Many questions have recently been asked about the relative importance of various mechanisms for moving heat to and from the surface, so this article covers a few basics.
One Fine Day – the Radiation Components
I added some color to help pick out the different elements, note that temperature variation is also superimposed on the graph (on its own axis). The blue line is net longwave radiation.
Not so easy to see with the size of graphic, here they are expanded:
Note that the night-time is not shown, which is why the net radiation is almost always positive. You can see that the downward longwave radiation measured from the sky (in clear violation of the Imaginary Second Law of Thermodynamics) doesn’t change very much – equally so for the upwards longwave radiation from the ground. You can see the terrestrial (upwards longwave) radiation follows the temperature changes – as you would expect.
Sensible and Latent Heat
The energy change at the surface is the sum of:
- Net radiation
- “Sensible” heat
- Latent heat
- Heat flux into the ground
“Sensible” heat is that caused by conduction and convection. For example, with a warm surface and a cooler atmosphere, at the boundary layer heat will be conducted into the atmosphere and then convection will move the heat higher up into the atmosphere.
Latent heat is the heat moved by water evaporating and condensing higher up in the atmosphere. Heat is absorbed in evaporation and released by condensation – so the result is a movement of heat from the surface to higher levels in the atmosphere.
Heat flux into the ground is usually low, except into water.
All of these observations were made under clear skies in light to moderate wind conditions.
Note the low latent heat for the dry lake – of course.
The negative sensible heat in Arizona (2nd graphic) is because it is being drawn from the surface to evaporate water. It is more usual to see positive sensible heat during the daytime as the surface warms the lower levels of the atmosphere.
The latent heat is higher in Arizona than Wisconsin because of the drier air in Arizona (lower relative humidity).
The ratio of sensible heat to latent heat is called the Bowen ratio and the physics of the various processes mean that this ratio is kept to a minimum – a moist surface will hardly increase in temperature while evaporation is occurring, but once it has dried out there will be a rapid rise in temperature as the sensible heat flux takes over.
Heat into the Ground
We can see that heat doesn’t get very far into soil – because it is not a good conductor of heat.
Here is a useful table of properties of various substances:
The rate of heat penetration (e.g. into the soil) is dependent on the thermal diffusivity. This is a combination of two factors – the thermal conductivity (how well heat is conducted through the substance) divided by the heat capacity (how much heat it takes to increase the temperature of the substance).
The lower the value of the thermal diffusivity the lower the temperature rise further into the substance. So heat doesn’t get very far into dry sand, or still water. But it does get 10x further into wet soil (correction thanks to Nullius in Verba- really it gets 3x further into wet soil because “Thickness penetrated is proportional to the square root of diffusivity times time” – and I didn’t just take his word for it..)
Why is still water so similar to dry sand? Water has 4x the ability to conduct heat, but also it takes almost 4x as much heat to lift the temperature of water by 1°C.
Note that stirred water is a much better conductor of heat – due to convection. The same applies to air, even more so – “stirred” air (= moving air) conducts heat a million times more effectively than still air.
Temperature Profiles Throughout a 24-Hour Period
I’ll cover more about temperature profiles in a later article about why the troposphere has the temperature profile it does.
During the day the ground is being heated up by the sun and by the longwave radiation from the atmosphere. Once the sun sets, the ground cools faster and starts to take the lower levels of the atmosphere with it.
Just some basic measurements of the various components that affect the surface temperature to help establish their relative importance.
Note: All of the graphics were taken from Contemporary Climatology by Peter Robinson and Ann Henderson-Sellers (1999)