The Sun is quite a noisy radio transmitter across a very wide range of frequencies. Downside of having a big, hot unshielded fusion reactor in the sky.

> How and why, that doesn't make sense at all?

Not sure. I remember reading an article some time ago that stated the Sun's electromagnetic radiation could interfere with Bluetooth and it's always worked with my observations. I've used multiple Plantronics headsets, Motorola headphones and an off brand of headphones all with an iPhone 5S, iPhone 6+, Nexus 6P, Moto X and a Note 3 and in all of these devices they work great indoors and going outside on a sunny day it would break up (but cloudy always seemed to work better).

It's entirely possible this is some crazy bullshit but my observations are real. So if it ends up being that the Sun does NOT provide interfere then it must be SOMETHING. I just had this issue this past week when I was working outside and had to grab my wired headphones.

Another, as likely, aspect, is that Bluetooth is designed around signals bouncing off surfaces. Try moving your phone from one side of the body to the other while outside. It may well be that your surroundings do not provide enough signal reflection. Keep in mind that most mobile bluetooth radios do not transmit with the same strength as wifi. Best i can tell, only USB dongles use a wifi equivalent radio class.

Interesting and sounds reasonable. I wonder if Bluetooth 5 will help in this regard.

I guess it could be possible. But the thing about range is that is most often reported under ideal conditions. Meaning clear line of sight between antennas and little to no other transmissions in that frequency range.

And likely your best results will be if both devices are v5 devices.