Outdoor Climate
Many conditions may change while you're outdoors that will affect your body temperature. Having the proper clothing will keep you at a comfortable temperature.
Weather
When we think of changing conditions, we generally think of the weather. As the temperature changes, you have to be concerned about being too warm, or not warm enough. Rain or snow may also move in getting you wet and making trails slippery or slushy. Winds may also change, either due to the weather or your location.
Sunshine
The temperature in the sun can be much warmer than that in the shade. You should assume, for any outing, that you may be in and out of the sun, so you should be prepared for both. If you start out for a long hike in the sunshine, where you are warm and dry, you might be tempted to leave your warmer layers behind, but don't be fooled; an extended hike into the woods can cool you off drastically. So can the clouds that weren't in sight when you started your hike.
Altitude
Temperatures and wind can change as you climb up and down the mountains
Activity Levels
Don't forget to plan for a variety of activities while you're outdoors. You may not feel the need for the warmer layers as you hike in the sunshine to your camp or fishing spot, but as you sit around after you're settled in you might not feel so warm.