Be very, very careful what you put into that head,
because you will never, ever get it out.

Thomas Cardinal Wolsey (1471-1530)

Bad
Rain
FAQ

Click on the symbol for its explanation.


 

This FAQ (Frequently Asked Questions) is written by Alistair B. Fraser. It is in response to questions posed over the years by readers of the Bad Meteorology pages. If you have arived on this page without having read those pages or the other Bad Science pages, then what follows, will probably make little sense.

Although the questions presented here are often ones asked by a specific person, each is chosen to characterize a group of similar questions which have been ask about the topic.

Issues discussed below

Terminal velocities
Artistic representation

 

Questions arising out of Bad Rain:

Terminal velocities

Question:
Reading your explanation as to the nature of raindrops I could not ignore the following sentence: "With increasing size, the fall velocity increases..." I would like to point out that according to Neutonian physics size and mass do not affect the speed of a falling object.

Answer:
I am always fascinated when someone sends me a message like this (sometimes Galileo is credited, sometime Newton, sometime others), because it represents a triumph of poorly understood theory taking precedence over observations.

Forget theory for a moment. Take observations. The fact of the matter is that big drops fall faster than small ones! This is verified by every observation (and there are thousands of them) that have ever been made on raindrops. Indeed, it is a simple thing to check yourself the next time there is drizzle. Then compare their fall speeds with those of rain. Sigh..., that is just the way it is, independent of the way you apply (actually misapply Newtonian physics).

But, how are the observations reconciled with the theory? Fairly easily actually. There are (at least) two forces involved (not one) and the behavior of a drop depends upon both. After all, Newton's law says that a NET force results in a change of momentum (F = ma is just a special case).

The force of gravity is proportional to the mass of the drop and thus proportional to about radius cubed. The drag on the drop (it is falling through air after all) is approximately proportional to the surface area and thus radius squared. Thus as the radius increases the significance of gravity increases relative to drag. The result is that the terminal velocity of the drop increases with drop size.

Newtonian physics does not say that size and mass have no influence on the speed of a falling object (as you claim) unless, the object is falling through a vacuum so that the only force on it is from gravity.

Rain does not fall through a vacuum.


Artistic representation

Question:
I didn't know that the shapes of falling raindrops are so different from that, we see on the weather-maps. But frankly, I think we should let the symbols stay as they are (a raindrop like a parachute or like a hamburger would lead to confusion).

Answer:
I have no more problem with the representation of a raindrop as a teardrop in art than I do with the presentation of the works of Picasso or El Greco (with their anatomically incorrect people). Yet, I would make a distinction between the scientific rendering of things and the artistic rendering. Would it not bother you if the works of Picasso were used as a serious representation of human anatomy in the teaching of surgeons in an a medical school?

In an supposedly scientific presentation such as a weather forecast in which the presenter apparently wishes us to believe in his understanding of the behavior of the natural world, I have considerable difficulty in believing he strengthens his claim to this understanding by presenting the world as it is not.


 


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written by Alistair B. Fraser
of the Department of Meteorology.
in the College of Earth and Mineral Sciences.
of the Pennsylvania State University

Alistair B. Fraser | abf1@psu.edu