Precipitation is one of the most important factors in the water cycle, which all life forms on Earth revolve around. However without the essential conditions and processes occurring, precipitation would not be possible.
Precipitation results when cloud particles become too heavy therefore they fall to the Earth’s surface as water that comes either in solid (Hail, ice-pellets, snow, diamond dust, sleet and snow grains) or liquid form (Rain, freezing rain and drizzle), falling from the atmosphere towards the Earth’s surface. In order for precipitation to occur, cloud formation is an essential process.
Figure 1 – Cloud formation
Sourced from: http://apollo.lsc.vsc.edu/ Clouds form from the process of condensation due to adiabatic processes. Unstable conditions result in an air parcel rising until it cools at dew-point temperature, therefore becoming saturated. A decrease in atmospheric pressure results in the expansion of air molecules in the parcel, therefore this causes a decrease in temperature. This process is known as adiabatic cooling and has a constant lapse rate of 10Co per 1000 metres. However air sinking down the atmosphere increases in temperature due to being compressed by the constant increase in air pressure, which is known as adiabatic heating. Further cooling results from the air parcel being lifted, thus condensation of water vapour occurs. The presence of cloud condensation nuclei is mandatory for water to condense amongst molecules of air. To summarise, cloud formation occurs based on the conditions of saturated air, cooling mechanisms and lastly the presence of cloud condensation nuclei. http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter06/adiabatic01.jpg
The form undertaken by precipitation depends mainly on the formation method and the present temperature during the formation. Two of the main theories accepted as being principally responsible for the formation of the precipitation particles are the Collision-coalescence process and the Bergeron/ ice crystal process.
Collision coalescence process:
A theory for warmer clouds in low latitudes, it states that as water droplets are cohesive, upon colliding with one another, they merge. Evidence for this theory has been seen in warmer sections of clouds where moisture exist solely in liquid state, with larger droplets overtaking smaller droplets whilst falling due to their less buoyant state, and merging with them to form a larger droplet. (See Figure 1) The increased mass of the droplets defy the updrafts of clouds and begin their descent to Earth’s surface. http://san.hufs.ac.kr/~gwlee/session8/images/collision.gif
Bergeron/Ice crystal process:
Figure 2: ‘Collision-coalescence process’
Sourced from: http:/ san.hufs.ac.krProposed in the 1920’s by Swedish meteorologist Tor Bergeron, the theory states that due to the lowered saturated pressure of the ice-crystal, as compared to the water vapour, this results in the ice-crystal growing larger in size as surrounding water vapour evaporates. Once the ice crystals are large enough to fall, they pass through the warm, lower portion of clouds where they absorb more moisture therefore increasing yet again in size. (Figure 2) The ice-crystals will then either melt and precipitate as rain or precipitate from the clouds as snowflakes.
Figure 3: ‘The Bergeron Process’- Sourced from: http://geography.hunter.cuny.eduhttp://www.geography.hunter.cuny.edu/~tbw/wc.notes/5.cond.precip/precipitation/bergeron.process.jpg
The forms of precipitation depend mainly on the air temperature and turbulences. The major forms of precipitation include Rain, Snow, Sleet (Also known as freezing rain) and hail.
The most common form of precipitate, it consists of droplets of liquid water. Though their sizes vary, droplets are usually between 2-5mm in diameter. Rain droplets may reduce in size and distance between each other as a result of the temperature of the air mass being slightly less than the dew point. This then is known as ‘Drizzle’.
The second most common form of precipitate, snow occurs when water vapour freezes directly prior to passing through sublimation, thereby forming tiny crystals around the freezing nuclei as seen in the Bergeron process. http://www.nssl.noaa.gov/primer/winter/images/snow.jpg
Figure 4- ‘Snow formation’
Sourced from: http://nssl.noaa.gov(Figure 4)
Rain falling down to Earth gets passed through a reasonably thick layer of cold air near Earth’s surface therefore freezing.
Figure 5- Temperature differences with precipitation forms
Sourced from: http://uiuc.edu/ forestry/publications/pdf/ urban_community_forestry/ trees_and_ice_storms_2006.pdfWhile Sleet is formed by raindrops freezing during descent, Glaze is formed the instant raindrops collide with a solid object.
A lesser-common precipitate, the formation of hail begins with the lifting of ice-crystals caused by strong updrafts in a cumulonimbus cloud. (Figure 6) Cooled by circulating inside the cloud, the water droplets attach together, forming layers. http://www.jamesspann.com/wordpress/wp-content/uploads/2011/05/SevereWx6.jpg
Figure 6-Hail formation
Sourced from: NC State University (http://hatteras.meas.ncsu.edu/secc_edu/SevereWeather/body)
A less common form of precipitate is:
Diamond dust/ ice crystal
A meteorological phenomenon, diamond dust is also known as a form of clear-sky precipitation as it forms under relatively clear skies.
Atmospheric lifting plays a major role the formation of precipitation as mentioned above. http://geospatial.gsu.edu/geog1112/files/Lab5/Convection.jpg
Figure 7- ‘Convective lifting’
Sourced from: Pearson prentice hall Inc. 2007/As different surface areas have various temperatures due to unequal heating, conduction causes the air parcel to be warmer than surrounding air (Figure 7). Heated air expands as its density decreases; therefore the air parcel rises towards the lower-density layer. As the air continues to rise, this results in the pressure of the unstable air decreasing. Thus the air cools adiabatically until the dew point temperature has been reached.
Precipitation that forms from convective lifting, convective precipitation, is common in warmer regions of the globe and is observed to be light showers with large raindrops. However convective precipitation usually last for short durations at a time. http://media-1.web.britannica.com/eb-media/51/135451-004-A6B6636B.jpg
Figure 8 Orographic lifting
Sourced from: http://britannica.comAscending air is forced upwards a land mass, usually due to a topographic barrier, where it cools adiabatically to dew point temperature and then condenses. When air stars to descend downhill, condensation and precipitation come to a halt as the air starts to warms adiabatically. Windward slope of the barrier is termed the wet side whereas the leeward slope is the dry side. The rain shadow area is considered to be from the dry side to the extent of the drying influence. Orographic precipitation is common in New Zealand due to the presence of many hills and mountain ranges in the landscape.
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This type of lifting occurs due to a front, which is defined by Whittow (2000) as being the boundary plane between two air masses with different meteorological characteristics. The warmer air mass is forced to rise over the cold air mass, thereby cooling to dew point temperature. This then allows for the formation of clouds and the resulting frontal precipitation. http://ocw.usu.edu/forest__range__and_wildlife_sciences/wildland_fire_management_and_planning/lifting3.jpg
Figure 9- ‘Frontal Lifting’
Sourced from: Idaho University http://www.sci.uidaho.edu
Figure 10- Convergent lifting
Sourced from: Idaho University
http://www.sci.uidaho.eduThe least common type of atmospheric lifting, it occurs with air converging thus, uplift is formed due to crowding. Instability is then increased therefore producing showery-type precipitation. Convergent lifting is linked to cyclonic storms and is far more common in lower latitudes, as well as the intertropical convergence zone, where tropical disturbances such as hurricanes are caused by it. http://www.sci.uidaho.edu/scripter/geog100/lect/05-atmos-water-wx/05-part-7-atmos-lifting-fronts/05-25a-lifting-convergent.jpg
The amount of precipitation received in any region on Earth’s surface depends on the characteristics of the air mass involved, such as humidity, temperature and stability which are also dependent on the originating latitudes and the trajectory of the air mass.
Figure 11- ‘Total annual rainfall’
sourced from: climate-charts ©
www.climate-charts.comWorldwide annual precipitation http://www.climate-charts.com/images/world-rainfall-map.png
Tropical latitudes are considered to contain most of the wettest regions in the globe. This may be due to the large moisture carrying capacity of the warm easterly trade winds, hence why very heavy rainfall is often produced in those areas.
Regions near the equator exhibit these circumstances, as warm and unstable air gets lifted by within the intertropical convergence zone, as the warmer ocean waters easily vaporizes. Large amounts of precipitations also form due to topographic obstacles forcing trade winds to rise. The orographic effect is exhibited in the eastern regions of tropical landmasses, such as Madagascar and N.E South America, due to the trade winds coming from the easterly direction.
Drier areas are more common on the western side of continents in subtropical latitudes (20o and 30o centred ) High pressure areas, where air sinks which does not promote condensation or precipitation, are common at the latitudes mentioned above which are closer to subtropical high-pressure cells.
The contrast between the precipitation received in coastal regions and interior regions of continents are also quite visible. (Figure 8) Coastal regions are able to receive a higher amount of precipitation due to being geographically closer to sources of moisture, such as oceans.
In order for precipitation to be formed, numerous conditions and processes have to be met. Adiabatic processes are key in cloud formation, which is essential for precipitation. The two main theories currently accepted as principally responsible for producing precipitation particles are the Bergeron/Ice-crystal process and the Collision-coalescence process. Forms of precipitation that arise from the processes above include, Rain, which is the most common form; Snow, Sleet, Glaze and Hail. A lesser common form is Diamond dust/Ice crystals which are known as a meteorological phenomenon.
Only through the processes of atmospheric lifting can precipitation occur. The four main principle types of atmospheric lifting include Convective lifting, which is due to unequal heating on the surface; Orographic lifting, which is caused by a topographic barrier; Frontal lifting, due to fronts; and lastly convergent lifting, the least common, occurring when air converges.
The distribution of precipitation varies greatly around the globe, depending however, on the characteristics of the air mass involved, such as humidity, temperature and stability which are also dependent on the originating latitudes and the trajectory of the air mass. Tropical latitudes receive the most amounts due to the moisture carrying capacity of the trade winds which also cause an orographic effect observed near the eastern regions of tropical landmasses, due to their easterly direction.
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