Because I am temporarily bereft of William Frey's book on crying, I am now perusing another book, Crying: The Natural and Cultural History of Tears, by Tom Lutz (Norton, 1999).
In my last posting, I promised to share my discovery of what exactly makes us cry—the metaphorical equivalent of the condensation nuclei (airborne dust, grit) water vapor condenses on to form a water droplet that may, under the right conditions, become visible to us as a cloud. I knew finding the answer or answers wasn’t going to be easy and that I would probably have to learn something about how tears are formed. I had written paper on the eye in elementary school (sadly, 5th, not 1st grade). I still have this paper and bring out occasionally to read to my family while cringing, laughing (and weeping) at my very unscientific approach to my topic.
Here is what I had to say about tears in my paper, written in my favorite blue felt-tip pen and in all caps:

“IF THE EYE IS VERY DRY OR SOME GERMS GET IN THE EYE, THE TEARS WILL FORM, BUT THEY WILL NOT APPEAR ON YOUR CHEEKS BECAUSE THERE ARE TWO CANALS BELOW THE EYE THAT CARRY THE TEARS OFF INTO THE NOSE. [LOL] THE TEARS WILL COME FROM THE TOP OF THE EYE AND RUN DOWN TO THE LOWER EYELID AND GO TOWARD THE NOSE UNTIL IT REACHES THE NOSE. IT WILL GO INTO THE CANAL AND INTO THE NOSE. IF A FOREIGN OBJECT GETS INTO THE EYE AND CAUSES PAIN OR EMOTIONAL STRESS THE TEAR GLAND WILL 'OVERFLOW' SO IT WILL PRODUCE TOO MUCH LIQUID WHICH WE CALL 'CRYING.'"

Wow.
I seem to be describing some kind of sewage system terminating at the nose.

In fact, the system in the body that produces tears, the lacrimal system, has secretory and excretory functions. That is, it produces tears and drains them away, so while the sewer metaphor is unfortunate, it is partially apt. The lacrimal system includes glands and ducts connected to a network of ganglia and nerves in the brain and spinal cord. Lutz's diagram of the lacrimal system (below, sorry about the focus) is not unlike a weather chart; the lines and direction arrows of neural pathways resembling the lines meteorologists use to indicate the direction and flow of frontal systems across the earth’s surface.

Not to force a metaphor, but dern if Lutz’s illustration of the lacrimal gland and excretory ducts (below) does not resemble a drawing, albeit a primitive one, of a cumulonimbus cloud with rain falling from its base. It even looks like it’s watering a tidy “lawn” of eyelashes!
Since I am interested in how tears are formed not drained, I will focus on the lacrimal gland and avoid the nose since, as I think you will agree, I gave it a pretty thorough treatment in my paper.
The main, cloud-like lacrimal gland is located between the frontal bone and the eyeball and, in conjuction with more than eighty much smaller lacrimal glands, produces basal tears continuously at the rate of 5-10 oz. a day. Basal tears lubricate the eye. Some of the basal tears evaporate between blinks of the eye and some is drained through the puncta—the little pink bump on the nose side of your eye where errant gnats often end up, drowned. From the puncta, the tears flow into the lacrimal sac (which looks just like a drainage canal!) and then into the nasolacrimal sac (another canal!) which empties into the…nose.

Now my mind has completely de-railed and I am thinking of re-writing Nicolai Gogol’s satirical short story, “The Nose,” as a tearjerker. Luckily I have my book of Gogol stories on my shelf downstairs; luckily, after re-reading the first few pages, I decide not to take on Yakovlevich, Kovalyov, and the evils of Russia’s upperclass symbolized by a giant snubbing nose.

Back to tears. In addition to producing 5-10 oz. of basal tears, the lacrimal glands produce exceptional quantities of emotional tears (I am hunting for estimates of exact quantity) causing the puncta to runneth over. Onto the cheeks.

Meteorologically, the runneth over part is the rain. At what point does a cloud runneth over? Several types of clouds produce rain, so my explanation here is simplified. According to my Main Rain Man, Michael Allaby, in his fabulous Encyclopedia of Weather and Climate, a cloud droplet 0.0008 inch across must increase its volume more than million times to attain the size of a raindrop 0.08 inches across.

Depending on the temperature inside the cloud, water droplets grow to this size by colliding with each other or by freezing into aggregations (snowflakes). When droplets are at least 0.04 inches across, gravity gets the best of them and they begins their descent from the base of the cloud to the earth. These droplets are still potential, not actual, rainfall. Like sad thoughts you can stifle before you choke up and get teary, some rain evaporates before it reaches the ground. Raindrops or ice crystals that fall from or are left behind a cloud but do not reach the ground are called virga (below).

Alas, dear reader, I must leave you here--high and dry--as I am now over my head in cloud physics. When I can explain to myself how exactly a cloud droplet becomes a successful, ground-reaching raindrop, I will let you know. Promise.