Atmospheric stability is the resistance of the atmosphere to the vertical motion of air. A stable atmosphere inhibits vertical motion. An unstable atmosphere encourages vertical motion. The stability depends on how the air temperature changes with altitude the temperature lapse rate. Convective initiation — how thunderstorms start. Very stable : temperature increases with altitude, a temperature inversion. Initially the rising parcel is colder and denser than the surrounding air.
If the parcel is lifted to 3 km it has the same temperature as the air around it. If lifted above 3 km the parcel air finds itself warmer and less than the air outside. If lifted just a little bit beyond 3 km altitude the parcel will be able to continue to rise on its own.
The atmosphere is conditionally unstable in this case. A rising parcel must first of all become saturated. Then it must be lifted to and just above the level of free convection. The value of the environmental lapse rate is one of the main factors that determines whether the atmosphere will be stable or unstable. The ground and the air above it cool during the night.
The atmosphere is usually most stable early in the morning. A temperature inversion represents an extremely stable situation. Rising parcels always cool with increasing altitude at either the dry or moist rate. In an inversion the surrounding air gets warmer and warmer with altitude. The difference between the cold parcel air and the warmer surroudings gets larger and larger with increasing altitude.
Sunlight warms the ground and the air next to it during the day. This steepens the environmental lapse rate and makes the atmosphere more unstable. Cooling air above the ground has the same effect. One last figure before we leave this topic. The figure shows the different types of clouds that form in stable and conditionally unstable conditions. The violet curve shows the environmental temperatures as a function of altitude.
The layer then becomes increasingly less stable at a rate faster than if condensation had not taken place. A descending subsiding layer of stable air becomes more stable as it lowers. The layer compresses, with the top sinking more and warming more than the bottom. The adiabatic processes involved are just the opposite of those that apply to rising air.
Since the lapse rate of the atmosphere is normally stable, there must be some processes by which air parcels or layers are lifted in spite of the resistance to lifting provided by the atmosphere. We will consider several such processes.
A common process by which air is lifted in the atmosphere, as is explained in detail in the next chapter , is convection. If the atmosphere remains stable, convection will be suppressed. But we have seen that surface heating makes the lower layers of the atmosphere unstable during the daytime.
Triggering mechanisms are required to begin convective action, and they usually are present. If the unstable layer is deep enough, so that the rising parcels reach their condensation level, cumulus-type clouds will form and may produce showers or thunderstorms if the atmosphere layer above the condensation level is conditionally unstable.
Wildfire also may be a source of heat which will initiate convection. At times, the fire convection column will reach the condensation level and produce clouds. Showers, though rare, have been known to occur.
Convection is a process by which air is lifted in the atmosphere. Surface heating during the daytime makes the surface layer of air unstable.
After its initial ineertia is overcome, the air is forced upward by the mom dense surrounding air. Layers of air commonly flow in response to pressure gradients. In doing so, if they are lifted up and over mountains, they are subjected to what is called orographic lifting. This is a very important process along our north-south mountain ranges in the western regions and the Appalachians in the East, because the general airflow is normally from a westerly direction.
If the air is initially stable, and if no condensation takes place, it sinks back to its original level after passing over a ridge. If it is neutrally stable, the air will remain at its new level after crossing the ridge.
In an unstable atmosphere, air given an initial uplift in this way keeps on rising, seeking a like temperature level, and is replaced by sinking colder air from above. If the condensation level is reached in the lifting process, and clouds form, initially stable air can become unstable. In each case, the internal depth and lapse rate of the layer will respond as indicated above.
As we will see in the chapter on air masses and fronts , warmer, lighter air layers frequently flow up and over colder, heavier air masses. This is referred to as frontal lifting and is similar in effect to orographic lifting. Stable and unstable air masses react the same way regardless of whether they are lifted by the slope of topography or by the slope of a heavier air mass.
As air is lifted over mountain, the resulting airflow depends to some extent upon the stability of the air. These simple airflows may be complicated considerably by daytime heating and, in some cases, by wave motion. Turbulence associated with strong winds results in mixing of the air through the turbulent layer. In this process, some of the air near the top of the layer is mixed downward, and that near the bottom is mixed upward, resulting in an adiabatic layer topped by an inversion. At times, the resultant cooling near the top of the layer is sufficient to produce condensation and the formation of stratus, or layerlike, clouds.
The airflow around surface low-pressure areas in the Northern Hemisphere is counterclockwise and spirals inward. In the next chapter we will see why this is so, but here we will need to consider the inflow only because it produces upward motion in low-pressure areas. Airflow into a Low from all sides is called convergence. Now, the air must move. It is prevented from going downward by the earth's surface, so it can only go upward.
Thus, low-pressure areas on a surface weather map are regions of upward motion in the lower atmosphere. In surface high-pressure areas, the airflow is clockwise and spirals outward.
This airflow away from a High is called divergence. The air must be replaced, and the only source is from aloft. Thus, surface high-pressure areas are regions of sinking air motion from aloft, or subsidence. We will consider subsidence in more detail later in this chapter. Frequently, two or more of the above processes will act together.
For example, the stronger heating of air over ridges during the daytime, compared to the warming of air at the same altitude away from the ridges, can aid orographic lifting in the development of deep convective currents, and frequently cumulus clouds, over ridges and mountain peaks.
Similarly, orographic and frontal lifting may act together, and frontal lifting may combine with convergence around a Low to produce more effective upward motion. Stability frequently varies through a wide range in different layers of the atmosphere for various reasons.
Layering aloft may be due to an air mass of certain source-region characteristics moving above or below another air mass with a different temperature structure. The inflow of warmer less dense air at the bottom, or colder more dense air at the top of an air mass promotes instability, while the inflow of warmer air at the top or colder air at the surface has a stabilizing effect.
The changes in lapse rate of a temperature sounding plotted on an adiabatic chart frequently correspond closely to the layering shown in upper-wind measurements. At lower levels, stability of the air changes with surface heating and cooling, amount of cloud cover, and surface wind all acting together.
We will consider first the changes in stability that take place during a daily cycle and the effects of various factors; then we will consider seasonal variations. On a typical fair-weather summer day, stability in the lower atmosphere goes through a regular cycle.
Cooling at night near the surface stabilizes the layer of air next to the ground. Warming during the daytime makes it unstable. Diurnal changes in surface heating and cooling, discussed in chapter 2 , and illustrated in particular on pages 27, 28, produce daily changes in stability, from night inversions to daytime superadiabatic lapse rates, that are common over local land surfaces. During a typical light-wind, fair-weather period, radiation cooling at night forms a stable inversion near the surface, which deepens until it reaches its maximum development at about daybreak.
After sunrise, the earth and air near the surface begin to heat, and a shallow superadiabatic layer is formed. Convective currents and mixing generated in this layer extend up to the barrier created by the inversion. As the day progresses, the unstable superadiabatic layer deepens, and heated air mixing upward creates an adiabatic layer, which eventually eliminates the inversion completely. This usually occurs by mid or late morning. Active mixing in warm seasons often extends the adiabatic layer to 4, or 5, feet above the surface by midafternoon.
The superadiabatie layer, maintained by intense heating, is usually confined to the lowest few hundreds of feet, occasionally reaching 1, to 2, feet over bare ground in midsummer. As the sun sets, the ground cools rapidly under clear skies and soon a shallow inversion is formed. The inversion continues to grow from the surface upward throughout the night as surface temperatures fall. The air within the inversion becomes increasingly stable.
Vertical motion in the inversion layer is suppressed, though mixing may well continue in the air above the inversion. This mixing allows radiational cooling above the inversion to lower temperatures in that layer only slightly during the night.
A night surface inversion is gradually eliminated by surface heating during the forenoon of a typical clear summer day. A surface superadiabatic layer and a dry-adiabatic layer above deepen until they reach their maximum depth about mid afternoon. The ground cools rapidly after sundown and a shallow surface inversion is formed This inversion deepens from the surface upward during the night, reaching its maximum depth just before sunrise This diurnal pattern of nighttime inversions and daytime superadiabatic layers near the surface can be expected to vary considerably.
Clear skies and low air moisture permit more intense heating at the surface by day and more intense cooling by radiation at night than do cloudy skies. The lower atmosphere tends to be more unstable on clear days and more stable on clear nights. Strong winds diminish or eliminate diurnal variations in stability near the surface. Turbulence associated with strong wind results in mixing, which tends to produce a dry-adiabatic lapse rate.
Mechanical turbulence at night prevents the formation of surface inversions, but it may produce an inversion at the top of the mixed layer.
During the day, thermal turbulence adds to the mechanical turbulence to produce effective mixing through a relatively deep layer. Consequently, great instability during the day, and stability at night occur when surface winds are light or absent. Stability in the lower atmosphere varies locally between surfaces that heat and cool at different rates. Thus, dark-colored, barren, and rocky soils that reach high daytime temperatures contribute to strong daytime instability and, conversely, to strong stability at night.
Areas recently blackened by fire are subject to about the maximum diurnal variation in surface temperature and the resulting changes in air stability. Vegetated areas that are interspersed with openings, outcrops, or other good absorbers and radiators have very spotty daytime stability conditions above them.
Topography also affects diurnal changes in the stability of the lower atmosphere. Air in mountain valleys and basins heats up faster during the daytime and cools more rapidly at night than the air over adjacent plains. This is due in part to the larger area of surface contact, and in part to differences in circulation systems in flat and mountainous topography.
The amount of air heating depends on orientation, inclination, and shape of topography, and on the type and distribution of ground cover. South-facing slopes reach higher temperatures and have greater instability above them during the day than do corresponding north slopes.
Both cool about the same at night. Instability resulting from superheating near the surface is the origin of many of the important convective winds which we will discuss in detail in chapter 7.
On mountain slopes, the onset of daytime heating initiates upslope wind systems. The rising heated air flows up the slopes and is swept aloft above the ridge tops in a more-or-less steady stream. Strong heating may produce a pool of superheated air in poorly ventilated basins. If upper winds are unable to provide the triggering mechanism needed to overcome interia and release the instability in this superadiabatic layer, a potentially explosive fire weather situation develops.
Over level ground, heated surface air, in the absence of strong winds to disperse it, can remain in a layer next to the ground until it is disturbed. The rising air frequently spirals upward in the form of a whirlwind or dust devil. In other cases, it moves upward as intermittent bubbles or in more-or-less continuous columns.
Pools of superheated air may also build up and intensify in poorly ventilated valleys to produce a highly unstable situation. They persist until released by some triggering mechanism which overcomes inertia, and they may move out violently. The amount of solar radiation received at the surface during the summer is considerably greater than in the winter.
As explained in chapter 1 , this is due to the difference in solar angle and the duration of sunshine. Temperature profiles and stability reflect seasonal variation accordingly. In the colder months, inversions become more pronounced and more persistent, and superadiabatic lapse rates occur only occasionally. In the summer months, superadiabatic conditions are the role on sunny days.
Greater variation in stability from day to day may be expected in the colder months because of the greater variety of air masses and weather situations that occur during this stormy season. In addition to the seasonal effects directly caused by changes in solar radiation, there is also an important effect that is caused by the lag in heating and cooling of the atmosphere as a whole.
The result is a predominance of cool air over warming land in the spring, and warm air over cooling surfaces in the fall. Thus, the steepest lapse rates frequently occur during the spring, whereas the strongest inversions occur during fall and early winter.
Air that rises in the troposphere must be replaced by air that sinks and flows in beneath that which rises. Local heating often results in small-scale updrafts and downdrafts in the same vicinity. On a larger scale, such as the up-flow in low-pressure systems, adjacent surface high-pressure systems with their divergent flow normally supply the replacement air.
The outflow at the surface from these high-pressure areas results in sinking of the atmosphere above them. This sinking from aloft is the common form of subsidence.
The sinking motion originates high in the troposphere when the high-pressure systems are deep. Sometimes these systems extend all the way from the surface up to the tropopause. Deep high-pressure systems are referred to as warm Highs , and subsidence through a deep layer is characteristic of warm Highs. Subsidence occurs in these warm highpressure systems as part of the return circulation compensating for the large upward transport of air in adjacent low-pressure areas. If the subsidence takes place without much horizontal mixing, air from the upper troposphere may reach the surface quite warm and extremely dry.
If no moisture were added to the air in its descent, the relative humidity would then be less than 2 percent. Subsiding air may reach the surface at times with only very little external modification or addition of moisture. Even with considerable gain in moisture, the final relative humidity can be quite low. The warming and drying of air sinking adiabatically is so pronounced that saturated air, sinking from even the middle troposphere to near sea level, will produce relative humidities of less than 5 percent.
Because of the warming and drying, subsiding air is characteristically very clear and cloudless. Subsidence in a warm high-pressure system progresses downward from its origin in the upper troposphere. In order for the sinking motion to take place, the air beneath must flow outward, or diverge.
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