What is the main difference between dew and rain?
Would it be correct if we say that rain is done in the upper troposphere while dew done in the lower troposphere? Or there are better definitions for the main difference between them?
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1$\begingroup$ More like subtle differences. Such as rain drops requiring CCN etc to form. There is also possibility of rain forming from ice melt or supercooled water and in the case of dew it is water vapor condensing into liquid water. So different phase transitions. $\endgroup$– user1066Jul 3, 2019 at 2:28
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4 Answers
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Dew is condensate that forms on the surface, usually by the surface cooling to a temperature below the dew point of the air. Thus the water vapr accumulates onto the surface.
Rain is formed by Collision-Coalescence or the Bergeron-Findelsen process. The cloud droplets are formed by the condensation of water vapor on aerosols, when the air reaches supersaturation via the Kelvin Effect and Raoult Effect (which makes up Kohler Theory).
answered Jul 3, 2019 at 15:13
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$\begingroup$ @BaroclinicCplusplus Is there any merit to the term dew point lapse rate? $\endgroup$– user1066Jul 5, 2019 at 1:42
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$\begingroup$ @gansub I don't think so, since the dewpoint is pressure dependent on moisture and pressure. A potential analog to the dry adiabatic lapse rate for moisture would be a mixing line (think about how you find the LCL on a skew-t). I don't know how physically useful a dewpoint lapse rate would be. A quick Google search lead me here: theweatherprediction.com/habyhints/160 $\endgroup$ Jul 5, 2019 at 3:23
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Both are condensation, but dew forms directly on a solid surface, rain forms in the air. In theory you could form dew in any part of the atmosphere if you had something for it to form on.
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$\begingroup$ So, in fact it's a correct definition what I suggested? $\endgroup$ Jul 3, 2019 at 4:34
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$\begingroup$ No, it isn't. Rain may form in clouds only 100 m above sea level, but it forms around aerosols. Dew on the other hand is water vapour directly deposited on surfaces. $\endgroup$– ErikJul 3, 2019 at 7:50
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Somebody posted that rain is either formed via collision-coalescence or the Bergeron-Findelsen process. This is misleading. The simplest classification of rain processes is into "warm-rain" and "cold rain" processes, where the former involves creation of rain by vapor and liquid phases, and the latter involves creation of rain by vapor, liquid, and solid phases of water. In the former, rain drops can form via "autoconversion" (vapor deposition onto liquid drops until they reach the size classified as rain), or collision-coalescence. In cold rain processes, rain can also be formed by melting ice particles, typically as they descend below the environmental 0 degree C level.
The Bergeron-Findelsen process is a process by which ice particles grow via vapor deposition in the presence of supercooled liquid water. Because of a difference in saturation vapor pressure over ice vs. over water, in these conditions ice will grow via vapor deposition at the expense of the water droplets, which decrease in size via evaporation. Ice particles can also grow via other processes such as aggregation (ice particles sticking to other ice particles), riming (liquid drops sticking to ice particles), or plain-old vapor deposition (i.e. not in the presence of supercooled liquid droplets). Ice particles (e.g. snow, hail, graupel) grown via any of these processes can then sediment and melt, becoming rain. While the Bergeron-Findelsen process is an important cold rain process, it is not the only one, and primarily acts to grow ice (that might later melt to become rain).
Hope this helps.
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Dew refers to direct condensation of water vapor to water without hygroscopic nuclei on surface objects. It's caused by condensation of moist air caused by radiation and contact cooling of moist air lying over it.
However, hygroscopic nuclei play an important role in case of rainfall as they play a crucial role in the formation of cloud droplets. For rainfall, the moist air must ascend, saturate and condense. Adiabatic cooling due to an upward movement of air is one of the most important mechanisms of condensation and related precipitation including rainfall.
