Question: A Car In The Rain

Discussion in 'Earth Science' started by jmpet, May 27, 2009.

  1. Oli Heute der Enteteich... Registered Senior Member

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    Where did you miss:
     
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  3. jmpet Valued Senior Member

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    I think a car in the rain driving at 60 will capture 101% the rain mass due to its acceleration relative to the wind-gusts (speed) of the rain... by being in motion, it captures more than at rest.
     
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  5. baftan ******* Valued Senior Member

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    I wonder which direction the wind is blowing in your rain scenario, but "per square mile" is "per square mile" and when you want to wet particular surface you can approach it from many angles, as long as you apply same amount of water, and/or same time. You can drop a bucket of water on the ground slowly, quickly, from top or from side. The result is the same amount of water contained by the bucket. If I do not move the car after I collect the first bucket of water, my car can get another bucket of water. So time can be important if water is not restricted to one bucket (or ten buckets of water). Imagine three or more possible angles towards the car (from side, directly from above, front and behind or crossway) In each situation they will be parts of car that get direct hit, while some other parts does not get direct hit. However you can assume the car size water absorbing spunch, so we can collect the water under each situation (windy or not) and compare them.

    If you walk enough period of time under the rain (you can run if the rain is heavy, doesn't matter) some parts of your body, which probably didn't get any sunshine for a long time, could get wet. It's not because you are so scared of rain and thunder and wet yourself, no; it is because of the variable of time that you spend under the rain. And there is no shadow, it's raining, there is no sunshine...http://www.sciforums.com/images/smilies/biggrin.gif
     
    Last edited: May 28, 2009
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  7. baftan ******* Valued Senior Member

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    This is perfect explanation: Driving at 60 will capture 101% the rain... If we develop this relation how fast should I go to capture 125.33% of water? Answer: None. Anything more than 100% is an abstraction used by celebrities and financial people. How can you get more than 100% of rain? Can you explain it to us please...
     
  8. baftan ******* Valued Senior Member

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    This is not an explanation. This is complication.
     
  9. Oli Heute der Enteteich... Registered Senior Member

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    Unless it means more than 100% of the rain than it would get if it were stood still.
    It's a comparative.

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    Only if you don't know what you're talking about.
    Which is becoming evident.
    There is no difference between "stationary car + moving rain" and "moving car + stationary rain" if they are the only things under consideration.
     
  10. Absane Rocket Surgeon Valued Senior Member

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    The way I like to present this is by explaining "flux."

    http://www.math.rutgers.edu/~greenfie/mill_courses/math251/gifstuff/flux2.gif
     
  11. Nasor Valued Senior Member

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    In the scenario you describe, 100% of the rain is equal to the rain that will hit the car if it's standing still. Obviously. If you didn't even understand that, good luck with the rest of it

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  12. leopold Valued Senior Member

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  13. Oli Heute der Enteteich... Registered Senior Member

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    Because it's not about the amount of rain coming down it's about the amount of rain hitting a given capture area: which varies with movement.
     
  14. leopold Valued Senior Member

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    it's still the same.
    if the car moves with the moving cell it cannot possibly get more rain than a car standing stationary under a stationary cell if both cells generate the same rainfall.
     
  15. jmpet Valued Senior Member

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    Yes. I am a financial celebrity. Does that answer it for you, smartass?
     
  16. baftan ******* Valued Senior Member

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    No, because question was not about your occupation. It was about "what is more than 100% rain?" Is it a kind of extra rain extraction from clouds? How does it work? My ass is not only smart, but it's beautiful too, although it has nothing to do with the question.
     
  17. hypewaders Save Changes Moderator

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    I've only skimmed this thread, so I apologize if the OP was already well answered.

    Maybe you didn't know it, but rain organizes itself into a consistent matrix of water spheres. The first time I saw this, I was blissfully astounded- I never expected the stunning scene of an endless, evenly-spaced colunnade of shining beads. It was in a hammerhead turn in an airplane, when I first fell for a moment at the very same speed as the rain, and found myself surrounded by indescribably beautiful, perfectly-arrayed rows of raindrops. Astounding, perfectly-spaced beads on a million invisible strings were all around me, stretching all the way to the ground far below. I could see the long rows of water-spheres bending lazily in the lower winds. Contemplating such scenes, I realize how small insects, many much smaller and lighter than a raindrop can fly among these meteors with impunity: In their simple but overclocked awareness, some flying insects can weave intuitively between beautiful and predictable columns of slowly-descending spheres. Falling with the rain, even for just an instant gives a magical perspective. Too soon relative motion increases, the spell is broken, and the scene blurs into the familiar everyday human vision of rain. But once you find yourself in the magic hall of rain for even a second, it's impossible to forget forget the expansive and intricately-ordered scene that is revealed.

    The number of raindrops impacting a given surface is proportional to our relative speed through the droplet field. Additional motion means getting wetter. Consider very small raindrops (mist) falling slowly. You get more damp (wet even) if you walk forward through a mist, or if a gentle wind is blowing it past. Stand still in a slowly-falling mist, and only the top of your head and shoulders get damp. It's the same with larger, faster-falling rain: Moving forward increases the relative velocity between a moving object and the droplet field, and increases the number of droplet impacts. Notice in your visualization that the droplet field and the wind do not have the same velocities or vectors. Heavy (rain-sized) droplets organize into columns, because they stabilize falling in the wake of preceding ones. Within the columns of large rain, there is an even distribution of smaller droplets in regular reactive impacts within their columns.

    Flying at great speed in heavy rain, we are fortunate about many things, including water's surface tension and impact behavior: Large-droplet impacts dissipate most of their energy into the surrounding droplet field in an always air-permeable boundary layer. We can bash through kilotons of water in an airplane, without being perceptibly slowed down. The slipstream's boundary-layer is little changed by the impacting water, but is instead visually revealed by millions of tiny droplets rolling slowly along the aircraft's skin, dramatically revealing areas of impact, stagnation, slow, and reversed flow. Sometimes at the trailing edges, a droplet flies just behind in formation until it grows too large to stay in the burble- then it falls away while a new levitating sphere grows in midair.

    There's a splattering dissipation of energy where there is impact- with enough force to cause sharp needles of pain to exposed skin, but it's nothing that light gloves, or the thin skin of an airplane can't handle at any droplet size or speed. It can be noisy, but very little drag is induced when moving fast through rain. Sheets of water (which would catastrophically spoil lift) cannot form in that energetic layer. The higher the speed and volume of water impacting, the more the rain is atomized into a beautiful layer that allows us to see the pressure gradients and flow fields around shapes in flight through heavy rain.

    These are some observations from many hours of flight through all kinds of rain. It's fascinating to study the rain's impact on an airplanes' various curves, playing a flashlight along them in the night, or glimpsing a snapshot of billions of stop-motion droplets, in a flash of lightning. I love flying in the rain. Now and then I can fall with it for a moment, and I'm always amazed at the rain's geometric organization. Often when a shower is coming, I smear a plane all over with dishwashing soap, and take her up for a glorious shower. Washing them that way is a clear demonstration of how moving fast through the rain gets things much wetter. And it gets the bugs off without scrubbing.
     
  18. cluelusshusbund + Public Dilemma + Valued Senior Member

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    The roof of a car settin still for 1 minute in a stedy rain woud collect the sam abount of water as the roof of a car movin at 60 mph for 1 minute... but the grill of the movin car woud collect water that the car settin still woud not colect.!!!
     
  19. hypewaders Save Changes Moderator

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    All true- the amount of vertical motion in the rain field is a constant, if we're only varying our horizontal velocity. If we energize a boundary of rushing air, we can of course deflect rain, as with a convertible car. On some large aircraft, you can open a forward window (a section of the windshield) in flight, and hardly a breath of wind or weather enters, because a well-streamlined nose section can compress and accelerate the airflow into a nearly impermeable boundary-layer or membrane of air.
     
  20. jmpet Valued Senior Member

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    So you're saying that because of aerodynamics, it's less rain.
     
  21. hypewaders Save Changes Moderator

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    No, the vertical component of rain is unchanged. A shielding roof or rain barrier can be made of something solid, or from an energized sheet of air. Just because we stay relatively dry just behind a convertible's windshield, or when we know the glory of an open-cockpit airplane in weather. An equivalent amount of rain can be deflected with laminar airflow, or canvas, or any other barrier splattering and deflecting falling spheres of water.

    Laminar airflow does have its limitations, compared with other weatherproofing- riding in the back seat of a large open convertible car in the rain, we might ponder why we get much wetter, the faster the car goes: It's because we're riding in the burble, where the sheet of air that flowed over the top of the windshield is billowing and collapsing into our laps, and all the full raindrops from above us are entering, along with the broken pieces of rain that would otherwise have fallen into the front seats.
     
    Last edited: May 31, 2009
  22. Captain Kremmen All aboard, me Hearties! Valued Senior Member

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    Yes.
    The faster the car goes, the more rain that the front of the car will hit.
    If the car was going at 100,000 miles per hour in a downpour, bucketfulls of rain would be hitting the front of the car each fraction of a second.
    Add a few noughts to the mph and it would be swimmingpoolfulls.
     
  23. hypewaders Save Changes Moderator

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    Yes, well, almost: Because a boundary layer forms, and gets firmer as we go faster, more raindrops are broken and deflected around the shape before they can impact. At 10kmph, the water droplets would be vaporized by the energy of the shock wave, and the hot surface of our vehicle would stay bone dry. But let's slow down a bit.

    It's harder to watch the effect in a car than in an airplane, but if you can watch the front slope of the side mirrors for example, you'll see a barrier form in front of the paint and metal, that breaks and deflects the onslaught of rain, most of it passing around the shape without actually impacting it.

    The size and curve of the shape is critical in this, and if you want to understand this principle, put your hand out into the rain at high speed. It's going to hurt, but you can learn a lot. A sloping large surface, like the palm of your hand deflecting a sheet of air hurts much less than a karate-chop narrow slice with the side of your hand. Similarly, thin structures like an antenna or propeller take a real beating from the rain, while large streamlined forms pass through with hardly any impact energy transfer. The difference is all about the formation of an energized boundary layer, which is the source of confusion about why moving faster means staying dryer in the front seat of a convertible, although moving faster brings the entire vehicle and its speed-induced air jacket into contact with more rain.

    OK now it's time for me to stop talking about motion through the air, and go do it. Woot! (full day of flying ahead)
     

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