Urgent help needed: can I take mean of correlation coeficients?

Historic market (Cash) prices and future contract prices are available for last 4 years. I have found correlation between Jan'11 market price with Jan'11,Feb'11 and March'11 future contract prices separately.I am planning to do the same for all months till Dec'14. So I will have 3 coefficients per month,36 coefficients per year and 144 coeficients for 4 years. Now I want to integrate the data,that means 1) To find only 3 coeficients for each month using total 12 coeficients of January of 4 years. Should I just take mean?Please let me know. 2) How can I incorporate effect of external factors like weather,volumes traded?

Please let me know if I am using wrong approach.I am ready to start fresh. Thanks for your time.