Can we call it a dry spell?
Based on the OED, it looks like the term "dry spell" originally meant an extended period of time without rain, and has since been figuratively extended to mean a period of time without something substantive or relieving.
The earliest attestation provided is from 1887:
Everybody found smoking on the streets..during the dry spell was liable to be arrested.
- The Boston Journal
However, the definition in OED draws attention to a 1920 attestation in a note, suggesting that it may have relevance to the term's origin or early meaning.
A Dry Spell is a period of fifteen or more consecutive days no one of which is a ‘Wet Day’.
- British Rainfall 1919 · 1920.
Particularly because the OED drew attention to this citation in a note, I wonder if the term originally had a far more technical meaning than we use today, referring to "fifteen days" in particular. Is this quote ascribing a unique and narrowly used meaning to the term "dry spell," or did it originally have such a technical meaning limited to "fifteen or more consecutive days?"
Bonus points if someone can enlighten me on the historical usages of "wet day" in their answer.
Solution 1:
A spell is the opposite of a fixed period of time— (MW)
3a. an indeterminate period of time; also a continuous period of time
b. a stretch of a specified type of weather
If my elderly neighbor asks me to come sit for a spell, I don't know whether she intends to keep me for two minutes or two hours.
As you know from the OED, this meaning derives from an old verb sense of spell meaning
To take the place of (a person) at some work or labour; to relieve (another) by taking a turn at work
which is said to persist in Australian and U.S. usage. This, in turn, is traced to spele, a rare or dialectical word meaning to take or stand in the place of (another); to represent.
This in turn is from Old English of obscure origin, but believed to be related to gespelia and spala; a spale was a sparing, respite or rest.
The use of spell to refer to a period of weather is attested from 1728—
1728 T. Smith Jrnl. (1849) 265 For several days past, there has been a spell of comfortable weather.
— and paired with adjectives from 1740 in the same source.
I believe no man ever knew so winter-like a spell so early in the year.
While dry spell comes late, Thomas Jefferson did write in 1797
You wish to know the state of the air here during the late cold spell.
The assignment of spell to a fixed period is almost certainly a modern invention for use within a specific domain, just as geologists began to use the term eon (æon) to refer specifically to a period of a billion years, while physicists adopted terms like shake and jiffy for very short times. In general usage, those terms remain inexact, and only represent fixed values when used as terms of art. It helps avoid prosaic terminology like heating degree days or survey townships, even as we abandon colorful measurements like the oxgang or the fistmele.
Solution 2:
"dry spell"--From 1915 Report of the Board of Trustees of the Agricultural College of Pennsylvania:
Also used in this 1889 report from the U.S. Department of Agriculture
"wet day", 1884, from Among the Clods: or phases of farm life as seen by the town mouse British--wet day
Solution 3:
The full text of the relevant passage of the reference cited by the OED answers your question:
"A Wet Day is a day ending at 9 h. (G. M. T.) on which 1 mm., or .04 in., or more, of rain is recorded. A Dry Spell is a period of fifteen or more consecutive days, no one of which is a "Wed Day" according to the foregoing definition. A Wet Spell is a period of fifteen or more consecutive days, each one of which is a "Wet Day" according to the foregoing definition."
British Rainfall, 1919, Part II, page 15.
But remember, this is a technical definition for a bureaucratic purpose, and not necessarily THE definition as understood by normal people who had been using the expression for decades at the time.
The expression was used as early as the 1830s.
"Recently the forms have fallen off very much owing to a heavy rain immediately succeeding a dry spell . . . ." Richmond Weekly Palladium (Richmond, Indiana), September 28, 1833, page 2.
"But alas for his reputation, a "dry spell of weather," to use the phrase of the venerable dogberry of the New York Gazette, succeeded the rainy season . . . ." The New York Evening Post, September 2, 1834, page 2.
The "spell" in "dry spell" is the noun sense of "spell," meaning a period of rest, which is not so technical. At the end of the theme song for the Beverly Hillbillies, for example, they say, "Sit a spell". Etymonline dates this use to 1845, but given the even earlier appearance of "dry spell," I would imagine that the sense was also used earlier.
A dry "spell" is simply a dry period of rest between rains.
And, as an aside, before I looked up the British Rainfall reference, my initial impulse was to read "wet day" as a reference to a day in which someone drank alcohol, a "dry day" being a day that they did not. And that expression was, in fact used, particularly during prohibition that started in the United States in 1920.
Solution 4:
The earliest use of 'dry spell' I can find, from a letter dated April 23, 1732, is relative; no standard definition is expressed or implied:
John Gunthorp to Abraham Redwood
Antigua, April the 23th, 1732Deare Abraham,
I should have wrote to you by several oppertunities since my arrival here, if my indisposition had not prevented me, which continued very severely upon me ever since I left you till about fourteene days agoe, from which time I have beene upon the recoverie and am now in hopes I shall live to see you again; We have had an exceeding dry spell of weather from November till the begining of this month, but thank God now its as favourable as we can desire:
Collections of the Masschusetts Historical Society: Seventh Series — Vol. IX, "Commerce of Rhode Island 1726 – 1800: Vol. I, 1726 – 1774"
This expresses definitely only that the weather in Antigua ("here"), as subjectively perceived by an 'indisposed' John Gunthorp, was drier than usual from sometime in November (or possibly October) until sometime toward the beginning of April, 1732.
The more roundabout collocation, 'spell of dry weather', with a sense as indefinite as that conveyed by the shorter collocation in the 1732 citation (foregoing), is in use at least as early as 1718,
...for such a long spell of dry weather has not been known in the memory of man...Read 18th June, 1718....
Calendar of State Papers, Colonial Series, August 1717 – Dec. 1718, 1930
notwithstanding the 1728 dates of the earliest citations for definitions 4a and 5a of spell, n.3 in OED (entry not fully updated since OED2, 1989),
4. a. A period or space of time of indefinite length; usually with adjs. denoting duration, as long, short, etc.
....
5. a. A continuous period or stretch of a specified kind of weather.
Regarding 'wet day', the earliest uses were almost certainly indefinite. The earliest use I found was this from a 1660 publication of The History of the Propagation & Improvement of Vegetables by the Concurrence of Art and Nature, by Robert Sharrock:
Mossiness of Trees, comes generally either from the barrenness or the coldness of the ground, and therefore I count it vain to attempt the removal of it, without taking away the cause, and making the ground better; which being done, it will be proper enough to rub down the trees in a wet day with an hair cloath.