See "(Entry 4 of 5)", the second noun definition of spell [1]. "an indeterminate period of time" or "a stretch of a specified type of weather" as in, "we've ha

See "(Entry 4 of 5)", the second noun definition of spell [1]. "an indeterminate period of time" or "a stretch of a specified type of weather" as in, "we've had a long dry spell; I hope it rains soon". It does not sound particularly strange or unscientific to me. [1] https://www.merriam-webster.com/dictionary/spell https://www.merriam-webster.com/dictionary/spell