17 ATMOSPHERIC MOISTURE III: INSTABILITY AND PRECIPITATION

Dr. Jitender Saroha

Objectives

define atmospheric stability and instability;
define and interrelate various lapse rates like environmental, dry and wet adiabatic;
differentiate various types of instabilities, like absolute,conditional,convectionaland mechanical;
explainthe factors enhancing stability and instability;
describethe type of clouds and associated precipitation; and
establish the relationship between distribution of precipitation and instability

Contents

Introduction

Learning Objectives

Atmospheric Stability

Absolute Stability

Absolute Instability

Conditional Instability

Convectional Instability

Mechanical Instability

Changes in Stability

Instability Enhancement Processes

Intense Surface Heating and Convection

Lifting Mechanisms

Horizontal Movement and Instability

Radiation Cooling from Cloud Tops

Ocean Currents

Instability and Daily Weather

Instability and Distribution of Precipitation

Summary and conclusions

Multiple Choice Questions

Answers:

References

Introduction

You must have seen a great variety of clouds in sky, and clear sky conditions also. Similarly, you have experienced heavy, moderate and light precipitation and of course, dry days. All these variations are basically associated with atmospheric stability and instability. Atmospheric stability represents condition of absence of vertical motions or resistance to such motions. Instability, on the other hand, represents prevalent tendencies for vertical motions. Instability causes various weather phenomena such as cloud formation, precipitation and its forms, and thunderstorms. In this module focus is on the concepts of stability and instability and their relationships with weather phenomena, especially precipitation.

Atmospheric Stability

Atmospheric stability represents the condition of absence or restriction of vertical motions in the atmosphere. According to Trewartha, “air is said to be stable, and consequently antagonistic to precipitation, if it is non-buoyant and resists vertical displacement. Voluntary vertical motions are largely absent in stable air. On the other hand, if displacement results in buoyancy and a tendency for further movement away from the original position, the air is unstable”. Hence, instability is protagonist to precipitation. Among the factors of precipitation, vertical motions in the atmosphere play the most vital role.

The stability of air depends upon the distribution of temperature at various altitudes in the atmosphere. In troposphere, temperature decreases with height at an average rate of 6.50C per 1000 m. This standard or normal lapse rate (NLR) is derived on the basis of difference in average temperature at surface (150C) and at the tropopause (-590C at 11 km). As it measures the temperature of the environment it is known as environmental lapse rate. This lapse rate varies substantially from place to place and time to time. On the other hand, the dry adiabatic remains constant and the rate is 100C per km. As explained in the Module17, after saturation in rising parcel of air wet, adiabatic rate operates and it varieswithin a narrow range. The stability and instability of atmosphere is determined by the relationships between environmental lapse rate and adiabatic lapse rates.

In case, an uprising dry parcel of air (with temperature 400C at surface) has higher dry adiabatic rate (100C per km) as compared to environmental lapse rate (assume 7 0C per km) and if it is does not achieve saturation and dew point, then it becomes colder than surrounding air at certain height (say at 1 km, it has 300C temperature and surrounding air has 330C). As it is colder than the surrounding air its vertical motion is restricted and the parcel of air would tend to come back to its original position, unless some outside force is applied to it. Such descending airis called to be in stable state.

Absolute Stability

Absolute stability prevails when the environmental lapse rate is less than the wet adiabatic rate. Assume at surface the temperature is 200C, and wet adiabatic rates and environmental lapse rates are, 60C and 50C per km, respectively (Figure 1). In this case, at 1 km altitude, the rising parcel of air cools down to 100Cbecause of dry adiabatic lapse rate, whereas the surrounding air temperature due to environment lapse rate is 150C. Therefore, it is colder and denser and would tend to sink to its original position, as discussed in the previous example of stability.In case, if this stable parcel of air is uplifted by some outside force upto the condensation level or above, it would remain cooler and denser than surrounding environment and would have tendency to sink to original position. For example, at the altitude of 3 km, the parcel of air due to wet adiabatic lapse rate cools down to -60C and surrounding environment temperature is 50C. Therefore, it would have strong tendency to return to its former position once the outside force ceases.

Figure 1: Absolute Stability

Source:http://www.ux1.eiu.edu/~cfjps/1400/FIG04_020.JPG

The most stable conditions are represented by the phenomenon of opposite to normal lapse rate i.e. inversion of temperature, which represents increase in temperature with altitude. It is an ideal example of absolute stability.

Absolute Instability

Absolute instability represents continued vertical motion of the ascending parcel of air till its temperature is not equal to that of surrounding environment.This, another extreme condition occurs when the environmental lapse rate is greater than the dry adiabatic rate and wet adiabatic rates (Figure 2). In case there is general instability in the atmosphere, a force, whatsoever, is required to aloft a given parcel of air. The intense surface heating by solar radiation or presence of a topographic barrier in the path of air streams can provide such a mechanism for initial upward motion.

Figure 2: Absolute Instability

Source:http://www.richhoffmanclass.com/images/chapter1/absoluteunstable.jpg

Instability requires higher environmental lapse rates as compared to adiabatic rates. Assume that environmental lapse rate and wet adiabatic rates are 120C and 60C per km respectively, and you remember that dry adiabatic rate is fixed i.e. 100C per km. Assume, at surface temperature is 400C, and due to initial trigger-effect by some impulse the parcel of air starts moving upward. At 2 km altitude, the temperature in this parcel of air becomes 200C and condensation starts because of saturation and dew point. The surrounding environment is cooler with 160C temperature. As this parcel of air is warmer it will continue upward movement at wet adiabatic rate. At one km height from level of condensation this rising parcel of air is 100C warmer than surrounding environment.As shown in the Figure 2, the ascending parcel of air remains always warmer than its surrounding environment and will continue to move upward because of its own buoyancy. The end result will be vertical clouds and precipitation.

Conditional Instability

A more frequent type of atmospheric instability is known as conditional instability. This occurs when the moist air has an environmental lapse rate between the dry and wet adiabatic rates. It simply means that the atmosphere is stable with respect to unsaturated parcel of air, but unstable in relation to the saturated parcel of air. Let us assume the same surface temperature 400C, same wet adiabatic rate 60C per km and dry adiabatic rate is 100C and in this situation environmental lapse rate is 80C per km, between wet and dry (Figure 3).The air is stable in the lower layers, but if the air is forced to rise, due to surface heating or orographic barrier or convergence, until the level of free convection. At this level the rising parcel of air becomes warmer than its surroundings and is free to rise.

Figure 3: Conditional Instability

Source:http://www.ux1.eiu.edu/~cfjps/1400/FIG04_022.JPG

In this example, at the level of condensation the rising air is 40C cooler than surrounding. It means it is stable, but if the lifting force continues and forces it above this level, wet adiabatic rate operates and at 4 km height the temperature of rising air becomes similar to surrounding. This represents the level of free convection, because this height onwards this parcel of air will move upward on self basis due to release of latent heat of condensation and it will remain warmer than environment. From this level along its ascent, the parcel will continue to rise without an outside force.

This situation is called conditional instability, because instability dependents on the air parcel becoming saturated.The term “conditional” is used because the air must be forced upward before it reaches the level where it becomes unstable and rises on self basis. Conditional instability is very common because the environmental lapse rate frequently prevails between the dry and wet adiabatic rates. The prerequisites for conditional instability are – the ascending parcel of air should be saturated, and it should get a very strong outside initial force to aloft it not only upto the height of level of condensation rather to the level of free convection.

Convectional Instability

So far in our discussion on, stability, absolute instability and conditional instability the situations were about a parcel of air uprising through a layer of air that was static vertically. But occasionally, there are conditions when an extensive layer (several hundreds of metres thick, extending over thousands of kilometers) is forced to uprise over an extensive orographic barrier or by some other mechanism. Within this extensive and thick layer of air the lower portion (labeled a, Figure 4) may be moist and upper portion may be dry (b).

Figure 4: Convective Instability

Source: http://images.slideplayer.com/37/10738477/slides/slide_14.jpg

In case, the whole layer is forced for vertical motion, the drier air at ‘b’ cools at the dry adiabatic rate, and same is the case at ‘a’, till the time condensation level is not achieved by lower portion. After reaching condensation level the lower part air mass cools at the wet adiabatic rate. This process finally increases the actual lapse rate of the total thickness of the raised layer, reflected in the figure by change from ab to a’b’. Here, ab represents the initial lapse rate (stable) and a’b’ represents final lapse rate (unstable) which is more than the saturated adiabatic lapse rate and that is why this line is steeper and it shows instability. Finally the layer becomes unstable and may overturn, as lower part (a) ascends faster than upper part (b). It is called convective or potential instability. Hence, it may be concluded that, in general, saturation lifting produces more instability than unsaturated lifting. The process of convective instability is vital in the dynamics of weather. When convectively or conditionally unstable air masses with conditions mentioned above are lifted, they produce vertical clouds, cumulonimbus and result in heavy precipitation with thunder and lightning. Further, convective instability also plays role in the formation of cyclones and thunderstorms.

Mechanical instability

Sometimes, abnormal conditions prevail and lapse rates in upper layers of troposphere become too high, lapse rates of 200C to 350C per km. This makes upper layers denser and heavier as compared to underlying layers. In case of such extraordinary conditions there is an automatic overturning of air without any initial trigger. This kind of situation represents mechanical instability. It contributes in the genesis of tornadoes, the most violent revolving storms.

Changes in Stability

It can be concluded that unstable parcel of air due to its own buoyancy ascends freely and on the other hand, stable air resists vertical motion. The most stable conditions are represented by the inversion of temperature. But steeper environmental lapse rates are associated with instability. Therefore, those factors which cause cooling of surface air increase stability and those which enhance warming effect in surface air in relation to air aloft, increases instability.

Stability is enhanced by the following conditions:

(i) Radiation cooling of surface during night.

(ii) Contact cooling (conduction) of an air mass from below as it passes over a cold surface.

Both these processes of cooling result into formation of fog as discussed in the Module 17on forms of condensation, and

(iii) Subsidence within an air column.

Subsidence results into substantial modifications in weather conditions. For example, a subsidence of about 300 m in an air mass may result into evaporation of all average size cloud droplets. The subsidence, due to horizontal convergence of air in the upper troposphere associated with high pressure belts, induces stability. Therefore, the warming effect of subsidence is enough to evaporate the clouds present in any part of the atmosphere.

Instability Enhancement Processes

The instability is enhanced by following processes:

Intense Surface Heating and Convection: Nocturnal cooling and stability are replaced by surface heating on a clear sunny day. The stability is replaced by instability. Due to this heating the lower atmosphere often becomes warmed enough to generate vertical motions in parcels of air. This heating results into onset of convectional process. The warm and moist parcels of air are cooled adiabatically below the dew point. The rising parcel of air gets transformed into a cumulus cloud and its flat base shows the level of condensation. There are many possibilities, one situation can be that this parcel of air is cooled enough and it ceases to rise and condensation is no more operational. This may result into dissolving of cumulus cloud after some downdraft. However, under a different set of conditions, favouring instability, convection may manifest itself in the form of great vertical clouds, cumulonimbus and associated heavy precipitation and lightning and thunder. Another outcome may be a convection cyclone energized by the latent heat of condensation. In the equatorial low pressure belts intensive surface heating results into formation of convectional currents and daily in the afternoon heavy rainfall is caused by cumulonimbus clouds (Figure 5a).

Figure 5: Atmospheric Instability and Lifting Mechanisms

Source:http://web.gccaz.edu/~lnewman/gph111/topic_units/moisture/moisture_stabil_prec/4_lifting.jpg

Lifting Mechanisms: Precipitation in substantial amount is induced by spontaneous convectional rise of warm and moist air and another mechanism enhancing instability is the forced rise of the moist air. The forced upward movement of air is caused by processes such as orographic lifting, frontal wedging, and convergence. All these lifting processes enhance instability and cause precipitation (Figure5b, c and d). The forced rise of moist air on the windward slope of orographic barrier results into adiabatic cooling, condensation and precipitation.

Convergence of winds results into lifting of air and enhances instability and causes precipitation. Convergence of two contrasting air masses results into instability along with formation of warm fronts, cold fronts and occluded fronts. Frontal lifting of air creates a great variety of clouds and precipitation. For instance, on the gentle slope of warm front nimbus, nimbostratus, altocumulus, cirrostratus and cirrus clouds are formed at different height ranges (Figure 6). The precipitation is moderate to gentle, of long duration and over wider area.

Figure 6: Fronts and Associated Weather

Source: https://sriutami88.files.wordpress.com/2012/02/cloudformation_fronts_large1.jpg?w=750

Horizontal Movement and Instability:The advection of cold air over warm surfaces enhances instability and may cause precipitation. For instance, the polar cold and dry air masses in winter while traversing over the Great Lakes acquire heat and moisture and become unstable. This instability causes cloud formation and snowfall known as “lake-effect snows”.

Radiation Cooling from Cloud Tops: Although on small scale, cloud droplets loss heat by radiation from cloud tops during evening,the radiation cooling at top of the clouds results into steeper lapse rate near the top and induces additional ascend from warmer lower side. This enhances their instability and growth. This mechanism is used to explain the nocturnal thunderstorms from clouds that ceased to grow prematurely at the end of day.

Ocean Currents: Warm ocean currents enhance instability and cold ocean currents enhance stability. For instance, majority deserts of the world are present in sub-tropical region mainly due to presence of high pressure belts, but stability and associated aridity is enhanced by cold ocean currents prevailing on western side of the continents.

Instability and Daily Weather

The daily weather conditions are normally determined by stability and instability operating in the atmosphere. The calm and clear sky conditions represent atmospheric stability. However, ascend of stable air, due to forced mechanism, results into formation of clouds which are fairly widespread but have limited vertical extent. Precipitation from such clouds, if any, is invariably light. Light drizzle and overcast sky indicate the forced uplift of the stable air.On the contrary,cauliflower-shaped cumulonimbus clouds associated with the convective unstable air currents have great vertical extent and are accompanied by heavy precipitation, thunder and lightning. Therefore, the type of clouds and nature of precipitation are symbols of type of atmospheric stability and instability.

On hot summer afternoons, due to intense surface heating, instability intensifies and, parcels of air get heated and through convection process move upward. In case, they are moist enough and able to rise to the level of condensation, clouds develop to give occasional mid afternoon showers. This precipitation is light and of short duration. Precipitation itself cools the surface and breaks the convectional cycle and secondly, surface heating of this type provides limited instability. But in equatorial low pressure belts surface heating (convection), convergence of winds and upper air divergence of Hadley cell intensify the level of instability. The result is heavy precipitation from cumulonimbus clouds extending upto tropopause with anvil head.

As noted earlier the inversion of temperature produces the most stable conditions in the atmosphere. In this situation, the air near surface becomes cooler and heavier than the air aloft.Radiation inversion, advection inversion, air drainage inversion and upper air inversion due to subsidence are common indicators of atmospheric stability. Long nights, clear sky, calm and dry air conditions are ideal for inversion of temperature. In winter, dew, frost and fog develop due to contact cooling. Radiation cooling, advection cooling and associated fogs represent stability.An extensive fog is guaranty of atmospheric stability. Subsidence can result in a temperature inversion aloft. Hence, the diurnal and seasonal characteristics of weather are strongly related to atmospheric stability and instability.

Instability and Distribution of Precipitation

In the distribution of the precipitation most important determinants are stability and instability of atmosphere. The global mean annual precipitation is about 97 cm. The areas of atmospheric instability receive very high precipitation; contrary to this the areas of atmospheric stability receive very low precipitation. In the latitudinal belts of atmospheric convergence due to adiabatic cooling cloud formation and precipitation are intense. The equatorial doldrums due to convergence and convection have instability and have precipitation maxima. The average annual precipitation in equatorial belt is about 175 – 200 cm, and majority precipitation is received in the afternoon and it occurs throughout the year. The second maxima of precipitation is in the mid latitude convergence zone. In this zone the instability is intensified by the convergence of warm moist tropical airmasses with cold polar air masses in frontal zone. The instability in these belts is associated with convergence, frontal wedging and temperate cyclones and results into throughout the year precipitation. Here, the average annual precipitation is about 100 – 125 cm. On the contrary, the subtropical high pressure areas of subsidence and divergence do not encourage cloud formation and precipitation due to atmospheric stability. These are the most arid areas and are predominated by deserts, with average annual rainfall less than 25 cm.Likewise, the stable polar areas due to high pressure or subsidence receive less than 25 cm average annual precipitation and represent cold deserts.

Monsoon areas receive high rainfall due to atmospheric instability in summer season. The differential rates of heating of land and sea, resultant moist and warm onshore winds, orographic lifting and local convective aloft are the reasons for instability in this climatic region. In the world, wherever the coastal areas are flanked by mountain ranges the onshore ascending winds due to adiabatic cooling cause cloud formation and heavy precipitation on the windward side and rain shadow regions on leeward sides. It is because of instability produced by orographic lifting that mountain areas record the highest precipitation in the world, for instance, Meghalaya plateau, Western Ghats, western Rockies and Andes, eastern Great Dividing Range and Southern Alps.

The air masses and prevailing winds moving over relatively warmer areas become unstable and may result into clouds and precipitation. In case of precipitation associated with tropical disturbances such as cyclones, hurricanes, tornadoes, the upper air divergence, lower air convergence and convection and adiabatic lifting result into heavy precipitation, by cumulonimbus or thunder clouds. The warm ocean currents are conducive to greater precipitation because they cause and intensify instability. On the contrary, cold ocean currents produce stability and extensive fog in suitable situations. Hence, there is intricate relationship between stability and instability and world distribution of precipitation.

Summary and Conclusions

Atmospheric stability represents condition of absence of vertical motions or resistance to such motions. Instability, on the other hand, represents prevalent tendencies for vertical motions. Hence, instability is protagonist to precipitation. Among the factors of precipitation, vertical motions in the atmosphere play the most vital role. Instability causes various weather phenomena such as cloud formation, precipitation and its forms, and thunderstorms.

The stability and instability of atmosphere is determined by the relationships between environmental lapse rate and adiabatic lapse rates. Stability occurs when the environmental lapse rate is less than dry adiabatic rate.Absolute stability prevails when the environmental lapse rate is less than the wet adiabatic rate. Under these conditions, the rising parcel of air becomes colder and denser and would tend to sink to its original position. The inversion of temperature is an ideal example of absolute stability.

Absolute instability represents continued vertical motion of the ascending parcel of air till its temperature is not equal to that of surrounding environment. This occurs when the environmental lapse rate is greater than the dry adiabatic rate and wet adiabatic rates. Under these conditionsthe ascending parcel of air remains always warmer than its surrounding environment and continuesto move upward because of its own buoyancy. The end result is, generally, vertical clouds and precipitation.

A more frequent type of atmospheric instability is known as conditional instability. This occurs when the moist air has an environmental lapse rate between the dry and wet adiabatic rates. It simply means that the atmosphere is stable with respect to unsaturated parcel of air, but unstable in relation to the saturated parcel of air.The term “conditional” is used because the air must be forced upward before it reaches the level of free convection where it becomes unstable and rises on self basis. Occasionally convective instability and mechanical instability also prevails.These are also significant in the dynamics of weather.

Stability is enhanced by the following conditions – (i) radiation cooling of surface during night, (ii) contact cooling (conduction) of an air mass from below, and (iii) subsidence within an air column. Subsidence results into substantial modifications in weather conditions. The warming effect of subsidence is enough to evaporate the clouds present in any part of the atmosphere.

Instability and precipitation depends on following conditions – precipitation in substantial amount is induced by spontaneous convectional rise of warm and moist air and another mechanism enhancing instability is the forced rise of the moist air. The forced upward movement of air is caused by processes such as orographic lifting, frontal wedging, and convergence. All these lifting processes enhance instability and cause precipitation.

The advection of cold air over warm surfaces enhances instability and may cause precipitation.Radiation cooling from cloud topsenhances their instability and growth. This mechanism is used to explain the nocturnal thunderstorms from clouds that cease to grow prematurely at the end of day.Warm ocean currents enhance instability and cold ocean currents enhance stability.

The daily weather conditions are normally determined by stability and instability operating in the atmosphere. The calm and clear sky conditions represent atmospheric stability. However, ascend of stable air, due to forced mechanism, results into formation of clouds which are fairly widespread but have limited vertical extent. Precipitation from such clouds, if any, is invariably light. Light drizzle and overcast sky indicate the forced uplift of the stable air. On the contrary, cauliflower-shaped cumulonimbus clouds associated with the convective unstable air currents have great vertical extent and are accompanied by heavy precipitation, thunder and lightning. Therefore, the type of clouds and nature of precipitation are symbols of type of atmospheric stability and instability. Further, the diurnal and seasonal characteristics of weather are strongly related to atmospheric stability and instability.

In the distribution of the precipitation most important determinants are stability and instability of atmosphere. The areas of atmospheric instability receive very high precipitation; contrary to this the areas of atmospheric stability receive very low precipitation. For instance, precipitation maxima prevails in equatorial zone and second maxima in mid latitude convergence zone and sub tropical high pressure belts are arid areas. Hence, there is intricate relationship between stability and instability and world distribution of precipitation.

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