Distance Casting In Hot-Humid-Calm Conditions: Thin Air Tough Casting?

[Richard Floggwater]

Hello All,

Distance casting at sea level in hot – humid – calm conditions is often found to be more difficult even though the air is actually thinner. References to the this thinner air situation can easily be found, although John Field’s Fly-Casting Finesse should easily suffice. I have a simple idea/hypothesis to put to the SexyLoops Board to discuss to ferret out whether this thought is “BS” or potentially holds some validity. Unfortunately, I have no experimental evidence to support the notion or good way to test it. The idea revolves around the tenet that down wind distance casting is easy “in comparison” to a calm conditions scenario, and that difference is what a caster notices – easy compared to more difficult. Please try to also envision a knowledgeable caster – using a fly rod as a flexible lever forming good loops with decent linespeed and a well cared for flyline – to distance cast in calm versus downwind conditions.

The idea is that downwind distance cast are potentially helped by the rod legs ability to travel more easily allowing the fly leg to unroll at a slightly slower rate as both legs travel further gaining more distance than a calm conditions cast. Although high wind speed may thwart a back cast, many experienced casters can punch a good backcast into a decent wind with good form. The forecast distance tends to be enhanced probably due to less drag on the line with wind traveling in the same direction as the cast. The idea here is that the downwind scenario reduces turnover speed by allowing the rod leg to travel in a less impeded style compared to a calm. Some have even suggested the ‘casting loop’ may act as a sail catching the wind. Contrasting that, during the calm/humid/hot scenario, the rod leg is potentially slowed fighting many variables (friction, drag, need for best form…) to travel and in turn causes a rapidly unrolling fly leg to reach the lines end with turnover occurring at shortened distances. Calm conditions demand optimal form for decent carry, and line speed has to be introduced continually to keep the system alive, which is not helped by any wind on back or fore cast. Even a head wind will help a back cast, where it might need to be checked to turnover on the forecast. Yes - optimal form is needed for any decent carry, but calm conditions present a situation where the ability to carry flyline is challenging due to wind unaided linespeed for control and to prevent grazing – more work. However, like a good carry where an unrolling loop travels over a longer line distance and longer time, a down wind cast presents a scenario where the rod leg travels allowing protracted unrolling of the fly leg over a greater distance down-field. If downwind = A, and calm/humid = B, then we really notice A does not equal B when distance casting. This idea may be wrong, but it’s a stab at why a calm soupy summer Florida morning is not a great time to expect a personal best distance cast.

Apologies for the document length - Richard Floggwater

[Paul Arden]

This is a really great question, Richard and welcome to the Board!!

It’s come up a few times over the years and there has been no definitive answers. It’s something I’ve thought about often and I really don’t understand.

I certainly follow your line of thinking here. I’ve cast indoors a few times and I’ve always found it difficult. We often talk about “dead air”. And definitely trying too hard results in a loss of technique.

However I sometimes cast here in breezy conditions. One would have expected even greater distances with the thinner air but I still struggle. I don’t think I have ever cast over 130’ here in any conditions. I’m happy if I throw in the low 120s, and sometimes the high one teens is all I can muster.

To my mind either there is something going on that we haven’t accounted for, or it’s the heat affecting the shooting ability of the fly lines. I have some tropical MED5 fly lines and while it helps I still don’t throw anything like as far as I do elsewhere. I agree that just being physical in humid conditions is a challenge. I bike indoors and it’s like bicycling in a sauna!! I certainly tire quicker here in a bucket of sweat.

Line carry is another interesting measurement for me. It’s less affected by weather conditions. Generally at sea level with a MED5 in England I can carry 95’ tops. This is the amount of flyline held to the hauling hand, excluding leader, for continuous casting cycles - not just one or two casts. I kind of lose it at that point. It’s right on the borderline of what I can do.

I’ve cast in Calgary where I spent a lot of fishing time. There I once measured my max carry as being ten feet further at 105’!! Calgary is 1000m above sea level… the line feels lighter, faster, more dynamic. In fact one trip I came down from West Yellowstone (2000m, also very long carry, even more dynamic casting!) and immediately afterwards gave a clinic in Denmark. I couldn’t cast!! The feel was totally different. It took me about 3 weeks to learn to cast again. The best I can explain it, is that it just felt heavier at sea level, and when we cast that subtle feel is obviously very important somewhere down in the subconscious.

Here in Malaysia I’m in high humidity, 250m above sea level and I can carry 92’ no problems (there is a video somewhere of that as a measured carry). I can probably peak at around 95’ but I haven’t measured this recently. One thing is for sure: I definitely can’t get to 105!!

Maybe a couple of air density charts would be useful here. Ie how does humidity compare to altitude?

Personally I find it quite bizarre, not simply that it’s a struggle, but it’s the opposite of what should happen.

Cheers, Paul

[Richard Floggwater]

Thanks for the valued input Paul! I believe you have mentioned in an email that you believe extremely humid conditions (above 90%) are puzzlingly harder to cast in. I find that type of humidity irritating to distance cast in, and locally those conditions happen with no wind. It’s interesting to me that you find carry less of an issue, indicative of your casting skills. I throw a lot of 7 and 8 wt outfits and still break out a T38 occasionally. During a calm, I really feel a difference continuing up the grain latter as though I need to input a certain amount effort for linespeed into false casting with a huge reprieve casting the 7 wt. This alone probably says something about my technique or lack there of. The T 38 is personally a great equalizer showing flaws, and occasionally is an exercise in masochism. Uniquely, also trying to keep 75-90 feet of an 8wt (SA;ED) in the air depending on conditions will also let you know work is being done in these conditions. It’s at these time a little bit of wind helps me a great deal.

[George C]

I don’t think you can make an assumption about air density based on humidity unless you account for differences in barometric pressure. Calm conditions are often associated with stable high pressure. Perhaps the high pressure effect exceeds the humidity effect and the air is actually denser.

[Richard Floggwater]

A point I have not considered. You are right about a high pressure and calm conditions correlation. Looking through some of the local meteorological information a typical summer July Florida day would have an average barometric pressure of 30.5 in Hg (around 775 mm Hg) that would fluctuate between 30.1 - 30.8 in Hg. I don’t know the significance or how trivial that fluctuation may be. I do know that the summer calm conditions are usually first thing in the morning where the air temperature and dew point converge resulting in high humidity (greater than 80%). Most settle for air conditioning and curse the fogging of their glasses if they where spectacles when walking outside in the morning as the windows are covered with condensation with an air temp already in the 80s (27-28C). The dew point would be in the mid 70s F. It’s under these summer conditions that a morning calm occurs. It’s like a tarpon fisherman’s prayer answered for calm water and hopefully happy rolling fish. Daytime heating to the 90s (32-34C) usually results in a sea breeze from the ocean refreshing the situation and often dropping the humidity to the 60-70% range. Casting in the morning calm or during the afternoon sea breeze conditions during the same day, I’d favor the later for a bunch or reasons. Those high humidity hot mornings I find the highest degree of distance casting difficulty. It’s rare in Florida to have humidity less than 40%, and when it happens it’s associated with a cold frontal passage and definitely high pressure. However there is usually some wind with the frontal weather and ironically- it’s these cooler days in Florida that I’ve cast my best distances. The barometric pressure then may exceed the July 30.5 in Hg.

LOL - Somebody has to point out, “Go fishing. Don’t distance cast when it’s a hot calm like that dough head”

[James9118]

I have another consideration that may (or may not) be relevant concerning the nature of drag in different airflows. Simplifying things somewhat, fluids (which air is for CFD concerns) move in laminar flows or turbulent flows (or more likely a mix of the two). Drag is higher in a laminar flow than a turbulent flow - the most quoted example of this is with racing cars, where once a car enters a straight and gets into the turbulent air coming of a car in front it is able to increase it's speed due to a reduction in drag (this same reduction in drag means that the following car cannot corner as quickly as drag also equates to downforce, which dominates in cornering).

I therefore wonder if on the days when our distance casts are going nowhere (relatively) that there is a greater proportion of laminar air movement than normal (it doesn't take much air movement on a macro scale to occur for the movement to be turbulent on a lower scale). It's also interesting to me that we describe these days as 'sticky' - perhaps there's more to that description than just the fact that sweat doesn't evaporate from our bodies?

James

[James9118]

I think what I've written above may be complete bollocks . Trying to find info about drag characteristics in a laminar flow versus a turbulent flow is difficult, most studies concentrate on what happens to the airflow around an object whilst starting with a laminar flow and perhaps separating in to a turbulent flow after the object has passed. Most references state that this separation leads to an increase in drag, however I'm not finding much written about where the incident airflow is turbulent as well as the airflow behind the object. So please ignore the above for the time being.

James

[Richard Floggwater]

Maybe not to look at a situation differently.

I’ve always found that casting in an open area with large fetch compared to a place with trees to break up wind patterns easier to cast in. The long fetch (the distance traveled by wind across open water or in this case a field) I always thought as more laminar in flow and gave me longer distance casts, where as with the the trees scenario a gust can cause turbulence; the wind can approach the caster in different directions (vectors). A sudden gust might cause a slight air movement into the casting arm that needs to be compensated for instantly with a decent carry. The closer to the trees - the greater the turbulence even with a possible lee provided by the trees. Distance casting is a slightly different beast than normal fishing distance casts which in my world would be 30-70 feet. I think many casters must intuitively adjust for alterations in wind direction by the millisecond. It’s probable that the longer fetch has a much greater complex dynamic than I have previously considered.

Sticky - yes! It would potentially mean more friction creating faster loop unrolling and less distance. Unfortunately, I have no meteorological/physics insight as to a mechanism. It certainly feels that way -

[James9118]

I do know from experience that there are some days where the drop in my ultimate casting distance seems to be totally out of proportion with the change in atmospheric conditions. I'd like to understand what was going on there also (those are the days I switch to practising accuracy out of frustration).

[Paul Arden]

Not just days but it can happen like a light switch too. Casting all day in Coggeshall mid-summer, and then just on evening, dew point and bang, lose 20’. Or the other way around; foggy morning, can’t see the end of the tape, don’t need to. Bit of wind swirls everything up and wallop, back to feeling like a distance caster again.

Maybe someone can explain fog and dew point for me and moreover why they are cast killers.

Thanks, Paul