I have family members hell bent on the Chemtrail conspiracy and aircraft spraying stuff on us.

I’m trying to explain it’s to do with altitude, moisture and temperature.

But I need a bit of help. We’re from Brisbane, Australia and we’re visiting Porto, Portugal. Porto has much longer and larger spread contrails than we have in Brisbane.

So my argument of “You get longer and wider contrails due to more moisture in the air” is failing on me. Brisbane (at sea level) has a lot more humidity than Porto.

So why are the contrails much longer and fatter and longer lasting in a less humid area like Porto - than Brisbane?

I’m guessing maybe the humidity is felt more in Brisbane as the moisture higher up - is lower? And up there - the humidity is somehow less?0

  • 1
    $\begingroup$ do you have actually more contrails over Porto than Brisbane? have you kept a log? what is your observation period? have you kept track of the meteorological details of your observations? do you have high-altitude data, and not only ground surface level data? have you cross-verified with sites like fr24.com or similar to verify that is not simply a case of "there's more flights when we look up"? (Europe tends to have a bit more flights in the air than Australia) $\endgroup$
    – Federico
    Dec 5, 2022 at 11:03
  • 2
    $\begingroup$ There's no point in trying to reason with a conspiracy theorist, they won't accept evidence. There are so many factors, one would be that there are just so many more flights over Porto than Brisbane, more flights are at lower altitudes as they are shorter hops - lower means they look bigger, etc. Save your breath. $\endgroup$
    – GdD
    Dec 5, 2022 at 11:25
  • 2
    $\begingroup$ That's not what I was saying at all @sophit. Craig says some of his family are, and he's trying to clue-bat them. I'm saying to him that it's a futile effort. They either believe it in which case no amount of logic will win out, or they don't and are using it as a way to be contrary or get attention, the best way to deal with either is just ignore it. $\endgroup$
    – GdD
    Dec 5, 2022 at 21:03
  • $\begingroup$ @GdD: oops sorry, I really missed the very first sentence of the question 🙈 Anyway I agree with you... $\endgroup$
    – sophit
    Dec 5, 2022 at 22:03
  • $\begingroup$ They actually do spray more stuff in Portugal than in Australia, that's why life's expectany is shorter in Portugal... $\endgroup$
    – sophit
    Dec 5, 2022 at 22:17

1 Answer 1


Humidity varies strongly with altitude and prevailing winds aloft which can be opposite winds at sea level. Of course, temperature matters too because to pull a contrail you need sub-freezing temperatures.

For the best contrails you need freezing air at altitude with as much water vapor already in it as possible. Then the water vapor in the jet exhaust will form ice crystals promptly and they will persist a long time before sublimating into invisibility.

The best way to get humid air is to blow it across a warm ocean for a few thousand miles but then you need vertical mixing to get that humidity up to 30,000 feet; the best way to get dry air is to blow it the same distance across a desert and then figure out how to mix it up to 30,000 feet too.

All these factors will make it very difficult to predict where to find the best contrails!

  • $\begingroup$ Thanks @niels. That kinda makes sense. What I’m. It getting though, is Porto is cold and wet. We have the cold Atlantic to the west. Sea level humidity is low. Day time temps at sea level are between 6 and 15 degrees. And yet I’m seeing long and spreading trails. In Brisbane, humidity is very high and we have the warmer pacific on our east. So would that not mean high temps at 30k. It’s always sun zero up there. Between -30 and -50 I think. With the sea level humidity high - I’d expect high level humidity to be high. Is that maybe false? There is MUCH more traffic over Porto though. $\endgroup$
    – Craig
    Dec 6, 2022 at 16:02
  • $\begingroup$ conditions at sea level are very different than at 30k feet, and are a poor indicator of what's going on "upstairs" at those altitudes the ambient temperature is dominated by the altitude effect and surface temps matter less. $\endgroup$ Dec 6, 2022 at 17:17

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .