Rasputitsa

The rasputitsa (Russian: распу́тица, IPA: [rɐsˈputʲɪt͡sə]) refers to the semiannual mud seasons when unpaved roads become difficult to traverse in most of Eastern Europe.

The term has been used historically to describe the condition in Belarus, Russia and Ukraine. The word may be translated as the "quagmire season", because during this period, the large flatlands become extremely muddy and marshy, as do most unpaved roads. The term applies to both the spring rasputitsa and autumn rasputitsa seasons, and to the condition of the roads during those seasons. The rasputitsa occurs more strongly in the spring due to the melting snow, but it usually recurs in the fall due to frequent heavy rains.

Rasputitsa in The War That Came Early
The rasputitsa caught the German Wehrmacht and Lüftwaffe off guard when they invaded the Soviet Union although their Polish allies expected it. During the "mud season" trucks and even panzers bogged down stalling the offensive. Likewise, aircraft were grounded since there were no paved runways and the dirt ones held the landing gear too strongly which sank into it.