Today I’m going to tackle one of the most important topics relating to the Ground to Sky interface. That is how our planet stays warm and what happens to heat and radiation at the Earth’s surface. I have touched on this before, explaining a little about the differences between vegetated and tarmaced surfaces. Today I will briefly describe the surface energy balance in much of its entirety.
The most important concept I will introduce here is that of the surface energy balance equation. H = K + (Sd – Sr – Su) + (Ld -Lu) – (G – λ E). This can be simplified as:
Heat = Storage + net (total including positives and negatives) Solar radiation + net longwave radiation + net heat flux.
Use the diagram below as reference as I introduce each of these terms and a small amount of discussion as to the meaning of each.
Heat term (H). This is the overall heat of the surface when all of the terms in the equation have been accounted for. This is the value we are trying to find out. How hot is our surface – how hot is it during the day? During the night? During cloudy or clear conditions? How hot the surface is affects our environment and is crucial for weather forecasting and other types of process modelling.
Storage term (K). This is how much heat is being stored by the surface. Different types of surfaces have a different ability to store heat. Man made surfaces are often capable of storing more heat than natural surfaces and this contributes to the urban heat island effect.
Net solar radiation (Sd – Su – Sr). Solar radiation (also called shortwave radiation) is the input of heat from the sun. Sd is the amount of sunlight coming down from the sun towards the patch of ground we’re interested in. Of Sd, some will be reflected by clouds and aerosols in the atmosphere before it even reaches the surface. This reflected solar radiation is called Sr. So we must subtract Sr from Sd. Some of the solar radiation does reach the surface but not all of it is absorbed by the surface and contributes to its heat balance. Su is the solar radiation that is reflected by the surface and must also be subtracted from the total incoming solar radiation, Sd. Su is controlled by the reflectivity of the surface, this is called the surface’s ‘albedo’. White surfaces reflect more sunlight than black surfaces. Some surfaces reflect only particular wavelengths of light, and this gives them their colour. For example an object that appears red to us reflects red light. The sky is blue because of the scattering of light in the ‘blue’ section of the visible light spectrum. But this is an aside. What matters here is that the amount of sunlight that reaches the surface, is absorbed and contributes to our heat term H, is reduced by reflection in the atmosphere by clouds and aerosols (Sr) and at the surface depending on surface albedo (Su).
Net longwave radiation (Ld – Lu). Longwave radiation is low frequency (infra-red) radiation. This is largely outgoing (Lu) heat from the Earth itself radiating into space. Ld is the term for the reflection of outgoing longwave radiation by clouds and greenhouse gases back down to the surface. Lu is quite close to balancing Sd overall, keeping the Earth’s atmosphere about the same average temperature. But the balance between the two varies from place to place (think the difference between summer and winter). Ld is the term that has caused such debate around the would – the greenhouse effect. The potential changes that could occur to the energy balance because Ld has been artificially altered by human activity is the subject of worldwide scientific attention.
Surface heat loss (G – λ E). These two terms represent the loss of heat from the surface itself. G is the sensible heat flux and λ E is the latent heat flux. Sensible heat (G) means that heat is being exchanged between the surface and the atmosphere and ONLY heat is being exchanged. That is, the only effect is a change in temperature. Minus G (-G) is loss of heat from surface to atmosphere (usually overnight) and +G is heating of a cold surface by warm air. When you think of air, you probably think of wind. Wind can come along in big parcels (air masses) originating from far away from our surface. Say the wind comes from a desert in the early morning? We will expect a +G as the air is hot but our surface is cool from the cold night. Interfaces of air masses and the ground can cause interesting weather patterns such as localised thunderstorms when the temperature difference between the two is high. λ E is the latent heat flux. Latent heat is different from sensible heat in that it is not only a change of temperature that is being brought about, but also a change of state. This term describes the heat that is used to evaporate water from a surface. This may or may not be mediated by vegetation depending on the surface. Plants can control evaporation using holes in their leaves called stomata. This way they can regulate how much water they have in their system to keep them alive. More heat is required to change the state of water from liquid to gas, so more energy is used up in this process than heating the ground. This explains why conditions feel cooler after it has rained, or near a lake or the sea.
When all these terms are totalled up we have H. H might be a hot surface or a cool surface depending on the balance of all the different terms. Is it a hot sunny day with no cloud? Then Sd will be high and Sr will be low. If we haven’t had much rain in a while λ E will be low too. What’s the difference between the energy balance during the day and during the night? What about a shaded bit of ground compared to an unshaded bit? A city compared to the countryside? Have a think about how H might change during the year. Understanding of the surface energy balance gives us a lot of insight into our environment.
I hope that this brief overview of how energy behaves in the lower atmosphere and at the surface helps you to understand some of the everyday observations that you make whilst wandering about in the ground to sky interface. In my next article I will return to soil science, and will be discussing how soil forms with another simple equation that you can see in action every day.