Editing Cumulus cloud (section)

== Formation ==
[[File:Bubbles in the Sky.ogv|thumb|right|Cumulus clouds forming over the [[Congo River basin]]]]
Cumulus clouds form via [[atmospheric convection]] as air warmed by the surface begins to rise. As the air rises, the temperature drops (following the [[lapse rate]]), causing the [[relative humidity]] (RH) to rise. If convection reaches a certain level the RH reaches one hundred percent, and the "wet-adiabatic" phase begins. At this point a positive feedback ensues: since the RH is above 100%, water vapor condenses, releasing [[latent heat]], warming the air and spurring further convection.

In this phase, water vapor condenses on various nuclei present in the air, forming the cumulus cloud. This creates the characteristic flat-bottomed puffy shape associated with cumulus clouds.<ref name="USA Today">{{cite news|url=http://usatoday30.usatoday.com/weather/wcumulus.htm|work=Weather|title=Cumulus clouds|access-date=16 October 2012|date=16 October 2005|archive-date=28 June 2017|archive-url=https://web.archive.org/web/20170628202558/http://usatoday30.usatoday.com/weather/wcumulus.htm|url-status=dead}}</ref><ref name="Stommel91">{{harvnb|Stommel|1947|p=91}}</ref> The height of the cloud (from its bottom to its top) depends on the temperature profile of the atmosphere and of the presence of any [[Inversion (meteorology)|inversions]].<ref name="Mossop632-634">{{harvnb|Mossop|Hallett|1974|pp=632&ndash;634}}</ref> During the convection, surrounding air is [[Entrainment (meteorology)|entrained]] (mixed) with the [[thermal]] and the total mass of the ascending air increases.<ref name="Langmuir175" />
Rain forms in a cumulus cloud via a [[Cloud physics|process]] involving two non-discrete stages. The first stage occurs after the droplets coalesce onto the various nuclei. Langmuir writes that surface tension in the water droplets provides a slightly higher pressure on the droplet, raising the [[vapor pressure]] by a small amount. The increased pressure results in those droplets evaporating and the resulting water vapor condensing on the larger droplets. Due to the extremely small size of the evaporating water droplets, this process becomes largely meaningless after the larger droplets have grown to around 20&nbsp;to 30&nbsp;[[micrometre]]s, and the second stage takes over.<ref name="Langmuir175">{{harvnb|Langmuir|1948|p=175}}</ref> In the accretion phase, the raindrop begins to fall, and other droplets collide and combine with it to increase the size of the raindrop. Langmuir was able to develop a formula<ref name="formula" group="note">The formula was <math>t={18\eta \over Egwr_0}</math>, with <math>t</math> being the time to infinite radius, <math>\eta</math> being the viscosity of air, <math>E</math> being the fractional percentage of water droplets accreted per unit volume of air that the drop falls through, <math>w</math> being the concentration of water in the cloud in grams per cubic metre, and <math>r_0</math> being the initial radius of the droplet.</ref> which predicted that the droplet radius would grow unboundedly within a discrete time period.<ref name="Langmuir177">{{harvnb|Langmuir|1948|p=177}}</ref>