Why is the equation for heat flow negative?

[Vitani11]

Homework Statement

q = -kdp/dx

Homework Equations

q = heat flow
k = constant
p = thermal energy density

The Attempt at a Solution

This equation says that when heat flows from hot to cold the thermal energy density increases but why? I thought heat meant a higher thermal energy density and hot to cold meant a lower energy density since the molecules that make up the air are slower when cold.

 

[Vitani11]

nvm