The term "January Barometer" refers to the belief held by some traders that the investment performance of the S&P 500 Index in January can predict its performance for the rest of the year. For example, proponents of this view believe that if the S&P 500 rises between Jan. 1 and Jan. 31, this will foretell a positive result for the remainder of the year. Similarly, it holds that if the market fares poorly in January, it will likely perform poorly thereafter as well.