Wednesday, March 29, 2017

How Fast are Raindrops?

What do we know, and not know, about rain? For one thing, raindrops do not taper to a pointed end at the top. That image is only an artistic means of conveying downward direction. Small water drops are round. This is due to surface tension - round being the smallest possible surface area for any given volume.

Raindrops lose round shape with increasing size. 
Dotted lines are circles; solid lines are actual shapes. 
As water droplets float about in the air they bump into each other and merge. Once a drop reaches a diameter of 0.02 inches it starts to fall as a round raindrop. However, above a certain size, air resistance causes the bottom to flatten. The drops are no longer round. More speed, more assimilated droplets, faster, more resistance from air and these large raindrops become concave on the bottom. And then, large drops fragment into several smaller drops, which revert to being rounder and slower.

The larger the raindrop, the faster it falls. Newtonian physics is not being circumvented here. Rather, as drops become larger their mass increases faster than the friction of falling through air. Only in a vacuum would a large and small raindrop fall at the same speed. As noted above, drops get only so large before fragmenting into smaller, slower-falling drops. As a consequence, the maximum velocity of a large raindrop approaches twenty miles an hour. Very small droplets may have little or no downward movement, i.e., drizzle and fog. There are exceptions to the twenty mile an hour maximum speed: at higher altitudes the air density is lower, so rainfall is faster, and also when the air itself is moving down, as in a thunderstorm downdraft.   
Real raindrops do not look like this

Rain can feel like it is falling faster without exceeding the speed limit. What we feel is an average of small to large drops. When a lot of rain is falling in a very short period of time - say two inches an hour - all the pieces of fragmenting large drops quickly combine with other fragments. At ground level, majority of raindrops are large and fast. Once the storm's rainfall slows there is less recombination.

Light rain is defined as a rate of about 0.1 inches per hour, moderate as 0.1 to 0.4 inches per hour, heavy as up to 2.0 inches per hour, and violent rain as exceeding 2.0 inches per hour. Severe thunderstorms can exceed a rate of 4.0 inches per hour for short periods of time. The known U.S. record is 12 inches of rain in 42 minutes, falling on Holt, Mississippi, on June 22, 1947.

Virga is a term used to describe rain that starts to fall from a cloud but evaporates before it reaches the ground. From a distance this phenomenon appears as dark, straight or slanted streaks extending below the base of a cloud. When looking at a televised weather radar map this can explain why an area is shown with rain while for the people on the ground nothing is happening.
  
Sleet happens in winter, when rain from warmer clouds aloft falls through colder air near the surface. Frozen raindrops reach the ground as ice pellets. If the cold air layer is thin, then freezing will be delayed until the raindrops actually reaches the ground, resulting in an ice storm. In this uncommon meteorological condition raindrops are supercooled to a temperature below the freezing point but do not actually freeze until impact with cold surfaces, such as car windshields and tree branches. The result is a coating of clear ice.

Hailstones (internet download). In a severe hailstorm the ground
can become covered inches deep. Click on photos to enlarge.
Hail is what happens when rain goes very, very, very bad. A frozen raindrop, caught in a thunderstorm's strong updraft, can spend many minutes traveling upward, accumulating layers of ice. The hailstones that finally breaks loose from the updraft and fall to the ground can range in size from a pea to a golf ball, and in rare instances, much larger. Large hailstones can exceed the weight of a baseball and impact at more than 80 miles per hour. U.S. farm and property damage exceeds one billion dollars a year.

In the book Isaac's Storm the author retells an observation of a Texas hailstorm with unusual consequences. Heavy rain and large amounts of hail combined into an ice laden flash flood. Fifty miles downstream the ice had melted but the fast-rising river was icy cold. Water spread across the river's flood plain, carrying cold-stunned fish with it. People were able to walk out into the shallows and pick up the cold fish.  

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