I'm a bit confused as to how to imagine a negative sensible heat flux near the Earth's surface. Negative means that sensible heat is moving towards the surface. My book states that that would mainly occur at nighttime. During day there is a positive sensible heat flux and sensible heat moves away from the surface. I can imagine that during the day, due to solar radiation, the surface warms up and air rises through convection and sensible heat is therefor moving away from the surface, being positive. During night the surface cools rapidly. Here, sensible heat is positive. But how does, during nighttime, sensible heat move downwards? I can't seem to grasp the concept. Thanks a lot!