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Does Cold air cause more Air Resistance?

  1. Jan 1, 2014 #1
    I know that Cold Air is more dense than the warm air, so for example: If a car was accelerating facing a hot air, it will face less air resistance. But if it was facing a Cold air, it will face more air resistance because the cold air contain more mass per volume for each particle. It will face more Air resistance even if there was little difference...
    Correct me if I'm wrong please.
    Thank You!
     
  2. jcsd
  3. Jan 1, 2014 #2

    SteamKing

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    Since drag is proportional to density, yes, colder denser air produces slightly more drag than warmer lighter air.
     
  4. Jan 1, 2014 #3

    rcgldr

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    The density of the air is a factor in air resistance, and it's common for colder air to occur due to low pressure weather systems if the comparason is made in the same location, as opposed to two different locations where one location has colder temperature and higher pressure.
     
  5. Jan 1, 2014 #4
    This may be slightly tangent to your question, but the car in your original thought experiment will get better fuel economy in the cold air then in the hot air despite the increased drag. The reason is that internal combustion engines operate more efficiently at lower ambient temperatures due to the increase in temperature difference between the combustion chamber and the ambient temperature.

    If your objective is speed the cold air is also preferable. Your engine can ingest and process a limited number of liters per minute of air depending on engine displacement, maximum RPM, and volumetric efficiency. If the air is more dense then that volume of air contains more oxygen. More oxygen + more fuel = more power.
     
  6. Jan 1, 2014 #5

    Ok, if it was a bicycle? There will only be more air resistance facing the bicycle without producing more power like you said previously about the combustion engines, because of course the bicycle doesn't have an engine... Am I right? Please answer me and thank you guys for answering you helped me so much!
    Im waiting for an answer asap!!
     
  7. Jan 1, 2014 #6

    Gfg

    Ok, if it was a bicycle? There will only be more air resistance facing the bicycle without producing more power like you said previously about the combustion engines, because of course the bicycle doesn't have an engine... Am I right? Please answer me and thank you guys for answering you helped me so much!
    Im waiting for an answer asap!!
     
  8. Jan 1, 2014 #7
    The bicycle does have an engine, the rider. there is an optimum temperature where the human rider will produce the most power.

    If you are just coasting, then you are correct in both the bicycle and car cases. Lower temperature -> higher density -> more drag.
     
  9. Jan 1, 2014 #8

    AlephZero

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    That is not necessarily true. It depends on the local climate and weather conditions.

    For example in the UK, colder weather (than average for the time of year) is correlated with HIGH pressure in winter, and with LOW pressure in summer. High pressure is associated with the greater temperature range of the continental climate (to the east of the UK) and low pressure with the smaller temperature range of low pressure weather systems coming from the Atlantic, which is the most common situation.

    For example in the recent series of weather systems from the atlantic, each low pressure system produced a RISE in temperature of about 5 to 7 degrees C as it crossed the UK.
     
    Last edited: Jan 1, 2014
  10. Jan 1, 2014 #9

    rcgldr

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    I forgot to take into account the effect of off shore versus on shore winds in locations near an ocean. The direction of wind flow would be affected by the location of a low (counter-clockwise flow in northern hemisphere) or high pressure zone (clockwise flow in northern hemisphere). I'm not sure if this explains the UK situation. I wasn't sure if the OP was interested in weather related cold temperatures (or humidity which I didn't mention before), or just the fact that under otherwise similar situations, cold air is denser than warm air.
     
  11. Jan 1, 2014 #10
    drag_coeff_2.GIF

    As you can see in the above representation, the drag coefficient is not a constant over values of Reynolds number.

    Fluid people have come up with ways to determine a fluids resistance to flow and one of these is Reynolds number.
    Reynolds number is the ratio of the inertia forces / viscous forces as an object moves through a fluid. Inertia forces relate to the resistance of the fluid to accelerate and are subject to the density of the fluid and the square of the velocity of the object through the fluid. The viscous forces result from the internal friction of the fluid, and are dependant upon the viscosity of the fluid and the velocity of the object through the fluid. In addition, and this may be surprising to you as it seems contrary to what one you would expect if they have experience with liquids, is that the viscosity of a gas increases with temperature.

    At low Reynolds number viscous forces predominate. At higher Reynolds number, the inertia forces predominate. For a given body moving in a given fluid, this would be the same as subsituting the phrase "Reynolds number" with "velocity".

    So as your car tries to accelerate on a run down the road, the air provides a drag opposing that acceleration. At the beginning, when the velocity of the car is low, the drag due to the density of the air is low and the drag due to viscosity is high ( in relation to one another and not the absolute drag on the car which will increase as the velocity increases ).

    Generally,
    Warm air being more viscous would give more percentage of drag than cold air at the beginning of the run.
    Cold air being more dense would give more percentage of drag than warm air as the velocity of the car increases.

    It could be that your would experience more total drag in denser but less viscous cold air than in a less dense but more viscous warm air, depending upon how fast you decide to go.

    Hopefully that makes sense.
     
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