Editing Cloud (section)

==Effects==
{{see also|Cloud cover|Cloud feedback|Global dimming|Climate change}}
[[File:Cumulus_clouds_panorama.jpg|alt=|thumb|400x400px|Cumuliform cloudscape over [[Swifts Creek]], Australia]]
Tropospheric clouds exert numerous influences on Earth's troposphere and climate. First and foremost, they are the source of precipitation, thereby greatly influencing the distribution and amount of precipitation. Because of their differential buoyancy relative to surrounding cloud-free air, clouds can be associated with vertical motions of the air that may be convective, frontal, or cyclonic. The motion is upward if the clouds are less dense because condensation of water vapor releases heat, warming the air and thereby decreasing its density. This can lead to downward motion because lifting of the air results in cooling that increases its density. All of these effects are subtly dependent on the vertical temperature and moisture structure of the atmosphere and result in major redistribution of heat that affect the Earth's climate.<ref name="cloud-heating" />

The complexity and diversity of clouds in the troposphere is a major reason for difficulty in quantifying the effects of clouds on climate and climate change. On the one hand, white cloud tops promote cooling of Earth's surface by reflecting shortwave radiation (visible and near infrared) from the Sun, diminishing the amount of solar radiation that is absorbed at the surface, enhancing the Earth's [[albedo]]. Most of the sunlight that reaches the ground is absorbed, warming the surface, which emits radiation upward at longer, infrared wavelengths. At these wavelengths, however, water in the clouds acts as an efficient absorber. The water reacts by radiating, also in the infrared, both upward and downward, and the downward longwave radiation results in increased warming at the surface. This is analogous to the [[greenhouse effect]] of [[greenhouse gases]] and [[water vapor]].<ref name="cloud-heating">{{Cite web |title=Cloud Climatology |url=http://isccp.giss.nasa.gov/role.html |access-date=12 July 2011 |website=International Satellite Cloud Climatology Program |publisher=National Aeronautics and Space Administration}}</ref>

High-level genus-types particularly show this duality with both short-wave albedo cooling and long-wave greenhouse warming effects. On the whole, ''ice-crystal'' clouds in the upper troposphere (cirrus) tend to favor net warming.<ref name="Clouds and the greenhouse effect" /><ref name="Nucleation">{{Cite journal |last=Franks |first=F. |year=2003 |title=Nucleation of ice and its management in ecosystems |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=361 |issue=1804 |pages=557–74 |bibcode=2003RSPTA.361..557F |doi=10.1098/rsta.2002.1141 |pmid=12662454 |s2cid=25606767}}</ref> However, the cooling effect is dominant with mid-level and low clouds, especially when they form in extensive sheets.<ref name="Clouds and the greenhouse effect" /> Measurements by NASA indicate that, on the whole, the effects of low and mid-level clouds that tend to promote cooling outweigh the warming effects of high layers and the variable outcomes associated with vertically developed clouds.<ref name="Clouds and the greenhouse effect">[[#Ackerman|Ackerman]], p. 124</ref>

As difficult as it is to evaluate the influences of current clouds on current climate, it is even more problematic to predict changes in cloud patterns and properties in a future, warmer climate, and the resultant cloud influences on future climate. In a warmer climate, more water would enter the atmosphere by evaporation at the surface; as clouds are formed from water vapor, cloudiness would be expected to increase. But in a warmer climate, higher temperatures would tend to evaporate clouds.<ref>{{Cite web |last=Wolchover |first=Natalie |date=25 February 2019 |title=A World Without Clouds |url=https://www.quantamagazine.org/cloud-loss-could-add-8-degrees-to-global-warming-20190225/ |website=Quanta Magazine}}</ref> Both of these statements are considered accurate, and both phenomena, known as cloud feedbacks, are found in climate model calculations. Broadly speaking, if clouds, especially low clouds, increase in a warmer climate, the resultant cooling effect leads to a negative feedback in climate response to increased greenhouse gases. But if low clouds decrease, or if high clouds increase, the feedback is positive. Differing amounts of these feedbacks are the principal reason for differences in [[Climate sensitivity|climate sensitivities]] of current global climate models. As a consequence, much research has focused on the response of low and vertical clouds to a changing climate. Leading global models produce quite different results, however, with some showing increasing low clouds and others showing decreases.<ref>{{Cite journal |last=Bony |first=S. |year=2005 |title=Marine boundary layer clouds at the heart of tropical cloud feedback uncertainties in climate models |journal=Geophysical Research Letters |volume=32 |issue=20 |pages=L20806 |bibcode=2005GeoRL..3220806B |doi=10.1029/2005GL023851 |doi-access=free}}</ref><ref>{{Cite journal |last1=Medeiros |first1=B. |last2=Stevens |first2=B. |last3=Held |first3=I. M. |last4=Zhao |first4=M. |last5=Williamson |first5=D. L. |last6=Olson |first6=J. G. |last7=Bretherton |first7=C. S. |year=2008 |title=Aquaplanets, Climate Sensitivity, and Low Clouds |journal=Journal of Climate |volume=21 |issue=19 |pages=4974–4991 |bibcode=2008JCli...21.4974M |citeseerx=10.1.1.620.6314 |doi=10.1175/2008JCLI1995.1}}</ref> For these reasons the role of tropospheric clouds in regulating [[weather]] and [[climate]] remains a leading source of uncertainty in [[global warming]] projections.<ref>{{Cite book |last1=Forster |first1=P. |title={{Harvnb|IPCC AR6 WG1|2021}} |last2=Storelvmo |first2=T. |last3=Armour |first3=K. |last4=Collins |first4=W. |year=2021 |chapter=Chapter 7: The Earth's energy budget, climate feedbacks, and climate sensitivity |ref={{harvid|IPCC AR6 WG1 Ch7|2021}} |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf}} From pp 1022: "The cloud feedback could either amplify or offset some of the future warming and has long been the biggest source of uncertainty in climate projections."</ref><ref>{{Cite web |title=Will Clouds Speed or Slow Global Warming? |url=https://www.nsf.gov/news/special_reports/clouds/question.jsp |url-status=dead |archive-url=https://archive.today/20111114194612/https://www.nsf.gov/news/special_reports/clouds/question.jsp |archive-date=14 November 2011 |access-date=23 October 2012 |publisher=National Science Foundation}}</ref>