Forums: Skydiving: General Skydiving Discussions:
Hot/Cold Air vs. Fallrate

 


Shredex

Dec 28, 2012, 1:54 PM
Post #1 of 21 (2987 views)
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Hot/Cold Air vs. Fallrate Can't Post

Will you fall slower in hot or cold air?

Cold air is more dense then hot air, so you think you would fall slower due to the drag on the higher density.

But at the same time, hot air rises and cold air descends...

So which is it?


jakee  (C License)

Dec 28, 2012, 3:15 PM
Post #2 of 21 (2894 views)
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
But at the same time, hot air rises and cold air descends...

Localised and relative.

Air doesn't all go up during the day and come back down at night.


theplummeter  (C License)

Dec 28, 2012, 3:34 PM
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Re: [jakee] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

You fall faster in hot air. Hot air is less dense so you have to fall further per parcel of time to hit the same number of air particles.


jakee  (C License)

Dec 28, 2012, 3:37 PM
Post #4 of 21 (2875 views)
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Re: [theplummeter] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

He's got that part already, he just needs to think a little more about meteorology.


labrys  (D 29848)

Dec 28, 2012, 3:56 PM
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

Quote:
Cold air is more dense then hot air, so you think you would fall slower due to the drag on the higher density.

But at the same time, hot air rises and cold air descends...

Crazy

Today's new word is "variable"

Tongue


kallend  (D 23151)

Dec 28, 2012, 4:38 PM
Post #6 of 21 (2810 views)
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
Will you fall slower in hot or cold air?

Cold air is more dense then hot air, so you think you would fall slower due to the drag on the higher density.

But at the same time, hot air rises and cold air descends...

So which is it?

The hot air can't all rise at the same time now, can it? THINK!


Premier Remster  (C License)

Dec 28, 2012, 5:46 PM
Post #7 of 21 (2760 views)
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Re: [kallend] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
...... THINK!

Actually, I'm going with "Think less, jump more".


JohnMitchell  (D 6462)

Dec 28, 2012, 7:26 PM
Post #8 of 21 (2717 views)
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Re: [kallend] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
THINK!
That's asking a lot of us, Doc. Smile


kallend  (D 23151)

Dec 28, 2012, 7:43 PM
Post #9 of 21 (2708 views)
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Re: [JohnMitchell] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
In reply to:
THINK!
That's asking a lot of us, Doc. Smile

I would hope ATC folks think occasionally.


almeister112  (D 31305)

Dec 28, 2012, 8:14 PM
Post #10 of 21 (2683 views)
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

I would imagine most people would see an increase in fallrate when it's colder. We tend to bulk up with winter clothing under jumpsuits, making them tighter and reducing drag, so I'd think that would be a much larger effect than air temperature itself. That's just coming out of my ass, though; that whole "science" thing is hard.


dream2fly

Dec 29, 2012, 2:20 PM
Post #11 of 21 (2528 views)
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

Assuming EVERYTHING is constant except for air temperature, you will fall faster in hot air and slower in cold air because the drag force will decrease with a decrease in air density, and air density will decrease with a decrease in air temperature.

Force of drag = 0.5 X density of fluid (in this case air) X velocity^2 X Cd (drag coefficient) X A (reference area)



Sources:
http://en.wikipedia.org/wiki/Drag_(physics)

http://www.engineeringtoolbox.com/air-properties-d_156.html


Shredex

Dec 29, 2012, 4:07 PM
Post #12 of 21 (2506 views)
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Re: [dream2fly] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

The reasoning I ask, is because normally in a track I can get to a good 82mph vertical speed with the lowest being 79 and an average of 86.
But last weekend it was a very cold day and I had one heck of a tailwind pushing me along and yet I couldn't fall slower then 95mph.
It confused me...still is...


jakee  (C License)

Dec 30, 2012, 8:49 AM
Post #13 of 21 (2405 views)
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
But last weekend it was a very cold day and I had one heck of a tailwind pushing me along and yet I couldn't fall slower then 95mph.

Why is the tailwind relevant?

(And you probably just had a less good jump than normal.)


(This post was edited by jakee on Dec 30, 2012, 8:51 AM)


kallend  (D 23151)

Dec 30, 2012, 9:57 AM
Post #14 of 21 (2377 views)
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
The reasoning I ask, is because normally in a track I can get to a good 82mph vertical speed with the lowest being 79 and an average of 86.
But last weekend it was a very cold day and I had one heck of a tailwind pushing me along and yet I couldn't fall slower then 95mph.
It confused me...still is...

How was the fall rate measured? GPS or barometric altimeter? That could have an influence on a cold day.


Shredex

Dec 30, 2012, 10:46 AM
Post #15 of 21 (2337 views)
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Re: [kallend] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

Alti-Track was used.


theonlyski  (D License)

Dec 31, 2012, 5:00 AM
Post #16 of 21 (2243 views)
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
Alti-Track was used.

I wouldn't consider that a very reliable form of measurement.


To answer your previous question, I fall at the same speed, just seems MUCH longer when it's fucking freezing.


Premier billvon  (D 16479)
Moderator
Jan 2, 2013, 9:29 AM
Post #17 of 21 (2085 views)
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Re: [almeister112] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

>I would imagine most people would see an increase in fallrate when it's colder. We
>tend to bulk up with winter clothing under jumpsuits, making them tighter and reducing
>drag . . .

?? More clothing increases drag. It's pretty common to have bigway fallrate problems on the first dive of the day when people add sweaters and thermals. Cold air is also more dense, and this decreases fall rate BUT it decreases fall rate for everyone and thus is not a factor for relative work.


skyjumpenfool  (Student)

Jan 2, 2013, 10:50 AM
Post #18 of 21 (2057 views)
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Re: [billvon] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

I tend to notice the difference more when landing my canopy's. Cool


rehmwa  (D 12816)

Jan 2, 2013, 11:35 AM
Post #19 of 21 (2029 views)
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Re: [skyjumpenfool] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

In reply to:
I tend to notice the difference more when landing my canopy's. Cool

it also seems easier to absorb air through my skin on hotter days (for breathing in freefall)


this question is one about the variability of a very minor factor in freefall speeds - the first question should really be "does the variability in air density even have enough effect to overcome the natural variability of freefall speeds"

- i.e., colder air = more resistance = slower terminal v with all other things held constant - which they aren't normally. But is it really even noticable when you realize that all other things aren't held constant from jump to jump, let alone on completely different temperature days?


(This post was edited by rehmwa on Jan 2, 2013, 11:35 AM)


skyjumpenfool  (Student)

Jan 2, 2013, 2:26 PM
Post #20 of 21 (1986 views)
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Re: [rehmwa] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

I'd agree with Bill for the free fall portion of this. But I think it's very noticable landijng a canopy. JMHO...


potatoman  (Student)

Jan 3, 2013, 12:11 AM
Post #21 of 21 (1912 views)
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Re: [Shredex] Hot/Cold Air vs. Fallrate [In reply to] Can't Post

Go and do the jump another 10 times, and then you compare. Alti issue, clothing, your body position, the "tailwind", hehe...

Did you check on your footage to see if you did not have a bootie that came off ie less drag.

Anyways, next time, screw tracking to check your speed, try speedskyjumpin.



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