Estimated? That was the best most could do. Mercury, the basis of the majority of thermometers at that time, freezes at 39 degrees. Young lads who had access to “quicksilver” found it entertaining to play marbles with frozen blobs of the stuff.*
Some intrepid weather trackers had spirit thermometers, which are alcohol based. One in Jordon Valley recorded 54 below.
Hundreds of cattle died along the Snake River. In Emmett, W.L. Fuller went out to milk his cow and found her frozen stiff, still standing.
The Idaho City World waxed poetic: “The kids of this place will regale the young generation of future years with tales of the awful cold weather in Boise County way back in ’88, and tell how the spirit thermometer registered down to the last degree, 99 below, and mercury froze on a red hot stove, and the kids played marbles with quicksilver.”
*Don’t try this at home. Mercury is highly toxic and handling is dangerous. Every. Single. One. Of those lads who played with quicksilver in 1888 is dead today.