Editing Cloud physics (section)

=====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>