MadSci Network: Physics

Re: why can you feel wind but you can't see it

Date: Sat Mar 18 15:18:02 2000
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Physics
ID: 953122388.Ph

Aaron, your question got a little complicated when I thought about it so I 
broke it into four questions: 1. Why can we feel the wind? 2 Why do we 
think that we cannot see the wind? 3. Why can we see things? and 4.When or 
how can we see the air or see the wind.

Why can we feel the wind?  Wind is air in motion.  Air is made up of  a 
mixture of very tiny particles called molecules.  A cup of air contains 
about 6 billion trillion molecules[6.000,000,000,000,000,000,000],  that's 
a lot more than the billions and billions served at MacDonalds.  When the 
air, the wind, moves against us or we move thru the air, we feel these 
trillions of molecules hitting us from every direction.  Since there are 
so many molecules the air feels smooth.  As the wind becomes faster,  it 
feels less smooth.  When we feel the the wind the numbers of molecules 
that hit us each instant are changing or are different on different parts 
of our body.  These changes are called pressure differences.  We feel the 
pressure changes;  we feel the wind.

Why do we think that we cannot see the wind?  This is because of two of 
the properties of air.[a property is something that describes or is part 
of something. properties that describe you: a boy, height, weight, hair 
color, etc.].  Air is almost completely "Transparent" to light. This means 
that light goes through it without being changed very much[we will see 
that it is changed a little bit].  Air is also "Homogeneous"
 [like homogenized milk].  This means that for short distances, say 100 
meters to a mile, the air doesn't change very much.  This means that when 
we look at it we see no changes and our brain simply dismisses it and says 
that we can't see it.  If you are standing beside a swimming pool and look 
at the water you easily see the water,  If you jump in, go under and open 
your eyes you don't see the water[if it is clean] any more because it is 
homogeneous[and transparent] and your brain dismisses it and gets busy 
looking for sharks.

Why can we see things?  We really can not see "things"!  We see light.  We 
see "things" because we either see the light they give off, things such as 
the sun , stars,  light bulbs, lasers, LEDs, lightning bugs, TV screens, 
movie projectors.  OR we see things because light [from one of the things 
that gives off light ]  bounces off or is reflected from an object.  Such 
objects are you, me, a movie screen, the moon, the planets, a book, LCDs, 
anything that doesn't give off light.   When you "see" something  think, 
"Am I seeing this thing because it is giving off light or because it is 
reflecting light?"  If it is reflecting light, Think, "Where is the light 
that it is reflecting coming from?"   An experiment to try to show that 
you really only see light can be done.  You will need your Mom or Dad or 
both to help out.  After dark go into a room in your house that has a 
large walk in closet with a door that can be securely closed.  Open the 
closet door with the room lights on and look in , you will be able to see 
what is in the closet. Go into the closet with your Mom or Dad and stand 
looking out the door.  Have the other person turn off all the room lights, 
but leave a light on in another room that you cannot see directly.  At 
first you will not be able to see much.  As your eyes adjust to the low 
light"adjust to the dark"  you will start to see things but not very well. 
Turn around in the closet and have your Mom close the door.  If the closet 
is truly light tight you now will not be able to see anything.  As you 
wait your eyes will not "adjust to the dark"  because there is no light 
for them to adjust to.  Conclusion: We see light, not things.

When or how can we see the air or the wind?  We must look for the times 
and places where the air changes.  The simplest place is to just look up 
into the clear blue sky.  Remember, we see things because they either give 
off light or they reflect light.  Well the atmosphere can give off light 
in lightning or in the northern lights or aurora borealis, but the sky is 
blue only in the daytime and only when the sky is clear.  So the 
atmosphere must also be reflecting light from the sun, the correct term is 
that the light is "scattered" by the molecules in the air.  Seeing the 
blue sky is seeing the air or wind.  The clouds racing across the sky  are 
moving with the wind so again we can see the wind.  If one looks along a 
hot road, railroad track, or sidewalk and sees wavy lines in the air, one 
sees the hotter air moving upward, an updraft.  The funnel cloud in a 
tornado would probably be visible even if it didn't have trees and 
rooftops spinning in it. Every chance you get, look around and see how 
many ways you can see the air , see the wind. 

Both Compton's and Encarta Encyclopedias have sections on wind, air and 
the molecules in air which are fair to good in content and accuracy.  An 
8th grade Earth Science text book from the school library would also be a 
good source of information.

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