Wow what a difference in consumption

[ClayBBQ]

I am smoking a brisket today. The temp is 38 degrees. In the summer I only use about 10 lbs of charcoal for 6-7 hours. I have already refilled the charcoal after 3.5 hours. I am aslo burning wood and it is being consumed big time. The humidity is about 78% also. Very little wind. For some reason I am having a really hard time holding temps. Tending to be on the low side. This is a 8.5 lb flat I don't want to be cooking until midnight. Do you think it matters that I have about 5-6 inches of ash on the bottom. Not near the charcoal basket because the ash falls through. It may be affecting the airflow but that would slow down the burn not speeding it up.

[Real Montana Charcoal]

You have several factors hurting your cook.
1. cooler air intake temperatures require more fuel to be burned to acquire the same cooking temperatures.
2. Moisture in the air hampers things is 2 ways, first it is harder to bring the fire up to temperature (the moisture cools) and two which will sound a little strange so I will explain it in another way. I grew up on a farm, come around sunset all the farm equipment seemed to gain 25 horses or so. The way it was explained to me is that cool moist air is more dense, (containing more oxygen because of the water content) and thus aids in ignition. Perhaps you have noticed something similar on a carborated vehicle. Your fire in the box will work in the same way, burning fuel faster, therefore consuming more fuel and at the same time heat is being stripped away faster, from the moisture in the air, cold temperature of the intake air, and colder temperatures outside the cooker. Try insulating it with a fire retardant blanket, fiberglass insulation works well if you can figure out how to make sure it does not get in the food. If you may figure out a way to pre-warm the air that will help as well. Hope this will help you in future cooks anyway, it does in cooking coal.