The Terra Cotta Geysers are a group of five geysers on the southwest (left) side of the Firehole River, downstream from the boardwalk bridge. In the early days of the park (indeed until some time around 1980), it was permissible to approach these geysers closely on foot, but that is now prohibited and they must be viewed from the boardwalk. Terra Cotta A is the most active, although not the most powerful, of the group, reaching a height of 10 feet or so at intervals averaging about 2 hours.
Mike Keller made an in-depth, long-term study of the Terra Cotta Group that was reported in The GOSA Transactions, the Association's technical journal (M. Keller, "Terra Cotta Complex," The GOSA Transactions v. XI (2010), pp. 4-13). One of the interesting observations in that report was that eruptions of less active Terra Cotta B Geyser (q.v.) appear to delay Terra Cotta A eruptions by a significant time, yielding intervals as great as 5 hours. Another was that several smaller vents in the Terra Cotta Group -- Keller dubs them Terra Cotta F through M -- also erupt during eruptions of Terra Cotta A, to heights usually no more than a few inches, but in the case of Terra Cotta F, sometimes reaching 1 or 2 feet. He also reports that Terra Cotta F has occasional, inches-high eruptions when Terra Cotta A is overflowing but not actually in eruption.
|This geyser is visible from the boardwalk, but is too distant for precursors to eruptions to be easily observed. DO NOT leave the boardwalk for a better view! This is illegal and can get you in serious trouble!
This said, if you observe the Terra Cotta group from the boardwalk near Scalloped Spring, you might get the chance to see Terra Cotta A in overflow before an eruption. According to Keller, overflow usually begins 30 to 45 minutes before an eruption, unless it is "reset" by an eruption of Terra Cotta B. Interestingly, if A is not in overflow when B erupts, A's eruption may be "accelerated" rather than delayed, and according to Keller, A's eruption may ensue from a crater not even in overflow yet.