Question: Why is the optimum snorkel length not longer than 40 centimeters?
About the article "Snorkeling", section "The snorkel".

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A swimmer's snorkel is a tube typically about long and with an inside diameter of between , usually L- or J-shaped and fitted with a mouthpiece at the lower end, and constructed of rubber or plastic.
It is used for breathing air from above the water surface when the wearer's mouth and nose are submerged.
The snorkel usually has a piece of rubber that attaches the snorkel to the outside of the strap of the diving mask.
An older technique is pushing the snorkel between the mask-strap and the head, but this practice increases the chances the mask will leak.
The optimum design length of the snorkel tube is at most 40 centimeters (about 16 inches).
A longer tube would not allow breathing when snorkelling deeper, since it would place the lungs in deeper water where the surrounding water pressure is higher.
The lungs would then be unable to inflate when the snorkeler inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure.
Snorkels also constitute respiratory dead space.
When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is recycled into the lungs, reducing breathing efficiency and causing a build up of carbon dioxide in the blood, which can result in hypercapnia.
The greater the volume of the tube, and the smaller the tidal volume of breathing, the more this problem is exacerbated.
A smaller diameter tube reduces the dead volume, but also increases resistance to airflow and so increases the work of breathing.
Occasional exhalation through the nose while snorkeling will reduce the build up of carbon dioxide, and help in keeping the mask clear of water.
 ""Oregon Willowherb" redirects here; not to be confused with Oregon Fireweed."
Epilobium oregonense is a species of flowering plant in the evening primrose family known by the common name Oregon willowherb.
It is native to western North America from British Columbia to Arizona, where it generally grows in moist places in several types of habitat.
It is a perennial herb growing spindly erect stems approaching 40 centimeters high or sometimes forming mats spreading via stolons.
The small leaves are rounded near the base of the plant and linear in shape farther up the stem.
The inflorescence bears flowers with four white to pink petals each a few millimeters long.
The fruit is an elongated capsule up to 5 centimeters in length which is borne on a long pedicel which may be longer than the capsule itself. 
The "Allium validum" bulb is three to five centimeters long, ovoid and clustered on the short end.
The outer coat of the stout rhizome is brown or gray in color, fibrous, and vertically lined.
The stem is 50 to 100 centimeters long and angled.
There are three to six leaves more or less equal to the stem and the leaves are flat or more or less keeled.
There are 15 to 40 flowers with pedicels being seven to twelve millimeters in length.
The flower itself is six to ten millimeters, its perianth parts are more or less erect, narrowly lanceolate, acuminate, and entire with a rose to white color.
The stamens are longer than the tepals, and there is no ovary crest. 
It normally grows to lengths of between 50 and 100 cm, and between 3 to 10 kg.
But it is not rare to find exemplars of more than 40 kg.
It can grow up to 150 centimeters in length and 60 kilograms in weight. 
"Astragalus subvestitus" is small, hairy, mat-forming perennial herb producing stems no longer than 8 centimeters.
The leaves are a few centimeters long and made up of several hairy oval-shaped leaflets.
The small inflorescence holds a few purple-tinged white flowers each just over a centimeter in length.
The fruit is a papery legume pod covered in short, curly white hairs and bearing a triangular beak at the tip.