First and foremost deer are "edge" creatures; they relate to edges, or changes in cover. This can be going from conifers (pines) to hardwoods or from the woods to a field (or in the case of suburban hunting where the woods meets yards.)
Fields, whether agricultural (corn for example) or natural and with some great browse such as clover) can be productive in the early season, especially before any oaks might start dropping acorns. Early season is also when deer are less jittery as it's been months since they've been hunted. Walk the entire field and look for trails coming from the woods. Back track those trails and you'll get to where the deer bed and as the season advances you should shift your hunting towards deeper into the woods.
Hunting the woods is my personal favorite as there's plenty of food sources to hang a stand around and generally, with more cover (and assuming you don't burn out a spot with hunting the wrong wind or some other error) will produce over a longer period of time than fields.
Hunting between a food source and bedding area can be great during the rut as well.
With regard to temperature, the early season can impact when deer move in either the field or the woods; when it's too hot, it's too hot-period. Early season my rule of thumb is if it's hot enough for me to notice I won't hunt; why risk contaminating a good spot for an "iffy" hunt?
One final note; it should go without saying that "cover" in the woods, especially hardwoods, will be impacted by the leaves dropping; what was once super-thick may be very open come late Oct/early Nov and you'll have to adjust accordingly.
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