There however a lot of different types of nylon polymers which result in different sounds. This makes that the sound can vary from brand to brand. Low-budget strings are produced through extrusion, i.e. pushing molten nylon polymer through holes with specific diameters. The same process is used for more expensive strings, but after the extrusion they are ground to produce a more consistent diameter. Nylon will stretch over time, so you might need to retune your ukulele. It is not effected by humidity, put temperature changes can cause the strings to stretch more or less. The most known brands for providing quality nylon ukulele strings are Aquila (of course) . They offer good stability, durability and a warm traditional ukulele sound.