When you think about it, it's kind of funny or maybe amazing that such a seemingly simple part of a fly fisher's gear, the line, is so important. The right line size impacts just about everything: how it loads the rod, the cast, the kind of fish, and how the fly is presented. For many fly fishers, using the right line size is about more than the manufacturer's instructions. Adapting line weight to various weather conditions, type of fish, size of the fly, and the water can make a big difference.
Fly lines size numbers have to do with weight- the higher the number, the heavier the line. Heavier lines are used with larger flies and fish, and lighter lines are used for smaller flies and fish. There are some handy charts out there to help figure out which line size they should be using.
Generally, the heavier the weight of line the greater the line speed so it is a good idea to take that into account. It will often have more impact as it hits the water as well. There are times when a more delicate hand is desired to not scare the beans out of the fish with the line slapping the water.
Wind and temperature also play into determining appropriate line weight. Windy conditions can make casting more difficult and casters don't always agree whether a lighter or heavier line is more efficient in these conditions.
That actually brings up an important point. A lot of a fly fisher's "success" (even that is measured subjectively) depends on their style and their skill level. What may work perfectly for one may not work at all for another. It is a great idea to talk to people who have experience and have tried different things and have found what works for them. Go to a local fly shop or ask questions in online forums. Another great resource is the smaller online fly fishing retailers. Often the smaller companies will take the extra time to answer your questions and give advice.
So while it is good to have some general guidelines regarding line size, it is also beneficial to get lots of good advice (even when it is contradictory) and experiment with what will work best for you. It is encouraging and perhaps a little overwhelming to know that with a simple line switch, everything can change (for better or for worse.)
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