How much does your hair grow in a month?

I found that as the weather got colder my hair grew slower, about 1/4 of an inch each month. I've been measuring my hair every month and keeping the information in a diary. Even though I'm retaining length and my hair has been getting longer each month, I would have loved for it to have been 1/2 inch each month.  As the weather warms up I plan to compare my hair growth as it gets hotter. It's easy to stay on track if you know if the products your using is helping or harming your hair.  Also, a measuring tape comes in handy.