The above graph explains heat transfer from a cylinder (your body) to the air based on the air's position around the cylinder. 0 on the X axis is directly in front of the cylinder and 180 is directly behind the cylinder. Higher values on the Y axis indicate more heat transferred from the cylinder to the air.

The bottom line on the chart represents a "laminar flow" (ie. not turbulent, low speed) scenario. The top line represents a higher speed scenario where laminar flow gives way to turbulent flow.

For both low and high speed scenarios significant heat transfer occurs not just at the front of your body, but also directly behind you. This is especially true for high speeds where laminar flow changes to turbulent flow (22-33mph). At this speed optimum heat transfer occurs at approximately 110 degrees from the front of the rider. Right in backpack territory.

This means wearing a backpack doesn't just trap sweat to your body, it has a significant impact on your body's ability to transfer heat to the air!

If you want to stay cool and dry, add some cargo capacity to your bike.