Negative Sensible Heat Flux at Earth's Surface

[AppeltjeBosheuvel]

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!

 

[jbriggs444]

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 had to Google the term sensible heat: "In meteorology, the term 'sensible heat flux' means the conductive heat flux from the Earth's surface to the atmosphere"

At night, the Earth's surface is able to cool by radiating heat away into the night sky. By contrast, the air is transparent at the relevant wavelengths and neither emits nor absorbs -- it cannot effectively radiate its heat away. This means that the Earth's surface becomes cooler than the air around it. With the surface colder than the air, heat naturally conducts and convects from air to Earth.

Among other things, radiative cooling and the associated local cooling of the air is responsible for dew on the grass in the morning.