Exits from Cambridge will be tough for vacationing students, especially if they plan to travel by air or automobile, weather reports indicated last night.

The United States Weather Bureau at Logan Airport said a heavy snow storm was forming in the mid-west and was bearing down on the New England area. Logan said the storm would effectively reduce air travel to the mid-west and would hamper cars severely.

No predictions were available yet about routes south.

The Associated Press said temperatures last night were below freezing over most of the northern states from the western mountains to the Atlantic.

Snow fell yesterday over the northern plains, northern Rockies, and many parts of New England. Heavy fall was predicted for Chicago.

Trains were reported crowded, with long distance reservations particularly hard to get. South Station had no precise reports available last night.

A hopeful development for vacationers in the national transport picture was the abandonment of a three-day strike by mechanics and flight service employees of Pan American World Airways. Through Tuesday, P.A.A. had managed to get 92 percent of its flights off despite the walkout but concern had been expressed if this degree of regular service could be kept up for long.