"The historical record says that the initial wave of molasses moved at 35 miles per hour... which sounds outrageously fast."

"At the time people thought there must have been an explosion in the tank, initially, to cause the molasses to move that fast," said Nicole Sharp, an aerospace engineer and science communications expert, who worked on a study of the Boston molasses flood of 1919. A 40-foot wall of molasses killed 21 and injured 150 more were left injured. No one had previously worked out the equations, accounting for the viscosity of molasses in cold weather.

If the tank had burst in warmer weather, it would have “flowed farther, but also thinner,” Mr. Rubinstein said. In the winter, however, after the initial burst — which lasted between 30 seconds and a few minutes, Ms. Sharp said — the cooler temperature of the outside air raised the viscosity of the molasses, essentially trapping people who had not been able to escape the wave.