[A humble request: If you can, please donate to help me carry on tracing word histories. Thank you.]

Of British-English origin, the phrase clerk of the weather (also clerk of the weather office), usually preceded by the definite article the, designates an imaginary functionary humorously supposed to control the state of the weather.

This phrase occurs, for example, in the following from No time for nameless, angry online cowards, by John Light, published in the Gloucestershire Gazette (Stroud, Gloucestershire, England) of Thursday 4^th July 2019 [page 32, column 4]:

I end with a huge apology.
Recently I posed the question, are the Cotswolds getting drier?
All evidence pointed to the fact they were.
Since then it has been extremely wet, with the clerk of the weather playing catch-up.

These are, in chronological order, the earliest occurrences that I have found of the phrase clerk of the weather (also clerk of the weather office):

1-: From The Gossiper. No. XVI. The humble Petition of the Umbrella to Mr. Gossiper, published in The Lady’s Monthly Museum; Or, Polite Repository of Amusement and Instruction; Being an Assemblage of whatever can tend to please the Fancy, interest the Mind, or exalt the Character of the British Fair (London, England) of October 1812 [page 207]:

Sir,
[…]
[…] Your petitioner formerly enjoyed the favour of the fair sex in a far more extensive and intimate degree than at present; a lady was then seldom abroad unaccompanied by your petitioner, who was then permitted to feel the cheering beams of the sun, which happiness he is now entirely deprived of. Your petitioner also at that time was allowed to attend divine service with his mistress, not as now-a-days only permitted to be present on wet days […].
But what your petitioner has most seriously to lament, and to request redress for, is, that an impertinent spruce coxcomb, yclept Parasol, has engrossed almost entirely the favour and protection of the fair sex, to the exclusion of your petitioner […]. Moreover, your petitioner is not exempted from any of the labour he formerly performed, save and except that of guarding the beautiful faces of the ladies from the rays of the sun, which employment constituted the principal happiness of your said petitioner.
[…]
Umbrella.
The Gossiper is sorry to inform Umbrella he can do but little for him; he has passed his petition to the proper office, and the clerk of the weather has promised to attend to it.

2-: From Private Letter from Dublin.—No. 8., dated Saturday 1^st September 1821, an unsigned letter published in The Statesman (London, England) of Friday 7^th September 1821 [page 2, column 3]—the Curragh, in County Kildare, Ireland, is the home of Irish horseracing; the phrase clerk of the course designates an official on a racecourse:

The last, my dear Pat, apprised you of the pelting rain that prevented his Majesty from visiting the Curragh on Wednesday last, and his postponement to Friday, notwithstanding the Clerk of the Course had every thing in prime order for the day’s sport. The clerk of the weather seemed to tell the King it should not be all sun-shine even in Ireland with him, which is a clear proof that we should not make up our minds with too much ardour, or we may be as much disappointed as his Majesty was in a fine day.

3-: From a letter to the Editor, by one John Williams, published in Felix Farley’s Bristol Journal (Bristol, England) of Saturday 24^th November 1821 [page 3, column 2]—John Williams was reacting to a letter by ‘Viator’:

Viator thinks it quite requisite that a printed paper should be put up, stating the different periods when the Boats are to cross; now as he must be aware this cannot be effected, unless we could command the wind and tide, he most likely could devise some means by which a communication might be gained with the clerk of the weather office.

4-: From Boxing at Moulsey Hurst. Between Dick Defoe and Reuben Martin, for 501. a-side, on Tuesday last, published in Pierce Egan’s Life in London, and Sporting Guide (London, England) of Sunday 21^st March 1824 [page 64, column 2]:

The morning was fine, and the road had a respectable appearance; lots of genteel members of society in the middling walks of life; a few tumblers full of commoners, enjoying all the happiness of rude health, laughing at the smiles and frowns of Fortune; not caring a fig whether the bright beams of Sol deigned to shine upon their careless mugs; asking of no favours from the clerk of the weather to keep off “the pitiless pelting storm,” as their greasy jackets were proof against all watery attacks; and as to the effects of Old Boreas “blowing up” to commoners, was no go!

5-: From a review of The Impracticability of a North-West Passage for Ships impartially considered (London, 1824), by ‘Scrutator’—review published in The Westminster Review (London, England) of April 1825 [page 540]:

It is no secret, that the Reviewer in the Quarterly, who generally writes upon the subject of the Polar voyages, is famous (we had almost said infamous) for the scantiness of his information and the short-sightedness of his views—we allude to his official station. […] He […] declares, “the solution of this important problem is the business of three months out and home!!,” without doubt to be performed in the flying Dutchman, for no other vessel could have accomplished it in the time, even supposing the passage perfectly free. The masses of ice are made to dissolve, by the harlequin flourish of the Reviewer’s pen, and the worthy secretary speaks as confidently as if he were also what seamen term “a clerk of the weather.”

6-: From Milling Correspondence, published in Pierce Egan’s Life in London, and Sporting Guide (London, England) of Sunday 28^th August 1825 [page 245, column 2]:

Pᴀʀᴛɪᴀʟɪᴛʏ Cʟᴜʙ.—This spirited Society, it appears, is under sailing orders, and will put to sea, full of fun and ꜰᴀɴᴄʏ on the 5^th of September, with a rare squad of TUFF Ones; who laugh at the rude elements, and who intend to give the Clerk of the Weather a holiday on that day, having made it “all right.”