When using EXCEL' built in regression function for a temperature project, I tried to mimic the way interest rates were regressed in the NEAS examples. I entered the Y variable as the current temperature and regressed the X variable as the 1 day lagged temperature (the temperature starting on day 2 instead of day 1). On most of the student projects they are getting a regression coefficient of ~ 0.59 and when i do it i constantly get a coefficient of 1.00

Is there some other variables that I should be regressing on to get a coefficient closer to 0.59?

[NEAS: Using many years and complete data, you should get an intercept between 80% and 95% and an R-squared of 70% to 90%. The actual results vary with the weather service location.]