the average winter outdoor temperature (taken in December or January for the US, since those are usually the coldest months). The figures shown in the table are ratios of wall surface to floor area. It's a bit confusing, so let's use an example to clarify things.

Imagine your house is built at a latitude of 44°N. Your average winter temperature is 25°F and the room behind the wall measures 350 square feet. So, you find the latitude column, the one before last. Look down that column to the 25°F line and you get the number 0.75. Multiply that by the room area to get the wall area: 350 x 0.75 = 262.5 square feet of Trombe wall surface area.

If your home isn't precisely at one of the latitude angles listed, you can extrapolate from the figures shown or pick the closest one that suits your climate and location.