What makes air unstable in the atmosphere

However, it is often possible to employ these concepts with somewhat greater confidence here than in the case of parcel-stability analyses. Let us first examine how the stability of an air layer changes internally as the layer is lifted or lowered. When an entire layer of stable air is lifted it becomes increasingly less stable. The layer stretches vertically as it is lifted, with the top rising farther and cooling more than the bottom.

If no part of the layer reaches condensation, the stable layer will eventually become dry-adiabatic. Let us consider an example:. We will begin with a layer extending from 6, to 8, feet with a lapse rate of 3. Because of the vertical stretching upon reaching lower pressures, the layer would be about 3, feet deep at its new altitude and the top would be at 20, feet.

If the air in the layer remained unsaturated, its temperature would have decreased at the dry-adiabatic rate. The temperature of the top of the layer would have decreased 5. The temperature of the bottom of the layer would have decreased 5. A lifted layer of air stretches vertically, with the top rising farther and cooling more than the bottom. If the layer is initially stable, it becomes increasingly less stable as it is lifted. Similarly, a subsidizing layer becomes more stable.

Whereas the original lapse rate was 3. The layer has become less stable. Occasionally, the bottom of a layer of air being lifted is more moist than the top and reaches its condensation level early in the lifting. Cooling of the bottom takes place at the slower moist-adiabatic rate, while the top continues to cool at the dry-adiabatic rate. 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. 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 other curve the temperature of a rising parcel of moist air. The rising parcel starts out unsaturated and follows the green portion of the curve. Once it becomes saturated it begins cooling at the moist rate and follows the orange line.

The top figure shows a conditionally unstable situation. Because the parcel is lifted above the Level of Free Convection LFC it is able to continue rising on its own and develops into cumuliform cloud perhaps a thunderstorm. The air in the lifted parcel does become saturated but never becomes warmer than the surrounding air. The parcel won't go any higher than it is lifted.

Satellite U. Great Plains Satellite - C. Great Plains Satellite - S. This is called moist convection. Normally in the atmosphere, if an air parcel rises, it will be colder than its environment due to expansion , and it will then sink back down again. The rate of cooling is known as the adiabatic lapse rate.

Warm air is less dense than cold air. Image 1: Graph of height against temperature in a stable situation with a temperature inversion a layer in which the atmosphere warms with height.

However, it becomes cooler than the atmosphere above the level where the lines intersect. Therefore, it stops rising at that point.

Any cloud formed in this situation will have a marked vertical extent, as in Image 2. Image 2: Graph of height against temperature in an unstable situation. What makes air rise in the first place? There are several mechanisms by which air can be encouraged to start rising. These include:.

Those April Showers made famous by the rhyme, are therefore the result of instability in the atmosphere together with a trigger to get them started.