The direction of the....Wind.
Important for establishing the likely feeding areas for fish and also for type of cast required. When described in terms of leeward and windward these terms can lead to some confusion. We have to define it in terms of position of shore and position of angler. You fish in the windward position from the Leeward shore and fish in the leeward position from the windward shore. Wind direction is also a factor in casting and a wind from the right (if right handed) should necessitate a cast from the left shoulder, or casting with your back to the water and using the back cast to place your flies. (opposite for left handed people). Fish will generally congregate at the leeward side of the lake as surface food will collect in this area. The 'thermocline' will be deeper here as the warmer surface water is blown to the leeward side of the lake.

Courtesy of Carl