How Come?

Archive for the ‘nature’ Category

Is it true that bumblebees shouldn’t be able to fly?  asks a reader.

No one’s sure where the myth started, but it has legs (er, wings):    Bumblebee flight is impossible.  According to the principles of aerodynamics, the story went, a big, fuzzy bumblebee, powered only by tiny wings, shouldn’t leave the ground.  A French book from the 1930s, for example, cited calculations that “proved” insects in general shouldn’t be able to fly.  But using mechanically-driven fixed aircraft wings to model the flight of bumblebees and other insects just didn’t work.

The mysteries of insect flight are still being unraveled, but today’s scientists say insects fly “in a sea of vortices,” using the swirling eddies of air created by their beating wings to stay aloft.

Bumblebees beat their wings up to 200 times a second, faster than the nerve impulses to their muscles can fire.  This works because bumblebee wing muscles don’t contract and expand with each electrical signal.  Instead, they continuously vibrate, like a repeatedly plucked guitar string.

When a bumblebee rests, its body temperature drops (or rises) to that of its surroundings. But according to entomologists, the temperature of a bumblebee’s wing muscles must be a toasty 86 degrees F. to lift the bee into the air.  So to fly to the nearest flower cafeteria, a bumblebee must dramatically raise its temperature in all but the hottest weather.

How?  Basking in the sun isn’t usually enough, so the bee will shiver her way to a higher temperature.  By rhythmically contracting and releasing her abdominal muscles (faster and faster as her temperature rises), a bee generates enough warmth to take flight.  The warmer the air temperature, the quicker the lift-off.  On a chilly, 42-degree day, a bumblebee must pump her thorax muscles for a tiring 15 minutes to reach 86 F.

Once in flight, a bumblebee’s muscle temperature remains in a range of about 86 to 111 F.  All of this activity expends enormous amounts of energy, provided by the sugars in flower nectar.  Not surprisingly, scientists have found that bumblebees are especially attracted to warm flowers, floral rooms whose color and shape keeps them heated in the sun.

According to Oxford University research published this month, bumblebee flight really is different from that of dragonflies and other insects.  Using a smoke-filled wind tunnel and cameras snapping 2,000 images a second, researchers found that bumblebee flight is surprisingly inefficient.  For example, a bumblebee’s left and right wings flap independently of each other.  And instead of using the varying air pressure at its wingtips to provide some lift, bumblebees use “brute force” to fly and hover.  Bumblebees, say scientists, are the “tanker trucks” of the flying insect world, using incredible amounts of energy to lumber (charmingly) through the sky.

In fact, bumblebees manage to fly even in places where human beings find it difficult to breathe.  On Earth’s highest peak, Mount Everest, bumblebees have been sighted at more than 18,000 feet up.  Although there’s much less air pressure for tiny wings to beat against, bumblebees power on, hot to the touch even in the frigid cold.

When you’re outdoors and it starts to rain, does running (rather than walking) to the nearest shelter really keep you any drier?  asks a reader.

It begins to pour.  You have no umbrella, or printed newspaper.  (The internet, alas, won’t keep your head dry.)  Never mind singing in the rain.  The question is, To run or not to run?

Assuming you’re healthy enough for a mad dash to that building across the street (or the shelter house in a public park), is it worth the energy?  Can you really slip between the drops?  Or will sprinting through a downpour actually make you wetter?

At first glance, the answer appears obvious:  Running means less time spent in the rain, and less-wet clothing.  But the walking advocates have arguments that seem to make sense, too.  If you run through falling raindrops, they point out, you’ll catch more raindrops on the front of your body — chest, stomach, fronts of legs.  If you walk, most of the drops will fall on your head.  Since the front of your body has more surface area than the top of your head and shoulders, you’ll get more water-logged if you run into the wall of rain.

Some have even done mathematical simulations that say you’ll be wetter if you run.  With rain falling straight down from the sky, and no wind, they argue, you’ll scoop up water through the rain field as you dash forward.  But actual experiments with real human beings have found otherwise.

According to physicist Jearl Walker, of Cleveland State University, running keeps you drier because you spend less time being pelted with water.  If there’s no wind, or the wind is blowing toward you, Walker says, you can minimize the number of drops your body encounters by leaning forward, while running as fast as you can.  With a wind at your back, he advises matching your speed to that of the horizontally blowing drops.  Moving with the rain, he says, you’ll avoid both front and back splatters; most drops will strike your head.

And in a 1997 report in the British journal Weather, two climate researchers in Asheville, NC also found that running trumps walking.  On a rainy day, Trevor Wallis and Thomas Peterson suited up in identical sweats, wearing trash bags underneath so that water wouldn’t soak through.  One ran and the other walked through the pouring rain over a 100-meter (328-ft.) course, weighing their track suits before and after their wet dash/stroll.  What they found:  The clothes of Wallis, who ran, were a full 40 percent drier.

If you’re already soaked, or the nearest shelter is very far away, it probably doesn’t matter whether you walk or jog.  To find out if it’s really worth breaking into a run in the rain, physicist Doug Craigen has devised a handy calculator.  You’ll need to enter your height, take some body measurements, and guess at the wind speed of your own rainy day.  Find out how wet you’ll get at   http://www.dctech.com/physics/features/0600.php.