On back-to-back days last week, incredible hail storms bombarded two Argentine cities. The first unleashed massive hail stones, the size of softballs, the second dispensed hail practically to the height of grown adults in a mere quarter hour.

On Oct. 25, humongous hail — up to 9 centimeters (3.5 inches) in diameter — hammered the city of Formosa, about 750 miles north of Buenos Aires, on the border with Paraguay.

The next day, a furious onslaught of hail poured down on La Cruz, a town on the southern outskirts of Córdoba, which is about 440 miles west-northwest of Buenos Aires, in north-central Argentina.

La Nacion, an Argentine news outlet, reported that the hail piled up to nearly five feet — all in the course of 15 minutes.

Photographs on social media show cars stuck in hail drifts above their wheels and up to their hoods.

La Nacion wrote that so much hail fell that mechanical shovels were required for its removal.

Although the icy drifts appear like snow, they are not. The deposits of ice are the output of thunderstorms whose vigorous vertical motions allow bits of ice to coalesce in the clouds. Once they become too heavy for the air currents to sustain, they plunge to the ground as hail. They are a warm season, not a cold season, phenomenon.

Hail storms that deposit inches to feet of ice taking on the appearance of snow are not uncommon in the United States, particularly in Colorado during late spring and early summer.

How in the hail did this happen?

Both Argentine hail storms formed along boundaries of converging air between areas of high and low pressures. Thunderstorms erupted along these boundaries.

When the giant hail fell in Formosa, the amount of atmospheric instability was through the roof. A meteorological quantity known as convective available potential energy, a fancy way to describe the amount of fuel of available to thunderstorms, exceeded 4,000 Joules per kilogram — a very large number. This amount of energy is similar to what’s common in significant hail and tornado events in the southern and central United States.

The atmosphere was wasn’t quite as explosive the next day, when the complex of storms unloaded the towering drifts of hail in La Cruz, but the air was still plenty unstable.

While it is autumn in the North America, it is spring in Argentina, and that is when it experiences its most violent storms, much like the United States.

Weather systems all move the opposite way in South America compared to North America. So the low pressure system that pumped in the warm, unstable air to help trigger last week’s storms spun clockwise — drawing in air from the north, sourced from the equator. Over North America, this entire situation works in reverse.