Saturday, November 5, 2011

The Great Raindrop Deception

You have been deceived...by magazines, newspapers, the digital media, by artists, school teachers, and yes, even children's books.  And it will stop here.

When raindrops are drawn, they almost always look the same way, as a teardrop.  Some samples:




The problems is that raindrops virtually NEVER look like this!  Northwesterns like us, with our persistent wet weather, should be expected to be fully educated in raindrop-ology, and thus this blog may serve as an important public service.  Perhaps the Mayor will establish a raindrop education day this year.

Well, if raindrops never look like teardrops, what DO they look like?  Well, it turns out that their shape depends on their size, a situation also obvious in humans.  Small droplets are spherical, bigger ones like pancakes, and very large ones like parachutes.   His is a graphical rendition:

Drizzle droplets (a Northwest favorite), with sizes of roughly  .3 to .5 millimeter (mm), up to small raindrops, of approximately 1-1.5 mm, look pretty spherical. (For the metrically challenged... 1 mm is about .04 inches)

Why spherical?  Surface tension--the attraction of water molecules to each other--pull the molecules together into a ball.  Furthermore, small droplets don't weight that much and their fall velocity (also known as the terminal velocity) is relatively small, so the disturbing influence of the air moving around the droplets is minimal.  I bet you have all noticed the difference in fall velocity of various size raindrops:  drizzle drops seem to slowly drift down while big rains come down REAL fast.

For bigger droplets (2-3 mm), their weight causes them to fall faster, and the greater speed of the air passing around them causes them to be distorted into a pancake shape, with a nice little dimple in the middle.

For really big droplets, the distortion caused by the fast moving air is huge...the droplets get hollowed out into a parachute-like shape.

There is a limit to how big a raindrop can get because eventually the aerodynamics forces of the passing air tears it apart.   What is the biggest raindrop ever observed?   Well my University of Washington colleagues (Dr. Peter Hobbs and Art Rangno) may have seen the largest---8.8 mm (.35 inches!!)--in tropical cumulus.   Imagine being hit by such a giant raindrops!

One final thing--our newly upgraded weather radars, with their fancy dual-polarization, have the capability to measure the shape of precipitation, including raindrops. Here is an example--the zdr product (stands for differential reflectivity).   Zdr measures the difference in return between an up-down and side to side view of precipitation, thus giving information on the shape.  When the returns in both directions are roughly the same (the difference between them being nearly zero), this product gives a value near zero. If the precipitation gets distorted, the number increases.  Pretty neat stuff--measuring raindrop shape remotely.  Looking at the image below, you can see a lot of near zero (blue) colors....not many teardrops here! The area of large values (white/gray colors) near Hoquiam is not meteorological--its ground clutter, where the radar beam is hitting the surface.
Zdr from the new NWS Langley Hill Doppler Radar

Finally, for those voters in Seattle, please keep in mind the Seattle School Board race and necessity to replace the current board members with those committed to better math education in our schools.  The math textbooks used in Seattle are terrible at all levels and I see the impact on Seattle students coming to the UW--many of them have very weak math skills.  We can do much better, this requires school board members that are willing to deal with the issue--the incumbents are not.  The challengers (Peaslee, McLaren, Martin, Buetow) are all committed to dealing with the problem. The latest school board poll is here.

16 comments:

Westside guy said...

One point you missed, Cliff - with humans, the trend is the opposite of raindrops. As we get bigger - unlike raindrops - we tend to become more spherical!

Cliff Mass Weather Blog said...

Westside...I was trying to be sensitive and PC about that issue...so I thought it was more tactful to say no more....

Christina Wilsdon said...

Another point, Cliff! Raindrops don't have faces! What an obvious flaw to have missed ;-)

strix27 said...

Cliff, I'm having a problem with these statements:

"Furthermore, small droplets don't weight that much and their fall velocity (also known as the terminal velocity) is relatively small, so the disturbing influence of the air moving around the droplets is minimal. . . ..

For bigger droplets (2-3 mm), their weight causes them to fall faster, . . . "

The acceleration of gravity is independent of
mass so it can't be weight that causes the bigger drops to fall faster. I would think increased diameter would increase drag due to increased surface area. So what gives?

Eric said...

Cliff for shame! All objects regardless of their mass fall (for all practical purposes) at the same rate.

The difference in speed of different size drops is due the drag of the fluid they are falling in (air), not their mass.

Cliff Mass Weather Blog said...

Strix et al,
I didn't want to get into a discussion of the force balance associated with terminal velocity, which as you suggest, is between the weight of the object and drag. Heavier objects need more drag to balance the weight and drag is proportional to the speed relative to the air...thus, heavier objects require a faster velocity to get a balance...cliff

MimiTabby said...

any child who has looked out a window at the raindrops dripping down can tell you that they do kind of look like that when they are adhering to the glass and moving.

TreeTimer said...

Your own diagram of the "parachute" shaped raindrop contradicts your whole premise. Each end of it looks just like the iconic storybook raindrop. Is this a cross section? Is it more like a jellyfish? Or is there a series of such pointy-top blobs around the rim of the 'chute?

Cliff Mass Weather Blog said...

TreeTimer....jellyfish or parachute..whatever you like! What is shown is a cross section....cliff

Charlie Phillips said...

haha, great post!

Mattster said...

Goodness gracious guys, give Dr. Mass a break! In keeping a post short and digestable, you don't always include every last point. Plus, if he wants to describe the shape as he sees it, let him. It's his blog after all.

On the weather front, wow, October felt like October, but so far November is about as ho-hum as you can get.

Lorac said...

thank you for stating the size in mm. keep it up and I hope to see temperatures in C soon. :)

Scrapycandy said...

I thought they were nature's 'tears'. Now they are nature's 'blobs'. No longer poetic. I think I like being ignorant.

sstelepro said...

In a perfect vacuum all objects of whatever size or shape will fall at the same rate. But our atmosphere is not a perfect vacuum and the fall rate is greatly influenced by friction. (a non technical term) or "wind resistance" another non technical term. That's why a feather doesn't fall at the same speed as a golf ball. And that is why a drop of dizzle does not fall as fast as a hail stone or a monster rain drop

Ansel said...

Concerning droplet shape, I want to add one minor point, however. There is a grain (or a drop!) of truth to the teardrop concept: High speed photography will show that a droplet, hanging on the underside of a solid surface, growing though accumulation of the water, does have a teardrop shape an instant after it breaks loose fron said surface. As the weight of the drop approaches that needed to break the surface tension holding it up, it is stretched out. When the drop first breaks free it will have a teardrop shape-for a few milliseconds- until surface tension makes it spherical.

James Henry said...

Very interesting! thank you for sharing This?
Rain drops are very interesting just like snow flakes!