VAPOR TRAILS - A Mystery to Me?

[Jeff D. Welker]

One of the many reasons I enjoy shooting at LAX is the opportunity to capture aircraft producing wonderful vapor trails. While I've had good luck on arrivals in the early morning on some visits, I've come up blank on other trips. As I ponder another trip to LAX, I wondered at how predictable "vapor trails" are?

Is it a daily occurrence?

Are certain times of the year better than others?

Are certain times of the day better than others?

Are certain weather conditions more conducive than others?

I'm hoping our members who are veteran shooters at LAX (i.e. Eric) can educate this old man. There is nothing like a DC-10 or A-380 squeezing the water out of the air on approach to RWY 24R. I need more cowbell [vapor]  

[Dave S.]

Ditto to Jeffe's request on when are condition best for squeezing water out of the thick air in LA.

[KeithS]

While I have not Googled the subject, I believe that a couple of factors are humidity (dew point?) and wing angle of attack.  During the recent monsoon activity, I saw several of the F35s making small vapor trails off the wing tips while in the landing pattern at Luke.  I have never seen this with the 16s or 15s at Luke.

[Dave S.]

Jeffe,

I went back to the photos of that trip last July.  Looks like the good, quality vapor was over by ~ 8:00.  Based on the historical weather data, 90% humidity is roughly our key metric for good vapor.  Of course there are other contributing factors (wing loading is one important factor that comes to mind), but those are beyond my ability to easily obtain.  I've found a website that has the historical weather data, in hourly increments, and I'm currently waiting to download that data.  Searching as far back as the beginning of 2012 and will look for days/time of year where >90% humidity occurs most often.

[Jeff D. Welker]

I agree with the notion that certain platforms produce vapor better than others. I would call the A-380 king with high marks for other heavies like the 747 and DC-10/MD-11. I also think certain wing designs squeeze water better than others. Many of the A2A photos I've seen of the F-35 would indicate it is a vapor-producing machine. While I'm ignorant of wing design/shape, I've seen photos of the F-35 producing vapor while it is flying next to a AV-8B that is not producing vapor. The wing root on the F-35 seems to produce vapor regardless of location or humidity.

I also agree with Dave that I've got my best vapor shots early in the morning. That being said, when Matt and I were at LAX in late December, there was little early morning vapor - go figure. I've also seen some great images after sunset where vapor trails are present. Just the other day, Joe made me aware of a killer night image of a 777 landing at LAX 24R that is pumping out some vapor - http://www.airliners.net/photo/Cathay-Pacific-Airways/Boeing-777-367/ER/2505950/L/. Another talented shooter I am acquainted with also caught some great night vapor at LAX - http://www.vortexaviationphotography.com/Civil-Aviation-Photography/Los-Angeles-2013/i-P9MfwFK/A.

I've also noticed that the vapor increases/decreases depending on where the arriving aircraft is relative to the EOR. For example, the massive vapor images we got on the A-380 occurred when it first appeared visually to the east. As it approached the vapor production decreased and almost disappeared as it crossed Sepulveda and then increased again as it crossed the piano keys prior to smoking the mains. I'm not sure if this is a function of air speed combined with AGL, or different pockets of water vapor/marine layer, or differences in ambient temperature?

Lastly, I've also noted that you can get vapor forming in the inlet of the turbo fans as they accelerate on take off. I can only assume this is a function of the compression squeezing water out of the air as it is sucked into the engine.

I guess where I'm headed is there a reasonable way for shooters (i.e. me) to predict vapor opportunities at LAX, or is it simply a roll of the dice?

[Chris V]

I think a big contributor to the vapor in the AM is the marine layer that finds its way inland over night and retreats back out to sea as the sun burns it off in the morning. Watching / reading local weather and surf reports can give a good idea of what to expect. Cooler temps I think also help?

[Scott Youmans]

The dew point and related relative humidity are definitely the major factor in predicting good conditions.  Dew point is the more reliable one to use since it stays more constant over time whereas relative humidity varies with temperature as the day progresses even as the dew point stays constant.  Higher air temperature=lower relative humidity assuming the same dew point.  When the air temperature is equal to the dew point the humidity is 100%.  So if the dew point is say 65 degrees and you know the morning temperature is going to be close to that you can be fairly certain that visible vortices will form at the points where the local velocity of the air is very high.  Ends of flaps are an obvious example as the air pressure under the flap is considerably higher than the pressure on the upper surface.  Consequently the air goes whipping around the tip from high to low pressure and forms vortices.  High velocity equates to low pressure and when the pressure is low the air can hold less moisture.  Since it's already nearly saturated (temp close to the dew point/high relative humidity) the water condenses into vapor (tiny visible drops).  Tornados are visible for the same reason.  Same thing on the upper wing surface to a lesser degree but over a large area.  Wingtips and leading edge root extensions are all sources of high velocity\low pressure vortices.  As we all know it gets colder as you get higher in altitude.  When you get high enough that the temperature drops to the dew point you will typically start to see clouds forming.

[Jeff D. Welker]

The last few times I have been out to LAX there has bee zip for vapor in the mornings or evenings as it is hot out here now.  

It is not a daily occurrence unless the conditions are right for it.  When Jay was out here earlier this year the one evening shoot from the hill yielded some vapor o a departing A380, B777 and A340 and a few other.  There was a marine layer off the coast and it was drifting inland.  That raised the relative humidity and dew point high enough to cause vapor on the wings and inside the nacelles on takeoff roll.  However the next evening shoot there was no vapor.  

I have not checked conditions before heading out to LAX for shooting.  There have been days with large amounts of vapor on the wings at landing and on departure, while other days you just see the streamers from the wingtips or flaps on approach.  

Hey Eric:

I appreciate your sharing your local knowledge and experience. Since it appears the marine layer plays a significant role in the amount of vapor present, have you found that there are different times of year that are better than others? A very quick check of the weather for the past twelve months would seem to indicate that (on average) the cooler months of the year had higher levels of humidity. A nimrod like me would conclude that winter months would provide better opportunities for vapor.

Conversely, I found an article on the marine layer/fog around the LA basin (http://www.theweatherprediction.com/weatherpapers/109/index.html) and it says this situation is most common May through the middle of October - the hotter/dryer months. While that doesn't make sense to me, it would support the very limited experience I had with lots of vapor on a July visit and very little in December.

What have you experienced on a seasonal basis?

[Scott Youmans]

Technical correction/clarification to earlier rambling. I incorrectly used the word "vapor" to describe the visible tiny water drops we see in the vortices. Water vapor is in fact invisible. When it condenses, as described, it becomes visible as tiny drops, such as clouds or fog. "Vapor trail" therefore is in an incorrect description.  We've probably all noticed that the exhaust plumes behind airliners at high altitude are invisible for some distance behind the engines's tailpipe. Only when the exhaust (which contains water vapor) has cooled to the dew point does the vapor condense and become visible.  Part of the combustion process yields H2O, thus the vapor.  Same reason we see water dripping out of car exhaust pipes especially on cool days when the engine is still warming up.  Bla bla bla...

[Jeff D. Welker]

Technical correction/clarification to earlier rambling. I incorrectly used the word "vapor" to describe the visible tiny water drops we see in the vortices. Water vapor is in fact invisible. When it condenses, as described, it becomes visible as tiny drops, such as clouds or fog. "Vapor trail" therefore is in an incorrect description.  We've probably all noticed that the exhaust plumes behind airliners at high altitude are invisible for some distance behind the engines's tailpipe. Only when the exhaust (which contains water vapor) has cooled to the dew point does the vapor condense and become visible.  Part of the combustion process yields H2O, thus the vapor.  Same reason we see water dripping out of car exhaust pipes especially on cool days when the engine is still warming up.  Bla bla bla...

Please tell me there is not going to be a test following this discussion

Thanks for your input Scott. We need to do lunch next week so you can explain this in terms a graduate of the Mesa Public School system can understand (i.e. pig latin)