Why are the horse latitudes called dead air?

The horse latitudes are called "dead air" because they are regions of high pressure located about 30 degrees north and south of the equator, where the trade winds converge and often result in calm winds. Sailors historically found themselves stranded in these areas with little to no wind to fill their sails, leading to ships being trapped for extended periods. The term is also thought to originate from the practice of sailors throwing overboard horses that they could no longer feed during these stagnant periods.