With regard to the dispute over what exactly sleet is, I note that Dialect Notes, volume 4, part 4 (1916), devotes an entire page to a communication received from the chief of the U.S. Department of Agriculture’s Weather Bureau, which states:
Sir: There has been some discussion in this Bureau as to the way the term “sleet” should be used for official purposes. A search of dictionaries and of a large amount of technical and non-technical literature appears to establish the following facts:
(1) In England “sleet” means usually, though not invariably, a mixture of raindrops and snowflakes.
(2) In this country the term “sleet” has nearly always been applied in the meteorological literature to some form of water which is in a frozen state before reaching the ground; viz., either small particles of clear ice (often mingled with rain or snow), or little snow-like pellets, differing in structure from true hailstones, but often called “winter hail,” or “soft hail.” (In German the latter form of precipitation is commonly called Graupel, and this name is sometimes used in English texts. The French equivalent is grésil.)
(3) Non-meteorological usage in this country varies; comprising the uses noted above under (1) and (2), and also another, in accordance with which the term “sleet” is applied to a coating of ice on terrestrial objects formed by rain which freezes after contact with such objects. When this coating is heavy, and especially when it results in the breaking of branches, wires, etc., the phenomenon as a whole is often called an “ice storm.” This use of the term “sleet” is common in the newspapers, and also in engineering literature, particularly in reference to accumulations of ice, due to rain, on wires and rails. In England the specific name for this form of ice is usually “glazed frost,” and this term is used officially by the British Meteorological Office. The name “silver thaw” has also been applied to it, in both Great Britain and the United States, but this expression is so inappropriate and misleading that it is avoided by most scientific writers.
The Bureau will feel indebted to you for any information you may be able to supply as to the use or uses of the term “sleet” current in your vicinity, and also as to the meaning which, in your experience, most commonly attaches to the term in contemporary speech and literature. Information would also be appreciated concerning the etymology and history of the word “sleet,” in case you are able to add anything to what is found in the latest editions of the New English, Century, New International, and Standard Dictionaries.
C. F. Marvin, Chief of Bureau
United States Department of Agriculture, Weather Bureau
So in all likelihood conflicting definitions of sleet go back at least a hundred years.