7.1: Introduction
- Page ID
- 46235
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Raindrops falling on a body of water (CC 0).
Sometimes rain feels like a gentle mist but at other times its a heavy downpour that floods streets and sidewalks. Many times, clouds cover the skies but never produce any precipitation at all. 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. We will explore why this is by examining cloud droplets and raindrops in more detail.
The average cloud droplet is very small with an average diameter of about 20 micrometers (μm), which is the same as \(20 \times 10^{-6} m\), 0.002 cm, or 0.02 mm. This diameter is about 100 times smaller than your average raindrop.
Pro Tip: 1 micron (\(\mu m\)) is the same as one-millionth of a meter (\(1 \times 10^{-6} m\)). In cloud microphysics, microns are the standard scale of measure.
The following image gives a sense of the difference in scale between raindrops (left), cloud droplets (center), and cloud condensation nuclei (right). The average raindrop has a diameter of 2 mm, and the average condensation nucleus has a diameter around 0.0002 mm.
When considering the volume of the droplets or particles, this differences quickly grows. The following image shows the volume of various cloud droplets and rain drops on a log scale.
Notice how cloud droplet sizes range from 2 \(\mu m\) to 50 \(\mu m\) and raindrop sizes range from 200 \(\mu m\) to 2500 \(\mu m\). Liquid drops exist on a size spectrum from about 1 \(\mu m\) to almost 5,000 \(\mu m\)(or 0.5 cm). The minimum size for a cloud droplet is effectively set by the surface tension required to keep the \(\ce{H2O}\) molecules together. The smaller the droplet, the higher the surface tension necessary. 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.
In general, the only difference between a cloud droplet and a raindrop is that a raindrop has a non-negligible fall velocity. On a continuous spectrum of sizes, at some point the gravitational pull on water drops in the atmosphere becomes large enough not to ignore. While all drops will fall, the larger the drops are, the faster they fall.