Precipitation Formation

Processes of Precipitation Formation

Some type of lifting mechanism is required to create rising air, which leads to cloud and precipitation formation. In the absence of a lifting mechanism, it is possible to experience light precipitation such as mist or a drizzle. Four common lifting mechanisms are (not in order of importance) convection, convergence, orographic, and frontal. Convection refers to the vertical transfer of heat energy through rising buoyant air near the surface. It is an important warm-season lifting mechanism leading to precipitation formation. Convergence refers to the lifting of air when there is an inflow of air into a region near the surface. Its spatial scale is normally larger than convection, but convergence is generally a weaker lifting mechanism than convection. Orographic lifting occurs when air is forced to ascend a mountain barrier, thus resulting in condensation and precipitation on the windward side of the mountain. Frontal lifting can occur with a variety of weather situations, but it is most common with a migrating wave cyclone where contrasting air masses are colliding.

If a lifting mechanism is in place, the next ingredient is something solid for tiny cloud droplets to attach onto. Condensation nuclei are small atmospheric particles (0.2 µm [micrometers]) with normal atmospheric concentrations ranging from 100 to 1,000 nuclei per cubic centimeter. They are composed of many different substances, with the most common being sea salt, dust (clay and silt), and gases such as sulfur and nitrogen oxide. They can be divided into two types: (1) hygroscopic, or attracted to water, such as sea salt and sulfates, and (2) hygro-phobic, or water repellant, such as soot and organic carbon. They are vital to the precipitation process because water in the form of cloud droplets (20 µm) requires a nongaseous surface to attach and transition from the gas phase to the liquid phase. Cloud condensation nuclei attract cloud droplets, and droplets then grow in size through coalescence, becoming larger cloud droplets (100 to 500 µm) and possibly raindrops (2,000 µm). The processes by which cloud droplets grow into raindrops are described in more detail in the following paragraph on precipitation formation processes.

There are two established processes by which precipitation forms: (1) collision and coalescence [Page 2288]and (2) Bergeron/Findeisin. Although these two processes have fundamental physical differences, precipitation frequently forms (especially in the midlatitudes) as a result of both processes working in unison. In the midlatitudes, precipitation may initially form as a result of the Bergeron process and then undergo collision and coalescence closer to the surface.

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