Precipitation Formation – Part 2

2011-11-20 14:44:53.000 – Rick Giard,  Weather Observer / Education Specialist

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On Wednesday we began the theme of precipitation formation. You now appreciate the fundamental roles of water vapor content, temperature and air capacity in cloud formation and eventual precipitation. Today, we continue our discussion by delving into the conditions and processes within clouds that determine the amount and type of precipitation that will occur. Recall that visible cloud droplets are the condensed liquid form of the invisible water vapor always present in the air. These are exceedingly small, and to precipitate they must grow much larger. The two principal ways in which droplets grow are Collision-Coalescence and the Bergeron Process.

In warmer clouds containing only water vapor and liquid droplets, Collision-Coalescence is the primary process involved. This predominates in tropical regions, but at mid-latitude locations like New Hampshire it occurs mainly under very warm conditions. Within any cloud there exists a range of droplet sizes moving at different velocities. The larger droplets tend to fall faster, encountering smaller, slower droplets to coalesce with. Smaller droplets are also blown about more readily by wind than larger droplets. Growth occurs as droplets of various sizes collide.

The Bergeron Process is a cold-cloud phenomenon, and the dominant means of precipitation formation at mid-latitude locations and higher elevations. It occurs within sub-freezing clouds containing a mixture of water vapor, liquid droplets and ice crystals. The water vapor capacity of air surrounding cold ice crystals is lower than that of air near liquid droplets. Due to lower capacity, the air in close proximity to the crystals becomes supersaturated with RH slightly greater than 100%. Meanwhile, the air around the droplets remains unsaturated. Water vapor moves from the area of higher concentration (droplets) toward lower levels (crystals).

The ice crystal then grows by deposition, as the water vapor changes phase directly to solid ice without going through the liquid phase. Growth occurs at the expense of liquid droplets which shrink by evaporation. As the crystal becomes large enough it begins to fall, encountering liquid droplets that provide additional water for growth. This process is enhanced within clouds of deep vertical extent, which present prospects for greater development. Remarkably, the Bergeron Process creates exclusively frozen precipitation particles in all seasons, which will then melt into rain if temperatures are sufficiently warm at lower levels. It should also be noted that both Bergeron and Collision-Coalescence processes are usually involved.

So, the next time you head above tree line and reach for your EMS Helix Jacket to fend off an icy blast of mixed precipitation, you can thank Mr. Bergeron!

Rick Giard,  Weather Observer / Education Specialist