unstable is when the latent heat released during cloud formation water vapor condensing to liquid cloud droplets is enough to make the temperature of the parcel warmer than the surrounding environmental air.
Meteorologists assess and compute the stability of the atmosphere by lifting hypothetical parcels of air upward from the surface and comparing the parcel temperature with the temperature of the surrounding air.
The temperature of the surrounding air from ground level upward is measured twice each day by releasing weather balloons with instruments attached. If the atmosphere is found to be unstable for lifted parcels, there is the possibility of thunderstorms; however, if the atmosphere is found to be stable for lifted parcels, thunderstorms will not form.
In this class, we will illustrate the concepts of cloud formation and stability using simplified numerical examples. The basic problem will be given the vertical temperature structure of the atmosphere and the water vapor content of air at the surface, lift a hypothetical parcel upward to determine a at what altitude will a cloud start to form and b at what altitude, if at all, will the parcel become unstable.
Now we add the concept of stability. To determine stability, compare the parcel temperature with the temperature of the surrounding air and think about what would happen if the parcel was "let go" or no longer forced upward.
An example of a "blank" table is given below. You are expected to use the rules for moving air parcels to keep track of the temperature and dew point temperature water vapor content in an air parcel that is moved upward from 0 meters to meters above sea level. The designation "neutral" is used to indicate that the parcel temperature is the same as the air temperature of the surrounding atmosphere and thus the parcel is the same density as the surrounding atmosphere. The rules for keeping track of the temperature and dew point temperature were given on the previous page.
We are specifically checking to see if a rising parcel becomes warmer than the surrounding environmental air. The next table has been filled in up to the lowest altitude where the air parcel becomes unstable.
You should realize that the rising parcel reaches saturation its temperature cooled to equal its dew point temperature at an altitude of meters. This is the level where a cloud would start to form in the parcel. At an altitude of meters, the temperature of the air in the lifted parcel is warmer than the temperature of the air surrounding the parcel, which is specified in the column "Environmental Temperature.
For the atmospheric coniditions specified in the table, if an air parcel were forced upward to meters, it would become unstable, i. An unstable parcel will accelerate upward as long as it remains warmer and less dense than the surrounding air. The remainder of the table is filled in below.
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