CHICAGO  For decades, wheat was a commodity no American needed to think much about, except the farmers who grew it. The grain was usually plentiful and prices were low.

All of a sudden, those assumptions have been turned upside down. With demand soaring abroad and droughts crimping supply, the world’s wheat stockpiles have fallen to their lowest level in 30 years, and stocks in the United States have dropped to levels unseen since 1948.

Prices have been gyrating in recent days as traders tried to figure out what to make of the situation. On Tuesday, prices for a sought-after variety, spring wheat, jumped to $16.73 a bushel on the Minneapolis Grain Exchange, the latest of several records.

Prices for common wheat are up nearly 50 percent since August, and they are up even more for the most sought-after varieties, leaving buyers, growers and longtime commodity traders shaking their heads.