How periodical cicada broods work

Periodical cicadas spend either 13 or 17 years underground as nymphs, then emerge together in huge numbers. Each synchronized population is a brood, numbered with Roman numerals (Brood I–XXII for 17-year, XIX, XXII, XXIII for 13-year).

• Because the cycle is fixed, the emergence years are predictable decades ahead — this site lists each brood's next year and the states it covers.
• The exact timing within the year isn't fixed: adults emerge once the soil ~20 cm down warms to about 18°C (64°F), usually mid-to-late spring.
• Some years feature rare dual emergences (e.g. 2024's Broods XIX + XIII) that draw national attention.

Brood years and ranges from the USDA Forest Service brood maps and the University of Connecticut cicada project (public data), linked on each brood page.