The January barometer is the hypothesis that stock market performance in January (particularly in the US) predicts its performance for the rest of the year. So if the stock market rises in January, it is likely to continue to rise by the end of December. The January barometer was first mentioned by Yale Hirsch in 1972.

Historically if the S&P 500

S&P 500

The S&P 500 is a free-float capitalization-weighted index published since 1957 of the prices of 500 large-cap common stocks actively traded in the United States. The stocks included in the S&P 500 are those of large publicly held companies that trade on either of the two largest American stock...

goes up in January the trend will follow for the rest of the year. Conversely if the S&P falls in January then it will fall for the rest of the year. From 1950 till 1984 both positive and negative prediction had a certainty of about 70% and 90% respectively with 75% in total. After 1985 however, the negative predictive power had been reduced to 50%, or in other words, no predictive powers at all.