Since this is Canada, with its infinite range of weather conditions, it's possible that some parts of the country have been afflicted with temperatures cold enough to freeze the freckles off Anne of Green Gables, in which case forgive what follows. For those who have been sweltering and perspiring because of the humidity, this is a brief look at why we speak of sweltering, perspiring and humidity.
Sweltering is easy enough. As noted in this column a while back, it derives from death. The word can be traced back to an Indo-European root ( swel) that meant to burn, but the association between burning and cremation ensured that by the time Beowulf and other Old English writings absorbed the verb sweltan from Germanic languages some time before 1000, it meant to die. Because English soon found itself up to its neck in synonyms for dying and death, it reassigned a few of them. Thus, by the 1300s, swelter meant to be overcome by the heat.
Swel, by the way, has nothing to do with swell, since neither sweltering nor dying has ever been particularly, well, swell. The sense of swell as terrific was a 20th-century twist on the 19th-century sense of swell meaning stylish or elegant, which spun off from the 18th-century noun for an elegantly dressed person of some distinction (he's a real swell), which was more positive than the sense earlier that century of arrogance and pomposity, which derived from the original sense of being swollen or distended, as a pompous person's chest might be.
It is best to keep a sense of humour about the humidity, since both words come from the same place. Humidity derives by way of French humide from the Latin humere, to be moist. From humere came the Latin word humorem, which referred to moisture or a liquid. Medieval practitioners supposed that a person's health and mood were directly related to the interaction of his or her blood, phlegm, bile (for melancholy) and choler (anger), and referred to them as the four humours. English picked up the word in that sense from the Anglo-Norman humour. Eventually, the sense of mood evolved into one of whimsy, and by the 1700s, humour described something that made people laugh. We still use "humour" to mean liquid when referring to the aqueous and vitreous humours in the eyeball, but the terms don't get much traction outside ophthalmological circles. (And no, those aren't circles under the eyes.)
Having been trained since childhood to say that horses sweat but humans perspire, I shall note first that perspiration comes from the French perspirer and the Latin perspirare, consisting of per (through) and spirare (breathe). Sweat is the Germanic alternative, which in Old English was spelled swat (when a noun) and sweaten (when a verb). It shares an Indo-European root with the Latin sudor, which meant sweat or, if you prefer, perspiration. From sudor we get the word for conveying sweat, as in the sudoriferous canal. I believe that is in the body. It might also be an offshoot of the Thames, and a dubious choice for a day of rowing.
The sun, which is implicated in the recent heat, is also of Germanic origin by way of the Old English sunne. It shares an Indo-European root with the Latin sol and the Greek helios, both meaning sun. Should you visit an amusement park to make the most of the heat, and should you purchase a helium-filled balloon, you may wish to know that helium derives from helios, since it was by analyzing the sun's spectrum that scientists first suspected that helium existed.
It is tempting in hellishly hot weather to assume that hell has something to do with helios, but it doesn't. Hell goes back to a Germanic base meaning to cover or conceal, which is good advice when going out in the sun. Wear sunglasses over your aqueous and vitreous humours.