High up in the atmosphere when ρ air is small, the speed of a falling raindrop, w T, rain, will be faster than near the surface when ρ air is similar magnitude to ρ 0. Again, “r” is the radius of the drop, given in meters. Where ρ 0 is a reference value of density, typically 1.2 kg m -3 and ρ air is the density of air where the raindrop exists. Raindropsįor raindrops, a different equation is used to approximate the fall speed. Terminal velocity implies that a steady state has been reached in the fall velocity with a balance between the downward gravitational force on the drops and the upward frictional drag of air. The friction provided by the air or even tiny upward air currents will keep cloud droplets suspended for long periods. A simple calculation will show that it takes hours, if not days, for a cloud droplet to fall from even low altitudes. Where “r” needs to be expressed in meters. The terminal velocity of a falling cloud droplet (with radius “r” less than 40 μm) is given by the following equation from Stoke’s Drag Law: At some later time, a cloud of many small cloud droplets, far too small to fall at any significant speed, will form. As the air nears saturation, condensation will begin to occur on the largest and most hygroscopic CCN. If the air were to be lifted by a mountain or in a rising thermal, it would cool, and the relative humidity would increase. Imagine there are many CCNs of differing sizes in a body of humid, but unsaturated air. For example, when condensation occurs on salt particles, which are extremely hygroscopic, condensation can begin at 80% relative humidity or lower. Many CCNs are hygroscopic, meaning they tend to absorb moisture, so condensation may start before the relative humidity reaches 100%. Recall from the previous chapter on clouds that cloud condensation nuclei (CCNs) are required for water vapor to condense onto. While all drops will fall, the larger the drops are, the faster they fall. On a continuous spectrum of sizes, at some point the gravitational pull on water drops in the atmosphere becomes large enough not to ignore. In general, the only difference between a cloud droplet and a raindrop is that a raindrop has a non-negligible fall velocity. 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.
#Calculate the velocity of a spherical raindrop pro#
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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