18 ATMOSPHERIC MOISTURE IV: FORMS AND TYPES OF PRECIPITATION

Dr. Jitender Saroha

Objectives

define precipitation,
explain the mechanisms of process of precipitation,
explain the forms of precipitation, and describe the types of precipitation
describe distribution of precipitation

Contents

Introduction

Learning Objectives

Precipitation

Bergeron-Fiendeisen Process

Collision-Coalescence Process

Forms of Precipitation

Rain

Drizzle

Freezing Rain and Freezing Drizzle

Snow

Sleet

Snow Pellets

Hail

Types of Precipitation

Convectional Precipitation

Orographic Precipitation

Frontal and Cyclonic Precipitation

Distribution of Precipitation

Summary and Conclusions

Multiple Choice Questions

Answers to MCQs

References

Web Links

Introduction

The process of precipitation represents falling down of water in liquid or solid forms on the earth surface. Rain and snow are the most common forms of precipitation. You all very well know that precipitation cannot take place without clouds. But all clouds do not bring precipitation; rather occasions of cloudiness without precipitation are common. Even when precipitation occurs, sometimes it is in the form of liquid droplets and at other occasions in variety of solid forms. Transformation of cloud droplets into precipitation droplets depends on certain mechanisms. In this module, our focus is on these mechanisms, forms of precipitation and types of precipitation.

Precipitation

Clouds as aggregates of water droplets and ice crystals are a form of condensation and a precondition for precipitation. The process of precipitation represents falling down of this condensed material in the form of rain, snow, hail, sleet or some other forms. Although all clouds contain water droplets or ice crystals or both, some move to the final step i.e. precipitation, while majority do not. Sometimes precipitation takes place from clouds but it gets evaporated in the atmosphere and fails to reach at surface. Therefore, clouds themselves are not enough for precipitation. The process of precipitation depends on fulfillment of certain conditions. The most significant requirement is related to the size of droplets which can result into fall of condensed material on surface.

The clouds are composed of billions of tiny droplets (diameter in the range of 10-20 micrometers). The size of cloud droplets remains tiny due to the presence of numerous condensation nuclei and competition among them for available water acquisition.Due to their small size these droplets fall at a very slow rate and likely to get evaporated before they fell a few meters below the cloud base or level of condensation. On the other hand, a typical rain drop has size about 100 times of that of the average size of cloud droplet and its volume is a million times of cloud droplet. Thus, millions of tiny cloud droplets constitute a single raindrop (Figure 1). For the transformation of cloud droplets into precipitation drops scholars have propounded two mechanisms,namely, Bergeron process and Collision-Coalescence process.

Figure 1: Comparison of Cloud Droplet with Raindrop

Bergeron-Fiendeisen Process

In 1933, two Norwegian meteorologists Tor Bergeron and Von Fiendeisen explained the mechanism of process of precipitation on the basis of ice crystal formation. It is also known as Bergeron process or ice crystal process.The Bergeron process involves ice-crystal growth, which depends on the simultaneous presence of water in three states – water vapour, liquid cloud droplets, and ice crystals. This process is based on two facts – (i)the water droplets in a cloud are found in supercooled state i.e. as liquid even at subzero temperature upto -400C, and (ii) different values of saturation vapour pressure prevail over water and ice crystals.

The supercooled droplets freeze when they come in contact with the freezing nuclei, which are limited in number as compared to condensation nuclei and are most active at -200C to -250C. Therefore, water vapour, supercooled droplets and ice crystals co-exist in the cloud. As there are numerous condensation nuclei, the number of supercooled droplets is far greater than ice crystals. In fact, a single ice-crystal is capable of rapidly transforming a cloud of supercooled droplets into an ice cloud due to variations in saturation vapour pressure, at the same temperature.

The saturation vapour pressure over ice crystals is lower as compared to supercooled water droplets. In the temperature range of -5 to -250C the vapour pressure variation is more than 0.2 mb. It means when air is saturated (100 per cent relative humidity) in relation to supercooled water droplets, it is supersaturated with respect to ice crystals. For instance, at 200C, when the relative humidity is 100 percent with respect to water, it is about 121 per cent with respect to ice.

As a consequence of vapour pressure variation, supercooled water droplets tend to evaporate and sublimation or direct deposition of water vapour occurs on ice crystals. Over the period of time, the size of ice crystals increases rapidly at the cost of water droplets. The enlarged ice crystals often join one another and produce large size snowflakes (Figure 2 and 3b). These snowflakes due to their weight fall towards earth surface in the form of raindrops, snow or hail depending upon the temperature conditions of layers of air between cloud base and earth surface.Within the supercooled cloud, during their descent,ice crystals grow in size by intercepting cloud drops that freeze on them, and riming occurs and it plays significant role in hail formation. Sometimes, these descending ice crystals break up and provide larger number of freezing nuclei for other droplets. The fast chain reactions produce glaciations in upper part of cumulonimbus clouds and shape changes from cauliflower to anvil-shaped.

Figure 2: Growth of Ice Crystals at the cost of Water droplets

The ice-crystal theory or the Bergeron process described above, explains satisfactorily the process of precipitation in the middle and higher latitudes.But is fails to explain precipitation from ‘warm clouds’ of tropical regions. Over tropical oceans cumulus clouds of about 2400 meter thickness produce copious rains though they have temperature on their top 70C or above. Therefore, ice crystals cannot be formed and some other mechanism of precipitation operates in such warm clouds and that is known as the Collision-coalescence process.

Collision-Coalescence Process

This idea,proposed by George Simpson and Mason, is based on the fact that precipitation requires formation of large enough (more than 20 micrometers diameter) droplets which can fall on surface. Hygroscopic particles such as sea salt work as ‘giant’ condensation nuclei for the development of such large droplets. Tropical warm clouds consist of water droplets of different sizes. The size of small droplets increases due to collision and coalescence. Due to these two processes the size of drops becomes so massive that the upward thrust of air fails to hold them and they fall on the earth surface as rainfall (Figure 3a).

Figure 3: Collision-Coalescence Process

Source:https://kaiserscience.files.wordpress.com/2015/03/raindrop-and-snowflake-formation-bergeron-process.jpg

Later on other scholars identified the additional factors contributing in collision and coalescence. One observation is that the terminal velocities of descending droplets are directly associated with the size of condensation nuclei. The large condensation nuclei generate large droplets and they descend faster and while overtaking, collide and coalesce with the small drops. As noted earlier, after aggregation of the equivalents of a million or so cloud droplets, the raindrops become large enough to fall on surface without evaporation in the transmission path.

The favourable conditions required for collision and coalescence are:

(i) The rate of expansion of droplets depends on factors such as size and distribution of cloud droplets;

(ii) Clouds which have great vertical thickness and abundant moisture have best potential for precipitation, because repeated updrafts of droplets make the droplets transverse the cloud many times and grow in size;

(iii) In the initial stage, the number of condensation nuclei should be limited so that they can acquiresufficient water from the available moisture and can become bigger in size;

(iv) Variations in size of the nuclei also favour collision and coalescence because, in case, size is uniform colloidal stability operates but variations in size produce colloidal instability due to different fall velocities;

(v) The breakup of droplets when they acquire the maximum size (about 5mm) due to failure of surface tension (holds the drop as a unit) of drops under frictional drag produces numerous smaller drops which indulge in further collision and coalescence; and finally

(vi) Coalescence is rapid in case droplets have opposite electric charges;

Forms of Precipitation

Due to spatial and temporal variations of atmospheric conditions, a variety of forms of precipitation occur. Precipitation occurs in many forms or phases, such as:

Liquid precipitation (Drizzle and rain),

Freezing precipitation (freezing drizzle, freezing rain and sleet) and Frozen precipitation (snow, ice pellets and hail etc.).

Drizzle, rainfall and snowfall are the most common, well known and widespread forms of precipitation. The other forms of precipitation such as freezing rain, sleet and hail etc are associated with specific weather conditions and are limited in occurrence and extent. The major forms of precipitation are described in the following sections.

Rain

Rain is the most common form of precipitation. The term is used for raindrops which have diameter in the range of 0.5 to 5 mm. Larger than this range raindrop fail to reach surface because while descending, the frictional drag exceeds the surface tension and large droplets breaks apart into smallerdrops. In tropical areas, cumulus clouds rain is the result ofcollision and coalescenceprocess. In temperate areas, most rainfall is produced by nimbostratus clouds. The vertical clouds, cumulonimbus produce heavy rainfall with lightning and thunder. In temperate areas and in cumulonimbus clouds most of the rainfall begins as snow crystals and due to presence of warmer air between cloud base and surface, changes into liquid state.

Drizzle

Drizzle represents light liquid precipitation in the form of uniform water droplets of diameter less than 0.5 mm. It is usually associated with stratus, nimbostratus and stratocumulus clouds. Precipitation in the form of drizzle continues for several hours or occasionally for days, but rate is about a millimeter per day or less. It is the most frequent form of precipitation over subtropical oceans.

Freezing Rain and Freezing Drizzle

Freezing rain occurs when raindrops pass through the subfreezing air near the surface, the raindrops become supercooled and freeze when they strike on surface features like plants and power lines etc. (Figure 4). Similarly, in case of small size droplets (less than 0.5 mm diameter), this supercooling produces freezing drizzle.

Figure 4: Forms of Precipitation

Source:http://archive.boston.com/news/weather/weather_wisdom/image005.gif

Snow

Snow represents precipitation in the solid form of water i.e. as white grains of ice or snowflakes.Generally, snow is powdery and feathery in appearance and primarily do not join together to form bigger crystal. When the accumulated ice is compacted, they form ice crystal and solidify.This term includes a great range of ice crystals. The shape, size and concentration of snowflakes reflect the processes by which they were formed. Snowfall takes place from ice clouds. In winter season, under subfreezing temperatures ice crystals fall from clouds and reach the surface as snow without melting.

Sleet

In Commonwealth countries, including Canada, precipitation composed of rain and partially melt snow is known as sleet. In American terminology, sleet represents small ice pellets as frozen rain or refrozen melted snow water. Sleet occurs in the presence of an above-freezing air layer overlying a subfreezing layer near the surface. When the raindrops, which are usually melted snow, pass out from the warmer layer and enter into colder air layer, they freeze and fall on the surface as small ice pellets of transparent or translucent ice, usually with diameter of 5 mm or less. Sleet is predominantly a wintertime phenomenon, in middle and higher latitudes. Sleet form of precipitation also occurs in frontal precipitation (Figure 5).

Figure 5: Sleet as Frontal Precipitation

Source:http://kvgktrailblazers.weebly.com/uploads/6/8/9/5/6895534/1487682_orig.jpg?331

Snow Pellets

Snow pellets consist of small, white and opaque compact grains of ice. The grains are mostly spherical and have a diameter of 2-5 mm. Snow pellets are also known as soft hail, the grains are brittle and bounce and break after striking surface. They are commonly associated with convective storms of winter and spring seasons.

Hail

Hail represents precipitation in the form of hard and rounded pellets of ice, mostly diameter between 1 cm and 5 cm. Strong, repeated ascend and descent as convective currents in cumulonimbus clouds results in the formation of nearly concentric layers of differing densities and degrees of opaqueness (Figure 6). Another mechanism of hail formation holds that a frozen nucleus descends and encounters and acquires supercooled water droplets and snow crystals as concentric shells to grow as hail stone. Its size depends on the content of ice and snow it acquires while falling down. Hailstones are common in violent summer thunderstorms.

Figure 6: Formation of Hail

Source: http://www.physicalgeography.net/fundamentals/images/hail_formation_model.jpg

Types of Precipitation

In the previous two Modules(17 and 18) on atmospheric humidity, it has been explained in detail that the process of adiabatic cooling plays the most significant role in process of condensation, especially cloud formation and precipitation. The atmospheric instability is enhanced by three mechanisms which force the moist and warm air to ascend above the level of condensation and form clouds. The Bergeron process and collision-coalescence process result into falling down of condensed material in the forms (rain, drizzle, snow etc.) described above.

The three mechanisms resulting into lifting of warm and moist air are:

(i) Thermal heating of surface and adjacent air and formation of convectional currents;

(ii) Forced uplift of air over an orographic barrier; and,

(iii) Frontal ascent or frontal wedging associated with convergence of different types of air massesin mid-latitude areas, and in tropical areas formation of low pressure cyclonic disturbances.

It is noteworthy that all these three mechanisms of lifting are not necessary independent of one another rather more than one functions, simultaneously. But the type of precipitation is identified on the basis of dominant mechanism. Therefore, the three major types of precipitation are following:

Convectional Precipitation

In this type of precipitation, the principal lifting force for the ascending warm and moist air is thermal convection produced by heating of surface and adjacent air by solar radiation. The two pre-conditions for convectional precipitation are:

(i) Intense heating of the surface by insolation so that the air in contact gets heated, expands and rises up; and

(ii) Ant supply of moisture to the ascending air to maintain high relative humidity.

These conditions result into upward movement of warm and moist air which initially cools at dry adiabatic rate. The adiabatic cooling increases relative humidity and saturation stage or level of condensation is achieved at some height. Further updraft of air occurs at wet adiabatic rate and cloud formation takes place. The ice crystal mechanism (in cumulonimbus) and collision-coalescence (in cumulus cloud) results into heavy showers in first case with occasional hailstones, and drizzle and rain in case of cumulus clouds. Thus, convectional precipitation is basically a warm weather phenomenon (Figure 7).

Figure 7: Convectional Rainfall

Source:http://geography.parkfieldprimary.com/_/rsrc/1344848458786/climate-types/equatorial-regions/climate/convectional.png

High temperature and high relative humidity of the equatorial regions provides ideal situation for convectional rainfall. Here, daily heating of surface results into onset of ascending warm and moist air convection currents. Consequently, the sky becomes overcast by thick vertical cumulonimbus clouds by 2 to 3 P.M., resulting into complete darkness, heavy rains, lightning and thunder, and sky becomes clear by 4 P.M. and rainfall stops. Therefore, convectional rainfall is a daily routine in equatorial zone, and doldrumsareas receive rainfallthroughout the year.

Away from equator, in tropical and temperate regions the convectional precipitation is associated more markedly with summer and the warmer times of the day. In temperate regions, it is usually not in the form of heavy showers rather it is light to moderate, slow and of longer duration.

In temperate regions, this type of precipitation is also produced due to movement of cold unstable air over relatively warmer surface. This situation results into the formation of convective cells, which move along with winds and produce precipitation in the form of showers of rain, snow or snow pellets. Therefore, this precipitation is widespread, but of limited duration for a particular place.

Another example of this type of precipitation is cumulonimbus convective cells of tropical cyclones, especially in the last stages over coastal lands. The associated precipitation is of high intensity, rain, hail, thunder and lightning and widespread. The convectional precipitation is predominantly the result of conditional and convective instabilities.

Orographic Precipitation

Orographic precipitation occurs when mountains or physical barriers force the flow of air to rise and cool adiabatically. It may results into condensation and precipitation as rain or snow, depending on level of instability. This type of precipitation predominantly occurs on the windward sides and leeward sides generally remain rain shadow areas.On the windward side, adiabatic cooling of warm and moist air results into condensation and precipitation, but after crossing the crest no lifting takes place and air becomes dry, asmost of the moisture is already precipitated. Therefore, the descending air gets adiabatically heated due to compression.Afterwards, condensation and precipitation remain negligible or low on the leeward side and it is called rain shadow region (Figure 8).

Figure 8: Orographic Precipitation

Source:https://image.slidesharecdn.com/wcppt-150831104047-lva1-app6891/95/weather-climate-12-638.jpg?cb=1441017737

The ideal situation for heavy orographic precipitation is when a highland lies generally, perpendicular the direction of the moisture laden wind coming from a warm ocean. Winds strike the obstruction at right angle. For instance, the coastal mountain ranges of North America on the western coast are parallel to the pacific coast and westerlies strike them at right angle, resulting into heavy orographic precipitation on windward (western) sides and relatively dry conditions of leeward side.

The best example of such a situation is represented by the Western Ghats of India, which lies parallel and in close proximity to Arabian Sea and in summer season warm and moist south west monsoon winds strike them directly. During this season, Mumbai on windward side receives 190 cm rainfall, whereas Pune on leeward side records only 50 cm rainfall. The crest part of the Western Ghats records about 400-500 cm average annual rainfall and it decreases all of a sudden to about 30-50 cm within limited range of about 80-100 km from the ridge-line. Likewise, in North India, in Himalayas the southern windward slopes get average annual precipitation about 200 cm whereas the northern leeward sides of these greatest mountain ranges of world receives only about 5 to 10 cm average annual precipitation. For instance, Ladakh region on the leeward side of the Greater Himalaya range is a cold desert.

The most significant feature of orographic rainfall is the inversion of rainfall. The amount of rainfall on windward side increases continuously up to a certain height known as line of maximum precipitation. Beyond this line it starts decreasing due to declining moisture content and adiabatic heating due to compression on leeward side. This represents inversion of rainfall. The location of line of maximum rainfall depends on latitude, distance from sea, moisture content in air, type of slope, season and exposure.Mawsynram and Cherrapunji are located on the southern slopes of the Khasi hills at the northern end of a funnel shaped valley running south to north in close proximity to the Bay of Bengal and both receive heavy rainfall. Mawsynram receives the world’s highest annual precipitation i.e. 1,221 cm.

It is noteworthy that orographic precipitation is not entirely due to the direct forced uplift by the physical barrier but it involves indirect effects of day time convective cells and cyclones also.

Frontal and Cyclonic Precipitation

This type of precipitation is associated with tropical and temperate cyclones and fronts. In temperate regions, when two different types of air masses with sharp contrasts in temperature and humidity come together by converging forces, zone of mixing or frontogenesis or front formation takes place. Lifting mechanism at fronts produces instability, resulting into condensation and precipitation. Extra-tropical cyclones or temperate cyclones or wave cyclones originate at polar fronts. These fronts have a great variety of clouds and also a great variety of forms of precipitation (Figure 9).

Figure 9: Frontal and Cyclonic Rainfall

Source: http://lessonslearned.faa.gov/Eastern66/Cold_Front.jpg

Along a cold front, the cold air invades the warm-air zone. The colder air mass being denser remains in contact with surface and forces the warmer air mass to rise over it. Depending upon level of instability cumulonimbus clouds and thunderstorms may develop along a line known as squall line. The precipitation associated with cold front is confined to narrow zone and is of short duration at a particular place.

In case of warm front, the warmer and moist air is forced to rise, as if it were ascending a long and gentle slope. Contact cooling and adiabatic cooling produces instability and precipitation. On the basis of level of instability of the slowly rising warm and moist air, convective clouds or even cumulonimbus thunderstorms may occur. Nimbostratus and altostratus clouds are also common at lower levels. Although the precipitation is moderate to gentle, it is widespread and of longer duration.

Therefore, the precipitation characteristics vary according to the type of fronts and stage of cyclogenesis. The frontal precipitation is generally moderate and widespread. The temperate cyclones move from west to east in 40 and 65 latitudes. In north India, they operate as western disturbances in winter season and bring snow fall in mountain areas and rainfall in plains.The winter precipitation in North America and Europe is predominantly of frontal origin.

In case of tropical cyclones, the convergence of air towards a low pressure centre is the initial trigger for updraft of air. The ascending air moves fast and almost vertical due to circulatory convectional currents. As the air rises, the moisture gets condensed and low pressure centre and uplift is maintained or energized by latent heat of condensation. Cumulonimbus clouds develop and produce heavy rainfall with occasional hail, lightning and thunder. Tropical cyclones often cause heavy rainfall and destruction in coastal areas of India and Bangladesh, as typhoons in China and Japan, as hurricanes and thunderstorms at Florida coast of USA and West Indies and as Willy willies in Australia.

Distribution of Precipitation

The distribution of precipitation in the world is highly uneven. The average annual precipitation in the world is 97 cm. At Mawsynram and Cherrapunji the average annual precipitation is 1,221 and 1,102 cm respectively. On the other hand, about one-third area of landmasses is arid and semi-arid in the form of subtropical hot deserts and cold deserts.

The equatorial zone receives maximum rainfall with a mean annual of 175 to 200 cm. The monsoon region also receives more than world average rainfall, mainly in summer season. For instance, average annual rainfall in India is 118 cm. The mid latitude region also receives high precipitation due to onshore westerlies and temperate cyclones associated with polar fronts (Figure 10). On shore, westerlies also result into winter precipitation in Mediterranean climate regions.

Figure 10: Zonal Distribution of Precipitation in the World

The subtropical high pressure and polar zones receive minimum precipitation. The hot deserts of subtropics receive less than 25 cm average annual precipitation (Figure 11). Likewise, polar areas are cold deserts, with limited precipitation mainly as snowfall.

Figure 11: Global Precipitation Distribution

Source: http://slideplayer.com/slide/6079172/18/images/36/Global+Precipitation+Distribution.jpg

Summary and Conclusions

Most precipitation occurs ultimately from a combination of cooling processes which result into condensation or deposition (sublimation). Additional processes such as the Bergeron or ice-crystal process and collision-coalescence process are necessary to cause raindrops or ice crystals to grow large enough to fall on earth surface. The form of precipitation (rain, drizzle, snow, sleet and hail) that reaches the surface depends on temperature conditions of clouds and the air layers between surface and cloud base.

The three mechanisms resulting into lifting of warm and moist air are – (i) thermal heating of surface and adjacent air and formation of convectional currents; (ii) forced uplift of air over an orographic barrier; and, (iii) frontal ascent or frontal wedging associated with convergence of different types of air masses, and in tropical areas formation of low pressure cyclonic disturbances. On the basis of these mechanisms three types of precipitation are – convectional, orographic, and frontal and cyclonic. It is noteworthy that all these three mechanisms of lifting are not necessary independent of one another rather more than one functions, simultaneously. The world distribution of precipitation is highly uneven.

you can view video on ATMOSPHERIC MOISTURE IV: FORMS AND TYPES OF PRECIPITATION

References

Barry, R.G. and Chorley, R.J. (1998) Atmosphere, Weather and Climate, Routledge, London.
Critchfield, H.J. (1983) General Climatology, PHI Learning Private Limited, New Delhi.
Frederick, K.L and Edward, J.T. (2010) The Atmosphere – An Introduction to Meteorology, PHI Learning Private Limited, New Delhi.
Khullar, D.R. (2012) Physical Geography, Kalyani Publishers, New Delhi.
Lal, D.S. (2009) Physical Geography, Sharda PustakBhawan, Allahabad.
Oliver, J.E and Hidore, J.J. (2003) Climatology: An Atmospheric Science, Pearson Education, Delhi.
Singh, S. (2015) Physical Geography, Pravalika Publications, Allahabad.
Strahler, A.N. and Strahler, A.N. (2001) Modern Physical Geography, John Wiley and Sons, Singapore.
Trewartha, G.T. and Horne, L.H.(1968) An Introduction to Climate, McGraw-Hill, New York.

Web Links