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Fast tigers and slightly salty rain
Q: What’s the average speed of a tiger? What’s the average
speed of a human? How fast do humans swim? (Steven, New York)
A:
Tigers can run about 35 miles per hour (56 kph) — roughly as fast as bears and
lions. They reach such speeds only over short distances (about 10 yards, 10 m)
during the attack.
Let sleeping tigers lay [Gary M. Stolz, US Fish & Wildlife
Service]
At night, a tigress silently stalks her prey — perhaps a deer
— through dense cover until close enough for the final rush. From the side or
rear, she bounds in great leaps toward the ill-fated prey, unsheathes her claws,
grabs the stag by the shoulder with one paw, bares her huge canines, bites the
deer’s throat, presses upwards, and either breaks his neck or suffocates him.
An average human, running at a paltry 17 mph (27 kph), would
have little chance of escaping. Even world-class sprinters only reach 24 mph (39
kph).
Olympic gold-medal swimmers slice the water at about 5 mph (8
kph).
By the way, tigers have webbing between their toes and can
paddle swiftly for long distances. Given a chance, they run hoofed prey (much
slower swimmers) into the water for the kill, says
Dave Stegenga of the
Honolulu Zoo.
Further Reading:
Burton, Maurice, and Robert Burton, eds.,
The International Wildlife Encyclopedia. New York: Marshall Cavendish
Corporation, 1970.
Honolulu Zoo:
Sumatran tiger
Q: When the sun draws water from the
ocean how come we don't get salt-water rain? (Elliot, Swansea, South
Carolina)
A:
The Sun draws water from the ocean only in the sense of heating the water enough
to evaporate it. The water molecules near the sea’s surface bop around. As the
Sun heats the water; the water molecules jiggle more. That’s what heating water
means — increasing the average speed of the molecules.
The Sun evaporates seawater and leaves the salt behind.
[John Bortniak, NOAA]
Seawater also contains dissolved salt (sodium chloride), which
is a collection of electrically charged particles called ions. Sodium ions have
a positive and chloride ions a negative charge. Water molecules, coincidentally,
also have slight end charges — plus on one end and minus on the other. (The
overall charges of a molecule balance but the positive-charge and
negative-charge centers don’t coincide. This permanent mismatch charges the
ends.)
Salt ions drift around in the water and attract the
oppositely-charged ends of water molecules. So, electrical bonds lightly hold
salt ions to water molecules.
The Sun heats the water molecules, they jiggle faster and kick
nearby water molecules. The extra kick energy breaks the electrical bonds
binding the water molecules with salt ions, and careens the water molecules into
the air. Almost all the salt ions stay behind.
Salt also evaporates but at a much slower rate — negligible at
sea temperatures. So essentially no salt accompanies the water as it evaporates
into the air.
Salt, however, does enter the atmosphere, just not through
evaporation. Instead, it comes in as tiny salty water droplets — for example,
via "bubbles formed by breaking waves," says
Craig
Bohren, author of Clouds in a Glass of Beer. "These droplets can
evaporate, leaving behind small salt grains."
So, when it rains, it rains salty water. Not very salty
— not enough to taste, but still salty. In fact, "cloud droplets form by
condensation of water vapor on small soluble particles, often salt (sodium
chloride)," says Bohren. "These droplets can coalesce to form much larger rain
drops.
Of course, much more junk makes its way into our atmosphere
and rains out — "disintegrating micrometeorites, industrial pollution, dust, and
an occasional small frog or fish that gets picked up by a particularly energetic
storm," says Bob
Harbort of Southern Polytechnic State University
Further Reading:
Craig Bohren.
Clouds in a glass of beer. New York: John Wiley & Sons, Inc, 1987.
ESPERE, Environmental Science Published for Everyone Round the
Earth:
Clouds and particles by Justine Gourdeau
Plymouth State College:
How
rain forms by Derek W. Brown
(Answered May 20, 2005)
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