How do we know when and where to catch a hatch?

Answers to this question take up many thousands of pages in the angling literature, on websites, and in hatch charts scrawled on blackboards in fly shops. Most are well-done and helpful, but even their authors acknowledge that nature does not always comply with such rigid descriptions.

We can use all this information more successfully if we understand the ways nature changes her playbook season by season, day by day, and hour by hour. Many of the changes are predictable if we know what to look for. Some books pepper the proper conditions and caveats throughout, but they don't gather these nuggets of wisdom into a single big picture. That is the intent of this article.

Read on to learn when, why, and how to expect the unexpected.