I'm considering designing and building a cold frame to use to grow arugula this winter. I want it to sit on a table or bench on my front porch: the only place I have that gets winter sun and is protected from the snow.

I've never used a cold frame before, so my first question is this: why do all have lids that open at the bottom rather than the top? Opening at the top seems most logical to me: it seems like that would allow more direct light when it's open, and hot air would escape more easily if open at the top. Maybe too much would escape?