Editing Drought (section)

== Causes ==
[[File:Drought.jpg|thumb|upright|Contraction and desiccation cracks in the dry earth of the [[Sonoran Desert]], northwestern Mexico]]

=== General precipitation deficiency ===
{{See also|Precipitation}}
Mechanisms of producing precipitation include [[convective]], [[Stratus cloud|stratiform]],<ref>{{cite journal|title=A convective/stratiform precipitation classification algorithm for volume scanning weather radar observations|author=Emmanouil N. Anagnostou|journal=[[Meteorological Applications]]|year=2004|volume=11|pages=291–300|doi=10.1017/S1350482704001409|issue=4|bibcode=2004MeApp..11..291A|doi-access=free}}</ref> and [[orographic lift|orographic]] rainfall.<ref>{{cite journal|title=A model of annual orographic precipitation and acid deposition and its application to Snowdonia|author1=A.J. Dore|author2=M. Mousavi-Baygi|author3=R.I. Smith|author4=J. Hall|author5=D. Fowler|author6=T.W. Choularton|journal=Atmospheric Environment|volume=40|date=June 2006|pages=3316–3326|doi=10.1016/j.atmosenv.2006.01.043|issue=18|bibcode=2006AtmEn..40.3316D}}</ref> Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation,<ref name="convection">{{cite book|author=Robert Penrose Pearce|year=2002|url=https://books.google.com/books?id=QECy_UBdyrcC&pg=PA66|title=Meteorology at the Millennium|publisher=Academic Press|page=66|isbn=978-0-12-548035-2|access-date=2009-01-02}}</ref> while stratiform processes involve weaker upward motions and less intense precipitation over a longer duration.<ref>{{Cite book|title=Cloud dynamics|last=Houze|first=Robert A. Jr.|date=1993|publisher=Academic Press|isbn=9780080502106|location=San Diego|oclc=427392836}}</ref>

Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice.

Droughts occur mainly in areas where normal levels of rainfall are, in themselves, low. If these factors do not support precipitation volumes sufficiently to reach the surface over a sufficient time, the result is a drought. Drought can be triggered by a high level of reflected sunlight and above average prevalence of high [[pressure system]]s, [[wind]]s carrying continental, rather than oceanic air masses, and ridges of [[high pressure area]]s aloft can prevent or restrict the developing of thunderstorm activity or rainfall over one certain region. Once a region is within drought, feedback mechanisms such as local arid air,<ref>{{cite book|url=https://books.google.com/books?id=3lbixDAw5DwC&pg=PA22|page=22|title=Greenhouse Effect, Sea Level and Drought|author1=Roland Paepe|author2=Rhodes Whitmore Fairbridge|author3=Saskia Jelgersma|date=1990|publisher=Springer Science & Business Media|isbn=978-0792310174}}</ref> hot conditions which can promote warm core ridging,<ref>{{cite book|url=https://books.google.com/books?id=PvJyVtw53Y4C&pg=PA48|pages=48–49|title=The Oryx Resource Guide to El Niño and La Niña|author1=Joseph S. D'Aleo|author2=Pamela G. Grube|date=2002|publisher=Greenwood Publishing Group|isbn=978-1573563789}}</ref> and minimal evapotranspiration can worsen drought conditions.

==== Dry season ====
{{See also|Dry season}}

Within the tropics, distinct, [[wet season|wet]] and dry [[season]]s emerge due to the movement of the [[Intertropical Convergence Zone]] or [[Monsoon trough]].<ref name="Wang">{{cite book|author=Bin Wang|url=https://books.google.com/books?id=yRT57TENzT8C&pg=PT206|title=The Asian Monsoon|date=2006-01-13|publisher=Springer Science & Business Media|isbn=978-3540406105|page=206}}</ref> The dry season greatly increases drought occurrence,<ref>{{cite book|url=https://books.google.com/books?id=y6r9jUTzJiYC&pg=PA349|page=349|title=Monitoring and Predicting Agricultural Drought : A Global Study: A Global Study|author1=Vijendra K. Boken|author2=Arthur P. Cracknell|author3=Ronald L. Heathcote|date=2005-03-24|isbn=978-0198036784|publisher=Oxford University Press}}</ref> and is characterized by its low humidity, with watering holes and rivers drying up. Because of the lack of these watering holes, many grazing animals are forced to migrate due to the lack of water in search of more fertile lands. Examples of such animals are [[zebras]], [[elephants]], and [[wildebeest]]. Because of the lack of water in the plants, bushfires are common.<ref>{{cite web|url=http://burarra.questacon.edu.au/pages/seasons.html|title=Wet & Dry Seasons|access-date=2018-12-23|archive-url=https://web.archive.org/web/20120320141721/http://burarra.questacon.edu.au/pages/seasons.html|archive-date=2012-03-20|url-status=dead}}</ref> Since water vapor becomes more energetic with increasing temperature, more water vapor is required to increase relative humidity values to 100% at higher temperatures (or to get the temperature to fall to the dew point).<ref>{{cite web|url=http://www.ems.psu.edu/~fraser/Bad/BadClouds.html|title=Bad Meteorology: The reason clouds form when air cools is because cold air cannot hold as much water vapor as warm air|author=Alistair B. Fraser|date=1994-11-27|access-date=2015-02-17|archive-url=https://web.archive.org/web/20150316122724/http://www.ems.psu.edu/~fraser/Bad/BadClouds.html|archive-date=2015-03-16|url-status=dead}}</ref> Periods of warmth quicken the pace of fruit and vegetable production,<ref>{{cite book|url=http://www2.ca.uky.edu/agc/pubs/id/id128/id128.pdf|archive-url=https://ghostarchive.org/archive/20221009/http://www2.ca.uky.edu/agc/pubs/id/id128/id128.pdf|archive-date=2022-10-09|url-status=live|page=19|title=Home Vegetable Gardening in Kentucky|author=Cooperative Extension Service|publisher=[[University of Kentucky]]|date=January 2014|access-date=2015-02-18}}</ref> increase evaporation and transpiration from plants,<ref>{{cite web|author=North Carolina State University|title=Evapotranspiration|url=https://www.nc-climate.ncsu.edu/edu/k12/.evapo|date=2013-08-09|access-date=2015-02-18|archive-url=https://web.archive.org/web/20150219045714/https://www.nc-climate.ncsu.edu/edu/k12/.evapo|archive-date=2015-02-19|url-status=dead|author-link=North Carolina State University}}</ref> and worsen drought conditions.<ref>{{cite web|url=http://www.noaanews.noaa.gov/stories/s909.htm|date=2002-05-16|access-date=2015-02-18|title=Warm Temperatures and Severe Drought Continued in April Throughout Parts of the United States; Global Temperature For April Second Warmest on Record|author=National Oceanic and Atmospheric Administration|url-status=dead|archive-url=https://web.archive.org/web/20150219055008/http://www.noaanews.noaa.gov/stories/s909.htm|archive-date=Feb 19, 2015}}</ref>

==== El Niño–Southern Oscillation (ENSO) ====
{{Further|El Niño–Southern Oscillation}}
The [[El Niño–Southern Oscillation]] (ENSO) phenomenon can sometimes play a significant role in drought. ENSO comprises two patterns of temperature anomalies in the central [[Pacific Ocean]], known as [[La Niña]] and [[El Niño]]. La Niña events are generally associated with drier and hotter conditions and further exacerbation of drought in [[droughts in California|California]] and the [[Southwestern United States]], and to some extent the [[Southeastern United States|U.S. Southeast]]. Meteorological scientists have observed that La Niñas have become more frequent over time.<ref>{{cite web|url=https://www.9news.com/article/news/nation-world/la-nina-west-coast-climate/507-f792ab4a-bc35-4e45-9de1-08d8379dfc9b|title=Weather's unwanted guest: Nasty La Niña keeps popping up|author=Seth Borenstein|publisher=9news.com.au.com|date=May 28, 2022|access-date=June 4, 2022|quote=Scientists are noticing that in the past 25 years the world seems to be getting more La Niñas than it used to...|url-status=live|archive-url=https://archive.today/20231206185338/https://www.9news.com/amp/article/news/nation-world/la-nina-west-coast-climate/507-f792ab4a-bc35-4e45-9de1-08d8379dfc9b|archive-date=6 December 2023}}</ref>

Conversely, during El Niño events, drier and hotter weather occurs in parts of the [[Amazon River]] Basin, [[Colombia]], and [[Central America]]. Winters during the El Niño are warmer and drier than average conditions in the Northwest, northern Midwest, and northern Mideast United States, so those regions experience reduced snowfalls. Conditions are also drier than normal from December to February in south-central Africa, mainly in [[Zambia]], [[Zimbabwe]], [[Mozambique]], and [[Botswana]]. Direct effects of El Niño resulting in drier conditions occur in parts of [[Southeast Asia]] and [[Northern Australia]], increasing [[bush fire]]s, worsening [[haze]], and decreasing air quality dramatically. Drier-than-normal conditions are also in general observed in [[Queensland]], inland [[Victoria (Australia)|Victoria]], inland [[New South Wales]], and eastern [[Tasmania]] from June to August. As warm water spreads from the west Pacific and the [[Indian Ocean]] to the east Pacific, it causes extensive drought in the western Pacific. Singapore experienced the driest February in 2014 since records began in 1869, with only 6.3&nbsp;mm of rain falling in the month and temperatures hitting as high as 35&nbsp;°C on 26 February. The years 1968 and 2005 had the next driest Februaries, when 8.4&nbsp;mm of rain fell.<ref>{{cite web|url=http://www.channelnewsasia.com/stories/singaporelocalnews/view/1040778/1/.html|publisher=Channel NewsAsia|title=February 2010 is driest month for S'pore since records began in 1869|first1=Joanne|last1=Chan|date=3 March 2010|access-date=5 November 2017|archive-url=https://web.archive.org/web/20100303222328/http://www.channelnewsasia.com/stories/singaporelocalnews/view/1040778/1/.html|archive-date=3 March 2010}}</ref>

=== Climate change{{anchor|Climatic_changes}} ===
{{See also|Effects of climate change on the water cycle|Effects of climate change on agriculture}}
[[File:Global warming and extreme weather.svg|thumb|upright=1.35| There will likely be multiplicative increases in the frequency of [[extreme weather]] events compared to the pre-industrial era for [[heat wave]]s, droughts and heavy precipitation events, for various climate change scenarios.<ref name="IPCC6AR_ExtremeEvents">{{cite web|title=Climate Change 2021 / The Physical Science Basis / Working Group I contribution to the WGI Sixth Assessment Report of the Intergovernmental Panel on Climate Change / Summary for Policymakers|url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM_final.pdf|publisher=Intergovernmental Panel on Climate Change|archive-url=https://web.archive.org/web/20211104175351/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM_final.pdf|archive-date=4 November 2021|page=SPM-23|date=9 August 2021|url-status=live}} Fig. SPM.6</ref>]]

Globally, the occurrence of droughts has increased as a result of the [[Instrumental temperature record|increase in temperature]] and atmospheric [[Potential evapotranspiration|evaporative demand]]. In addition, increased [[Climate variability and change|climate variability]] has increased the frequency and severity of drought events. Moreover, the occurrence and impact of droughts are aggravated by anthropogenic activities such as land use change and water management and demand.<ref name=":3" />

The [[IPCC Sixth Assessment Report]] also pointed out that "Warming over land drives an increase in atmospheric evaporative demand and in the severity of drought events"<ref>Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, and O.  Zolina, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter08.pdf Chapter 8: Water Cycle Changes]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I  to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1055–1210, doi:10.1017/9781009157896.010.</ref>{{rp|1057}} and "Increased atmospheric evaporative demand increases plant water stress, leading to agricultural and ecological drought".<ref>Caretta, M.A., A. Mukherji, M. Arfanuzzaman, R.A. Betts, A. Gelfan, Y. Hirabayashi, T.K. Lissner, J. Liu, E. Lopez Gunn, R. Morgan, S. Mwanga, and S. Supratid, 2022: [https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Chapter04.pdf Chapter 4: Water]. In: [https://www.ipcc.ch/report/ar6/wg2/ Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 551–712, doi:10.1017/9781009325844.006.</ref>{{rp|578}}

There is a rise of compound warm-season droughts in Europe that are concurrent with an increase in potential [[evapotranspiration]].<ref>{{Cite journal|last1=Markonis|first1=Yannis|last2=Kumar|first2=Rohini|last3=Hanel|first3=Martin|last4=Rakovec|first4=Oldrich|last5=Máca|first5=Petr|last6=AghaKouchak|first6=Amir|year=2021|title=The rise of compound warm-season droughts in Europe|journal=Science Advances|volume=7|issue=6|pages=eabb9668|bibcode=2021SciA....7.9668M|doi=10.1126/sciadv.abb9668|issn=2375-2548|pmc=7857689|pmid=33536204}}</ref>

{{excerpt|Effects of climate change#Droughts}}

=== Erosion and human activities ===
{{See also|Aeolian processes}}

Human activity can directly trigger exacerbating factors such as over-farming, excessive [[irrigation]],<ref name="Galilee">{{cite news|title=A biblical tragedy as Sea of Galilee faces drought|newspaper=Belfast Telegraph|url=http://www.belfasttelegraph.co.uk/news/world-news/a-biblical-tragedy-as-sea-of-galilee-faces-drought-13956116.html}}</ref> [[deforestation]], and [[erosion]] adversely impact the ability of the land to capture and hold water.<ref>{{cite web|title=Kenya: Deforestation exacerbates droughts, floods|url=http://forests.org/shared/reader/welcome.aspx?linkid=63511&keybold=climate%20drought%20water%20catchment|url-status=dead|archive-url=https://web.archive.org/web/20110927024422/http://forests.org/shared/reader/welcome.aspx?linkid=63511&keybold=climate%20drought%20water%20catchment|archive-date=2011-09-27|access-date=2008-05-24|work=forests.org}}</ref> In arid climates, the main source of erosion is wind.<ref name="Erosion">{{cite web|author1=Vern Hofman|author2=Dave Franzen|year=1997|title=Emergency Tillage to Control Wind Erosion|url=http://www.ag.ndsu.edu/disaster/drought/emergencytillagetocontrolerosion.html|access-date=2009-03-21|publisher=[[North Dakota State University]] Extension Service|archive-date=2012-04-17|archive-url=https://web.archive.org/web/20120417232250/http://www.ag.ndsu.edu/disaster/drought/emergencytillagetocontrolerosion.html|url-status=dead}}</ref> Erosion can be the result of material movement by the wind. The wind can cause small particles to be lifted and therefore moved to another region (deflation). Suspended particles within the wind may impact on solid objects causing erosion by abrasion (ecological succession). Wind erosion generally occurs in areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation.<ref>{{cite web|author=United States Geological Survey|author-link=United States Geological Survey|year=2004|title=Dunes&nbsp;– Getting Started|url=http://geomaps.wr.usgs.gov/parks/coast/dunes/index.html|url-status=dead|archive-url=https://web.archive.org/web/20120427204448/http://geomaps.wr.usgs.gov/parks/coast/dunes/index.html|archive-date=2012-04-27|access-date=2009-03-21}}</ref>