A fly fishing thermometer is an integral piece of gear to me, I use just like the doctor uses it when you arrive in his or her office. Your temperature is taken and that can be an overall indicator of what is going on. A fishing thermometer is used very similarly. The water temperature is a primary indicator of fish activity, how active they may be, where they may be based upon how warm or cold the water is, what bugs are hatching, spawning time, etc.. For decades I have used thermometers to track many of our hatches on local waters, and consider an angler fishing the tailwaters and seeking to fish hatches to be at an extreme disadvantage if they do not own one. This thermometer is tough, housed in an aluminum cylinder, is 4 3/4" long, and has a temperature range of 20F to 120F (10C - 50C).