The maximum size for a raindrop is limited by drop breakup because when the drop becomes too large, air friction will break it up into a bunch of smaller droplets. The smaller the droplet, the higher the surface tension necessary. The minimum size for a cloud droplet is effectively set by the surface tension required to keep the H 2O molecules together. Liquid drops exist on a size spectrum from about 1 μm to almost 5,000 μm (or 0.5 cm). Notice how cloud droplet sizes range from 2 μm to 50 μm and raindrop sizes range from 200 μm to 2500 μm. radius “R”, given in microns, and drop volume (mm 3) on a log scale ( CC BY-NC-SA). The following image shows the volume of various cloud droplets and rain drops on a log scale. When considering the volume of the droplets or particles, this differences quickly grows. Comparison of raindrop, cloud droplet, and condensation nucleus sizes, given as diameter in mm (Image Created by Britt Seifert). The average raindrop has a diameter of 2 mm, and the average condensation nucleus has a diameter around 0.0002 mm. The following image gives a sense of the difference in scale between raindrops (left), cloud droplets (center), and cloud condensation nuclei (right). In cloud microphysics, microns are the standard scale of measure.
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Pro Tip: 1 micron (μm) is the same as one-millionth of a meter (1*10 -6 m). This diameter is about 100 times smaller than your average raindrop.
The average cloud droplet is very small with an average diameter of about 20 micrometers (μm), which is the same as 20*10 -6 m, 0.002 cm, or 0.02 mm. We will explore why this is by examining cloud droplets and raindrops in more detail. This leads us to question: why does it rain and do raindrop sizes vary? What is the relationship between raindrops and cloud droplets, and by what processes do each form? You know that clouds form by condensation but, apparently, condensation by itself is a necessary but insufficient condition for rain. Many times, clouds cover the skies but never produce any precipitation at all. Sometimes rain feels like a gentle mist but at other times its a heavy downpour that floods streets and sidewalks. Raindrops falling on a body of water ( CC 0). Describe the Ice Phase (Wegener-Bergeron-Findeisen) process.Describe the Collision-Coalescence process.Calculate the speed of a falling cloud droplet and raindrop.Recall the importance of cloud condensation nuclei and aerosols.By the end of this chapter, you should be able to:
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