Editing Cloud physics (section)

====Adiabatic cooling====
{{See also|Adiabatic process}}

As water evaporates from an area of Earth's surface, the air over that area becomes moist.  Moist air is lighter than the surrounding dry air, creating an unstable situation.  When enough moist air has accumulated, all the moist air rises as a single packet, without mixing with the surrounding air.  As more moist air forms along the surface, the process repeats, resulting in a series of discrete packets of moist air rising to form clouds.<ref>{{cite journal 
|title=Why do clouds always appear to form in distinct clumps? Why isn't there a uniform fog of condensation, especially on windy days when one would expect mixing? |date=August 4, 1997
|author=Harvey Wichman |journal= [[Scientific American]] 
|url=http://www.scientificamerican.com/article/why-do-clouds-always-appe/ |access-date=2016-03-19
}}</ref>

This process occurs when one or more of three possible lifting agents—cyclonic/frontal, convective, or [[Orography|orographic]]—causes air containing invisible [[water vapor]] to rise and cool to its [[dew point]], the [[temperature]] at which the air becomes saturated. The main mechanism behind this process is [[Adiabatic lapse rate#Dry adiabatic lapse rate|adiabatic cooling]].<ref name="adiabatic process">{{cite web |author=Nave, R. |year=2013 |url=http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/adiab.html |title=Adiabatic Process |publisher=[[Georgia State University]]|website= [[HyperPhysics]]|access-date=February 5, 2018}}</ref> [[Atmospheric pressure]] decreases with altitude, so the rising air expands in a process that expends [[energy]] and causes the air to cool, which makes water vapor condense into cloud.<ref>{{Cite web|url=http://www.ems.psu.edu/~fraser/Bad/BadClouds.html|title=Bad Clouds|website=[[Penn State College of Earth and Mineral Sciences]]|access-date=February 5, 2018|archive-url=https://web.archive.org/web/20150316122724/http://www.ems.psu.edu/~fraser/Bad/BadClouds.html|archive-date=March 16, 2015|url-status=dead}}</ref> Water vapor in saturated air is normally attracted to [[Cloud condensation nuclei|condensation nuclei]] such as dust and [[salt]] particles that are small enough to be held aloft by normal [[Atmospheric circulation|circulation]] of the air. The water droplets in a cloud have a normal radius of about 0.002&nbsp;mm (0.00008&nbsp;in). The droplets may collide to form larger droplets, which remain aloft as long as the velocity of the rising air within the cloud is equal to or greater than the terminal velocity of the droplets.<ref name="cloud drops">{{cite web | author = Horstmeyer, Steve | title = Cloud Drops, Rain Drops | year = 2008 | url = http://www.shorstmeyer.com/wxfaqs/float/float.html | access-date = 19 March 2012 }}</ref>

For non-convective cloud, the altitude at which condensation begins to happen is called the [[lifted condensation level]] (LCL), which roughly determines the height of the cloud base.  Free convective clouds generally form at the altitude of the [[convective condensation level]] (CCL). Water vapor in saturated air is normally attracted to [[Cloud condensation nuclei|condensation nuclei]] such as [[salt]] particles that are small enough to be held aloft by normal [[Atmospheric circulation|circulation]] of the air. If the condensation process occurs below the [[freezing level]] in the troposphere, the nuclei help transform the vapor into very small water droplets. Clouds that form just above the freezing level are composed mostly of supercooled liquid droplets, while those that condense out at higher altitudes where the air is much colder generally take the form of [[ice crystals]]. An absence of sufficient condensation particles at and above the condensation level causes the rising air to become supersaturated and the formation of cloud tends to be inhibited.<ref name="humidity, saturation, and stability">{{cite web |author=Elementary Meteorology Online |year=2013 |url=http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter06/ |title=Humidity, Saturation, and Stability |publisher=vsc.edu |access-date=18 November 2013 |url-status=dead |archive-url=https://web.archive.org/web/20140502055741/http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter06/ |archive-date=2 May 2014 }}</ref>

=====Frontal and cyclonic lift=====
{{see also|Extratropical cyclone|Warm front|Cold front|Precipitation}}

Frontal and [[cyclonic]] lift occur in their purest manifestations when [[Atmospheric instability|stable]] air, which has been subjected to little or no surface heating, is forced aloft at [[weather fronts]] and around centers of [[low-pressure area|low pressure]].<ref name="frontal clouds">{{cite web|url=http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/dvlp/frnt.rxml/~wintelsw/MET1010LOL/chapter06/|title=Lifting Along Frontal Boundaries|author=Elementary Meteorology Online|year=2013|publisher=Department of Atmospheric Sciences (DAS) at the [[University of Illinois at Urbana–Champaign]]|access-date=February 5, 2018}}</ref> [[Warm front]]s associated with extratropical cyclones tend to generate mostly cirriform and stratiform clouds over a wide area unless the approaching warm airmass is unstable, in which case cumulus congestus or cumulonimbus clouds will usually be embedded in the main precipitating cloud layer.<ref name="Mackerel sky">{{cite web|url=http://www.weatheronline.co.uk/reports/wxfacts/Sometimes-a-bit-fishy.htm|title=Mackerel sky|publisher=Weather Online|access-date=21 November 2013}}</ref> [[Cold front]]s are usually faster moving and generate a narrower line of clouds which are mostly stratocumuliform, cumuliform, or cumulonimbiform depending on the stability of the warm air mass just ahead of the front.<ref name="G&N:207-212">{{cite book|title=A World of Weather: Fundamentals of Meteorology: A Text / Laboratory Manual|year=2001|publisher=Kendall/Hunt Publishing Company|isbn=978-0-7872-7716-1|oclc=51160155|url=https://books.google.com/books?id=oh8lqM5obuYC&pg=PA212|author1=Lee M. Grenci |author2=Jon M. Nese |edition=3|pages=207–212}}</ref>

=====Convective lift=====
{{See also|Atmospheric convection}}

Another agent is the buoyant convective upward motion caused by significant daytime solar heating at surface level, or by relatively high absolute humidity.<ref name="humidity, saturation, and stability"/>  Incoming short-wave radiation generated by the sun is re-emitted as long-wave radiation when it reaches Earth's surface. This process warms the air closest to ground and increases air mass instability by creating a steeper temperature [[gradient]] from warm or hot at surface level to cold aloft.  This causes it to rise and cool until temperature equilibrium is achieved with the surrounding air aloft.  Moderate instability allows for the formation of cumuliform clouds of moderate size that can produce light showers if the airmass is sufficiently moist. Typical [[convection]] upcurrents may allow the droplets to grow to a radius of about {{convert|0.015|mm|in|sigfig=1}} before [[precipitation|precipitating]] as showers.<ref>{{cite journal |doi=10.1029/2011JD016457 |title=Linear relation between convective cloud drop number concentration and depth for rain initiation |journal=[[Journal of Geophysical Research: Atmospheres]] |volume=117 |issue=D2 |pages=D02207 |year=2012 |last1=Freud |first1=E |last2=Rosenfeld |first2=D |bibcode=2012JGRD..117.2207F |doi-access=free }}</ref> The equivalent diameter of these droplets is about {{convert|0.03|mm|in|sigfig=1}}.

If air near the surface becomes extremely warm and unstable, its upward motion can become quite explosive, resulting in towering cumulonimbiform clouds that can cause [[severe weather]]. As tiny water particles that make up the cloud group together to form droplets of rain, they are pulled down to earth by the force of [[gravity]].  The droplets would normally evaporate below the condensation level, but strong [[updraft]]s buffer the falling droplets, and can keep them aloft much longer than they would otherwise.  Violent updrafts can reach speeds of up to {{convert|180|mph|km/h}}.<ref>{{cite journal |author=O'Niell, Dan |title=Hail Formation |journal=Alaska Science Forum |id=328 |date=9 August 1979 |url=http://www.gi.alaska.edu/ScienceForum/ASF3/328.html |access-date=23 May 2007 |archive-url=https://web.archive.org/web/20070611100843/http://gi.alaska.edu/ScienceForum/ASF3/328.html |archive-date=11 June 2007 |url-status=dead }}</ref>  The longer the rain droplets remain aloft, the more time they have to grow into larger droplets that eventually fall as heavy showers.

Rain droplets that are carried well above the freezing level become supercooled at first then freeze into small hail. A frozen ice nucleus can pick up {{convert|0.5|in|cm}} in size traveling through one of these updrafts and can cycle through several updrafts and downdrafts before finally becoming so heavy that it falls to the ground as large hail.  Cutting a hailstone in half shows onion-like layers of ice, indicating distinct times when it passed through a layer of [[super-cooled]] water. Hailstones have been found with diameters of up to {{convert|7|in|cm}}.<ref>{{cite web |title=Largest Hailstone in U.S. History Found |year=2003 |url=http://news.nationalgeographic.com/news/2003/08/0804_030804_largesthailstone.html|archive-url=https://web.archive.org/web/20030807092143/http://news.nationalgeographic.com/news/2003/08/0804_030804_largesthailstone.html|url-status=dead|archive-date=August 7, 2003}}</ref>

Convective lift can occur in an unstable air mass well away from any fronts. However, very warm unstable air can also be present around fronts and low-pressure centers, often producing cumuliform and cumulonimbiform clouds in heavier and more active concentrations because of the combined frontal and convective lifting agents. As with non-frontal convective lift, increasing instability promotes upward vertical cloud growth and raises the potential for severe weather. On comparatively rare occasions, convective lift can be powerful enough to penetrate the tropopause and push the cloud top into the stratosphere.<ref name="Tropopause penetrations">{{cite web|author1=Long, Michael J.|author2=Hanks, Howard H.|author3=Beebe, Robert G.|title=TROPOPAUSE PENETRATIONS BY CUMULONIMBUS CLOUDS|date=June 1965|url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0621573|access-date=9 November 2014|archive-url=https://web.archive.org/web/20160303235551/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0621573|archive-date=3 March 2016|url-status=dead}}</ref>

=====Orographic lift=====
{{Main|Orographic lift}}

A third source of lift is wind circulation forcing air over a physical barrier such as a [[mountain]] ([[orographic lift]]).<ref name="humidity, saturation, and stability"/> If the air is generally stable, nothing more than lenticular cap clouds will form. However, if the air becomes sufficiently moist and unstable, orographic showers or [[thunderstorm]]s may appear.<ref name="MT">Pidwirny, M. (2006). [http://www.physicalgeography.net/fundamentals/8e.html "Cloud Formation Processes"] {{webarchive|url=https://web.archive.org/web/20081220230524/http://www.physicalgeography.net/fundamentals/8e.html |date=2008-12-20 }}, chapter 8 in ''Fundamentals of Physical Geography'', 2nd ed.</ref>
[[File:Dreamy Twilight.jpg|thumb|Windy evening [[twilight]] enhanced by the Sun's angle, can visually mimic a [[tornado]] resulting from orographic lift]]