Old wives' summer
From AMS Glossary
Old Wives' summer
A period of calm, clear weather, with cold nights and misty mornings but fine warm days, which sets in over central Europe toward the end of September; comparable to Indian summer.
It has been explained as a transition between the summer and winter pressure types. In summer, central Europe is dominated by the Azores high, from which a wedge of high pressure extends to southwestern Germany. In winter, the dominant feature is the Siberian high, from which a ridge extends across Switzerland. Between these two stages there is often a period, on the average occurring between 18 and 22 September, during which an independent anticyclone forms over Germany. As this gradually drifts away eastward, the Old Wives' summer tends to be delayed until October in the western part of the former Soviet Union. The term itself probably stems from the widespread existence of "old wives' tales" concerning this striking feature of autumn weather.