Humidity, Air Conditions and Casting

[Walter]

I used to park my truck on the side of a cliff overlooking the Waiau River in Fiordland, New Zealand. And in the evening I would stand on the roof and play with my maximum carry. There was music involved and red wine.

The point is I used to throw the forward cast downwards. Now it may have been my perspective but there was a big difference between the loop unrolling downwards, and loop straight and free fall.

I’ll find a cliff to reenact this entertainment and try to film it this time.

The same thing happens with a crosswind (I believe) so I don’t think it’s loop lift.

Paul

At a cliff face I've visited a number of times in New Mexico (Sandia Peak) you could stick your arm out over the edge and feel the rush of air directly upwards (depending on the direction the wind was coming from). It was also the case that as the air cooled as it was travelling upwards the moisture in it would condense into a mist, thus you could stick your hand into an upward shower of mist. Paragliders would often launch from the same place.

James

This is from the loop dynamics topic but I think it makes a great segue into a topic Paul and I have discussed a number of times.

Paul tells me that when casting in high humidity and especially in fog that he can't cast as far - a difference of 10 feet. I know it's anecdotal but when someone spends as much time as Paul does practicing and pays as much attention to anything that could be affecting his cast I tend to listen to what he says.

The problem is that the science I've read tells me that it should be the opposite.

I'm sure we've all seen the formula for form drag:

Force of Drag = 0.5 ρACdv^2

in which ρ represents the density of air.

This formula tells us that the drag force is directly proportional to the density of air. Intuitively we think that humid air is more dense than dry air because water is denser than air, right? The problem is that water vapor is less dense than air so humid air is less dense than dry air. That should mean that there is less drag on the line in humid air and, therefore, it should go farther.

So, assuming Paul is correct about the effect of humidity on his cast, there must be something else affecting his cast and I have no idea what it would be. (i'm ruling out arthritis for now... )

So, two questions:

1. Has anyone else noted a similar effect?
2. Does anybody have any idea what the reason for this would be? The only other thing I can think of is that humidity affects the line itself. Maybe that's farfetched but we know that monofilament nylon absorbs water which results in it becoming weaker after it has been submerged for some time. If fly line has been sitting in higher humidity could it absorb some of the humidity affecting its castability? Given the recent discussions about losses in fly line during the cast I'm wondering if something that affects the fly line itself and causing additional losses could be a cause? Would the dynamic test Torsten described in another thread help to confirm or deny this?

Paul also tells me that he can cast farther in higher temperatures as well. Something else to consider - what other air condition factors could affect the cast?

Cheers

Walter

[Lasse Karlsson]

Hi Walter

1. Yep, distance goes down in wet calm air.
2. My lines throw as far at the end of a fishing day when they've been in water all day, as they do when they are dry in the field.
I think its both because the air is still, and the psycological effect that has on us. We know it won't for as far, so we hit it harder and mess more up.

Cheers
Lasse

[George C]

Isn’t fog different than just humid air?
With fog the humidity has reached a point where it has exceeded the air’s ability to carry it and has begun to form droplets.
At that point isn’t the line is colliding with not just air but also water droplets? Seems like that ought to slow it down.

[gordonjudd]

Isn’t fog different than just humid air?

George,
Yes it is, but you will have difficulty in getting a quantitative answer to the density of heavy fog. The best estimate that I could find was in this paper on the difficulty that mosquitoes have in flying in fog of all things that states:

Mosquitoes fail to sustain flight in humidifier fog, which has a density up to 2 kg/m3.

My best guess is that the density of lighter fog is in the range of 1.7 Kg/m3 which is about 1.4 times the density of air.

Gordy

[Paul Arden]

Well that explains fog then where I lose about 20’ and not 10. But it doesn’t explain humidity. Sometimes the wind blows here too Lasse! I’ve noticed this in Florida, top end of Australia, and all through the tropics. I lose about 10’.

My thoughts on temp were comparing zero to 25C with a MED. As well as reports from Estonia. Cold air is more dense which would explain that.

The distance loss in the tropics may have to do with line stiffness. Coldwater lines are uncastable; they are limp and don’t shoot well through the rings. Perhaps the tropical lines simply aren’t as good when compared to the Coldwater lines in temperate regions?

Cheers, Paul

[VGB]

I proposed a hypothesis a while back. Unfortunately, when the thread was rearranged the description of drag effects was disconnected :

https://www.sexyloops.co.uk/theboard/vi ... 862#p68747

[VGB]

The difference in drag depending on free stream flow conditions

[attachment=0]IMG_2060.png[/attachment]

[Paul Arden]

That’s very interesting Vince. Can I put it in my own words to see if I understand it? Surface drag on the flyline is less when the air is more turbulent. The air is less turbulent in high humidity of cold conditions. Hence the feeling of “dead air” is actually true. (?)

Cheers, Paul

[VGB]

Hi Paul,

Yes that is correct, I've experienced myself many times, yesterday being the most recent.

Regards

Vince

[James9118]

Hi Walter,

I can confirm that I've experienced this many times also, the most recent being last weekend. I practice my carry with a shooting head set up (an old #5 MED cut to length), on the running line I put a 'flag' of tape to mark my exact carry. This 'flag' on my current practice line is set at 90ft. Last weekend, in still, humid (but cold), conditions I simply could not get to the point where I was holding the flag. On other days, with some air movement, I find I can get to the flag with ease. On a day where I can't get to the flag I would struggle to cast a full MED line much over 100ft, on a day when I can get to the flag on the shooting head then I'd be expecting to be in the 130s with a full line.

The interesting part of this is that with some wind you'd sort of expect the backcast (casting downwind) to become harder as you're trying to fire the line into an unhelpful airflow. However, often I find it's actually easier - especially when compared to a flat calm.

Many years ago I hypothesised that this may be due to differences in the flow characteristics of the air I'm casting in, i.e. perhaps a transition from laminar ambient conditions (in the case of a flat calm) to turbulent conditions as the airflow (wind) picks up. I was also aware that drag can (not always) reduce in turbulence - an example of this being race cars, the car behind the one in front is faster as it is travelling in the turbulent wake.

I discussed this on a number of occasions with Vince, who found the reference he posted above, showing that drag can be significantly reduced if the approaching airflow is turbulent.

As such, I think the 'distance falls in high humidity' observation has some truth in it, but it's not simply a result of the humidity which, as you rightly point out, would actually lower drag. I suspect observations of poor distances in sticky, high humidity conditions are more linked with lack of wind and perhaps a transition to a more laminar flow.

James