HEATING AND COOLING OF THE EARTH’S SURFACE
There is a branch of climatology, called microclimatology, that deals with the climate at a particular locality on the Earth. One important part of microclimatology has to do with what could be described as “the climate near the ground”. Does that not seem like a suitable topic for a course on the environment of the Earth’s surface?
Most of the standard weather observations made at weather stations are taken a standard height above the ground that’s a little greater than the height of the average person. At most times, the conditions right at the ground surface are rather different, in terms of wind speed, temperature, and humidity. The climate near the ground, and its effect on the uppermost layer of the soil, is of obvious importance not just for agriculture but also for civil and environmental engineering, architecture, planning, and even medicine.
1.2.2 The Seasons
I often find myself wondering how much the average person understands about why there are seasons, and why sun height and day length change over the course of a year. Because I don’t know much about that (my basis for judgment is only anecdotal), I risk being either too elementary or too advanced for you in this brief section on the seasons.
First, some facts about the geometry of the Earth–Sun system. The e\Earth revolves around the Sun, once in a full year (that’s how a year is defined!), in a plane passing through the earth’s orbit, called the plane of the ecliptic. The earth’s orbit is nearly, but not quite, a circle; it’s actually an ellipse with a slight eccentricity. To give you an idea of the degree of non-circularity of the orbit, the maximum Earth–Sun distance is about 152 million kilometers and the minimum distance is about 147 million kilometers.
The only easy way to detect by direct observation that the earth is revolving around the sun is astronomical: view the stars at the same time every night throughout the year, and see how their position changes. They make one complete circuit through the heavens in the course of a year.
You know, of course, that the earth rotates about its axis once a day, and that’s what causes the difference between night and day. (!) If the Earth’s axis of rotation were exactly perpendicular to the plane of the ecliptic, the length of the day and the height of the sun above the horizon at a given time of day (local noon, say) would not change through the year, and things would not be nearly as exciting as they are in reality. (By “day” here I mean the time during the 24-hour day that the sun is above the horizon. The word “day” is rather ambiguous in the English language; think of this aspect of “day” as “daylight day”)
The earth’s axis of rotation is in fact inclined to the plane of the ecliptic by about 23° 27', usually rounded off to 23-1/2°. This has far-reaching consequences for conditions at the Earth’s surface. As you can see from Figure 1- 4, there are two times during the year, called the equinoxes (one in spring, on 20 or 21 March,and one in autumn,on 22 or 23 September),when the earth’s axis of rotation is exactly perpendicular to the line between the earth and the sun. At those times of the year, and only at those times, the daylight day is exactly twelve hours long at every point on the earth.
The earth’s north pole and south pole represent special cases, in the sense that at the time of the equinoxes the sun is seen to lie right on the horizon and make a complete circle around the sky every 24 hours!
At times midway between the equinoxes, called the solstices, the earth’saxisofrotationliespreciselyintheplanethatpassesthroughboththesun and the earth and is perpendicular to the plane of the ecliptic (Figure 1-4). At those times, one of the Earth’s poles points closest to the sun and the other pole points farthest from the sun. At the time of the summer solstice in the Northern Hemisphere, on 21 or 22 June, the sun is highest in the sky at a given time of day, as at noon, and the daylight day is longest. Conversely, at the time of the winter solstice in the northern hemisphere, on 21 or 22 December, the sun is lowest in the sky at a given time of day, and the daylight day is depressingly short, even nonexistent.
(If you think that things around the time of the winter solstice are bad at the latitude of Boston, think about what they are like in the latitude of Scandinavian cities, or Nome or Fairbanks in Alaska. On the other hand, during the legendary “white nights” of St. Petersburg, in early summer, it is said that one can read a newspaper at midnight without artificial light.)
But the angle at which the sun rises above the horizon at sunrise, and the equal angle at which the sun sets below the horizon at sunset, remains the same through the year (Figure 1-5). Think carefully about the geometry of the Earth– Sun system to convince yourself of that fact.
Incidentally, can you think of a reason why the equinoxes and solstices do not fall on exactly the same days, in March and September and in June and December, respectively, every year? It’s related to the existence of leap years. The astronomical duration of the year—the time it takes for the earth to make on complete revolution around the sun—is close to 365-1/4 days, whereas the calendar year has 365 days, by definition. Every four years they add 29 February to the year, and it makes things come out almost okay, except not quite, so once every century they have to have an extra adjustment of one day, etc., etc. That causes the precise times of the year when the equinoxes and solstices occur to vary depending upon the position of the given year relative to the leap years.
One final point: the time of maximum distance from the sun to the earth, called aphelion, falls on 4 July, and the time of minimum distance from the sun to the earth, called perihelion, falls on 3 January. Note (Figure 1-6) that (1) these times are out of sync with the seasons and (2) the earth is closest to the sun in the northern-hemisphere winter, by a factor of about 1.03 (152 x 106 km divided by 147 x 106 km), which because of the inverse-square decrease in the sun’s radiant energy with distance from the sun translates to a factor of about 1.07 (or seven percent, in what I think is the conventional way of stating percentages). It’s a good thing, too, except perhaps for ski fanatics: if the situation were the reverse, with the Earth farthest from the Sun in the northern-hemisphere winter, winters in New England would be even tougher than they are.
As you are probably aware, the Sun is fueled by thermonuclear fusion, by which hydrogen is fused to helium with enormous release of energy. The Sun’s surface, at a temperature of about 6000°C, radiates energy in all directions over a wide range of wavelengths. The Earth intercepts a tiny fraction of that radiant energy, in the form of sunlight. Most of the ultraviolet part of the radiation is absorbed before it reaches the Earth’s surface, although in recent times the degradation of the ozone layer as a consequence of certain man-made refrigerant gases in the upper atmosphere has led to greater ultraviolet radiation at the surface, especially at high latitudes. The effects of man-made gases on the ozone layer, and its consequences for ultraviolet radiation, is a field of active research by atmospheric chemists and physicists nowadays.
How much of the Sun’s energy reaches the earth? Think about one square centimeter of area, oriented perpendicular to the line between the Earth and the Sun and outside the Earth’s atmosphere. The rate of delivery of radiant energy to that square centimeter is very nearly two gram calories per minute. This is often referred to as two langleys per minute, a langley being one gram calorie per square centimeter. This value is called the solar constant, although in fact it’s not quite constant: it varies slightly depending upon two effects:
• changes in the sun’s output, connected with things like solar flares; and
• changes over the course of a year, because the earth’s orbit around the sun is not a circle but rather an ellipse with a small eccentricity, as you have already seen.
How can I give you some feel for what the solar constant means in everyday life? First of all, I should point out that the energy flux (by that I mean rate of delivery of energy per unit area perpendicular to the direction of radiation) is at most about half that just outside the atmosphere, or about one langley, because of the inevitable absorption and reflection, even on the clearest of days. On cloudy days, or when the sun shines at a low angle through the atmosphere, the value is correspondingly much smaller.
All of you have some experience with how much energy it takes to heat water. You put a pan of water on the stovetop, turn on the burner, and wait and wait for the water to boil. As you will learn more in a later chapter, water has a very high specific heat capacity: it takes a lot of energy to raise the temperature of water by a given amount.
Suppose that you had a little cube, one centimeter on each side, filled with water and oriented with one face facing directly into the sunlight. The other five faces are perfectly insulated from their surroundings, and the face that catches the sunlight is completely transparent to the sun’s incoming energy but can pass no heat back out to its surroundings. How long would it take to heat water contained in the cube from the freezing temperature (0°C) to the boiling temperature (100°C)? Answer: about 100 minutes.
1.2.4 Heating and Cooling of the Surface
The ground surface is the interface between the atmosphere and the solid and liquid material of the earth. The ground surface is where solar radiation is intercepted and converted to heat. It is also the source of outgoing long-wave radiation. It is where liquid water is evaporated and where incoming rainfall is stored as soil moisture and groundwater. First, in the following two paragraphs, are some very general things about heating and cooling. Then, after some more physics background, there is more detail on the heating and cooling of the Earth’s surface.
Low-lying areas are commonly colder at night than higher ground nearby. On clear nights, the ground is chilled as its heat is radiated out to space. The cold ground then chills the air near the ground. The chilled air is slightly more dense than the overlying air, so it tends to flow slowly downhill, in the same way that water flows downhill. The cold air “ponds” in low areas. These are places where the first frosts of autumn are earliest and where the last frosts of spring are latest. If you ever have a chance to plant fruit trees, plant them on the highest ground around!
In hilly areas, north-facing slopes get less sunshine than south-facing slopes. Local temperatures on the north-facing slopes are colder than on south- facing slopes in both summer and winter. In areas with winter snows, the snow melts much later on north-facing slopes. Even over distances of a few meters, the difference in microclimate between a sunny, open area with low herbaceous vegetation and a nearby grove of tall trees can be spectacular.
Everybody knows that the Earth’s surface tends to be warmed on a sunny day and cooled on a clear night. Let’s look more deeply into how this happens. Many different and interesting effects must be taken into account.
If there were no atmosphere, things would be fairly simple: the Sun radiates energy to the Earth’s surface at short wavelengths, most of it absorbed but some of it reflected, and the Earth would re-radiate that energy back out to space at longer wavelengths, and the temperature of the surface would become adjusted so that the outgoing long-wave radiation would balance the incoming short-wave radiation over a long time. (For that statement to make more sense to you, you need to be aware that the intensity of emission of radiation from a body increases with the temperature of the body—to the fourth power! That’s called the Stefan- Boltzmann law.)
A few additional words about the reflection of incoming solar radiation by the Earth’s surface back into space are in order here. The local reflectivity varies greatly depending upon what’s covering the surface. It ranges from as much as 95%, for fresh snow cover, to as low as just a few percent, for water surfaces at high sun. Figure 1-7 is a table giving approximate values of reflectivity for various kinds of surface. The overall grand average for the entire earth (that is, the percentage of incoming solar radiation intercepted by the entire earth that’s reflected back to space, on a long-term average), is called the earth’s albedo; its value is about 30%.
In the real world, with a thick atmospheric cover, the situation at the ground surface is much more complicated, largely by virtue of two effects:
• The atmosphere reflects and scatters some of the incoming solar radiation. (Scattering is the process whereby the atoms, molecules, and tiny particle in the atmosphere interact with passing electromagnetic waves whose wavelengths are approximately the same as the size of the particles, causing a part of the waves to be diverted in a wide range of directions.) What arrives at the surface is a combination of direct radiation and indirect, downscattered radiation. The atmosphere itself absorbs some of the incoming solar radiation, but, perhaps surprisingly, not much. (Incidental note: the shorter wavelengths are more susceptible to scattering than the longer wavelengths, so the indirect downscattered radiation we see at the earth’s surface tends to be in the shorter- wavelength part of the visible spectrum—hence the blue sky on a clear day. By the same token, the yellow or orange or even red sun seen at sunrise or sunset is a consequence of the greater scattering of the shorter wavelengths during the long slanting passage of the Sun’s rays through the atmosphere, leaving mainly the longer-wavelength radiation to come through to our eyes.)
• A lot of the long-wavelength back-radiation to space from the earth’s surface is absorbed by the atmosphere. Some of the absorbed energy is re-radiated back to the earth’s surface, and some is re-radiated out to space. The important effect here is that there is a net radiation of long-wavelength energy to space, but the almost-equal magnitudes of back-and-forth long-wavelength radiation between the surface and the atmosphere is much larger than the net radiation to space. This is the famous greenhouse effect.
Certain atmospheric gases figure most prominently in the absorption of outgoing long-wave terrestrial radiation by the atmosphere, foremost among them being water vapor, carbon dioxide, methane, ozone, and certain man-made gases like chlorofluorocarbons. It’s this strengthened greenhouse effect, caused by the increasing concentration of anthropogenic greenhouse gases, that is thought to be the cause of global warming (but the situation is in reality more complicated, in large part because of the still poorly understood potential changes in cloud cover; for such reasons, global warming has its responsible skeptics). Each greenhouse gas absorbs outgoing terrestrial long-wave radiation in different segments of the electromagnetic spectrum; they combine to leave only narrow windows of transparency (Figure 1-8).
Water vapor, because it is an effective absorber of long-wave terrestrial radiation and because it is present in the atmosphere in far greater concentrations than any of the other greenhouse gases, is the number-one greenhouse gas (but the others, most importantly carbon dioxide, humankind has some control over, which is not the case with water vapor). Beware of what you read in the news media about carbon dioxide being the most important greenhouse gas! I could also point out here that the greenhouse effect is much more our friend than our enemy: if it were not for the greenhouse effect, the Earth would be a frozen and lifeless planet.
You need also to be aware of several readily understandable processes specific to the earth’s surface, which are important factors in the heat budget of the surface:
• When the surface is warmer than the immediately overlying atmosphere, heat is conducted from the ground to the atmosphere; when the surface is cooler than the air above, heat is conducted downward from the air to the ground.
• As the ground surface is warmed, heat is conducted downward into deeper levels of the soil; as the ground surface is cooled, heat is conducted upward from the deeper levels.
• Provided that the relative humidity is less than 100%, soil moisture present at the ground surface, or drawn upward to the ground surface by capillary action, is evaporated into the atmosphere, thereby delivering latent heat to the atmosphere. (As you will learn in Section 3 below, it takes a lot of heat energy to evaporate liquid water; that energy is released into the atmosphere when the water vapor condenses to liquid water.)
• Some heat is added to the ground surface by condensation of dew on clear nights, but that’s not an important effect; what’s more important is delivery of precipitation to the ground surface from above. If the temperature of rain is greater than that of the surface, the surface is warmed, and vice versa.
It’s not easy to make any simple statement about the state of the ground surface in terms of heat, because all of the above effects and processes vary with time of day, state of the weather, time of year, and climatic zone. But the next time you are outdoors and about, looking at the ground surface, I want you to think about all of the foregoing processes, acting simultaneously to set the temperature of the surface. Just for a summary, look at Figure 1-9, which shows the various processes, and in a qualitative way their typical magnitudes, on a sunny summer day and on a clear summer night. The major effects are: incoming solar radiation absorbed by the ground (S), which is a combination of direct and downscattered radiation; incoming solar radiation reflected from the ground (R); incoming long-wave radiation from the atmosphere (LI); outgoing long-wave radiation to the atmosphere and outer space (LO); conduction of heat from the ground to the overlying air or from the overlying air to the ground (C); and loss (or minor gain) of heat from the ground by evaporation of soil moisture (V).
On average, over the course of a year, there is a net gain of heat by insolation (the term used to describe the totality of solar radiation reaching the Earth’satmosphere)atlowlatitudesandanetlossathighlatitudes.That imbalance has to be made up somehow, because the Earth’s average surface temperature changes only very slowly with time. That happens by net transport of heat from low latitudes to high latitudes by the Earth’s wind systems and ocean currents. One consequence of the existence of seasons is that the solar energy received by the Earth varies not only with latitude, as described above, but also with the seasons. Does it surprise you to learn that around the time of the summer solstice insolation at the north pole is even greater than at the equator at that same time, because the sun shines there twenty-four hours a day! Figures 1-10 and 1-11 show, in two different ways, how insolation varies as a function of both latitude and season.
What about the temperature of the soil (or, more generally, regolith and bedrock) beneath the ground surface? Despite songs about the cold, cold ground, the substrate is not necessarily colder than the ground surface. The vertical distribution of temperature as a function of depth below the surface depends upon a multitude of factors:
• ambient temperature of the air above the surface
• clarity of the sky
• state of the ground surface, especially moisture content and vegetative cover
• composition of the substrate
• moisture content of the substrate
• past history (because the temperature profile does not adjust instantaneously to changing surface temperature: there’s a long time lag)
When the Sun shines on the Earth’s cool land surface and heats it, that heat is conducted downward into the Earth, at a rate that depends on the temperature difference between the surface and the subsurface and also on the heat capacity and the thermal conductivity of the Earth’s soil and rock materials. Likewise, when the Earth’s surface cools by radiation of energy back out to space, heat from the still-warm subsurface materials is transferred by conduction back out to the surface. Such changes are mainly on time scales of day to night and of winter to summer seasons. Figures 1-12, 1-13, and 1-14 show some interesting results of actual measurement of the cyclic changes in temperature with depth.
Obviously the temperature near the surface is greatest in summer and least in winter, but there are two significant effects below the surface:
• The changes in temperature over one diurnal (that is, daily) cycle or over one yearly cycle decrease with depth in the Earth. Below a depth of about a meter, the day-to-day changes are negligible, and below a depth of about ten meters the temperature is almost the same year-round.
• The timing of the change in temperature at some depth lags behind the timing of the change in temperature at the surface. That’s because it takes time for the heat to be conducted downward. The time lag increases with depth. At a certain depth, the cycle is out of phase with the surface by a whole half cycle: that is, the maximum temperature attained at depth happens at the time of minimum temperature at the surface, and vice versa!
In colder climates the effects would be qualitatively the same, but the temperatures would be lower. In such areas, as in New England, the upper layer of the substrate, to a significant depth, becomes frozen for quite some time in the winter. Building codes in cold regions are specified to ensure that footings for foundations are placed at a depth great enough to be below the deepest freezing level to be expected in the given climate.