You’ve Been Warned
Carbon monoxide and other toxic gasses are among the many occupational dangers to coal miners, and the practice of bringing canaries into coal mines persisted from 1911 to 1986. If the bird suddenly got sick or died, it suggested to the miners that a deadly gas was present, and they should get out, hence the metaphor “canary in the coal mine” as an early indicator. Animals that serve an environmental warning role to humans are sometimes called “sentinel species.”
The old saying “Red sky at night, sailor’s delight; red sky in the morning, sailor’s warning” is a very old one, and variants appear in Shakespeare and the Bible. Since this red color is attributable to water vapor or dust splitting sun rays into their color spectrums when they pass sideways through the most atmosphere, color hints at the atmosphere’s contents at these times, which play a big role in weather. However, the saying is most accurate in the middle latitudes when weather systems move from west to east.
Talking about a “shot across the bow” is similar to saying “a warning shot,” or a warning gesture to show that you’re serious, even about using force. In the nautical sense, this means deliberately firing in front of another vessel, sometimes forcing that ship to change course or stop. This gesture has also been used to signal to an unknown ship to fly its flag, but in the modern time these efforts at identification also include attempts at radio contact.