IN MANY PARTS OF THE U.S., IT'S NOT UNCOMMON for a summery afternoon to transform into a below-zero night. But no place has felt a greater 24-hour change in the mercury than Loma, Montana. Between January 14 and 15, 1972, thermometers surged from -54 to 49 degrees Fahrenheit in a record-breaking 103-degree swing. In fact, that entire month in Loma and its nearest neighboring city, Great Falls, serves as an exemplar of the sudden temperature spikes and drops that can happen anywhere. The cause: dueling blasts of air.