Law, and assuming that the objects are very good emitters (“black body” emission rates).
Incoming Sunlight Decreases in Winter and at Higher Latitudes
The emission of light from the sun is essentially constant through a year, and the distance of the Earth from the sun differs by only a few percent (a bit closer during winter in the Northern Hemisphere). The strong patterns of variation in incoming sunlight with latitude and season of the year result from a tilted planet doing an annual revolution around the sun. Summers are warmer because incoming light is several‐fold greater than in winters; the tilt of the Earth is toward the sun in summer, giving high‐angle incoming light that lasts for more hours in the day (Figure 2.7). The incoming light for a flat site at 23° latitude in mid‐summer is 2.5 times the amount received in January. At higher latitudes the difference between mid‐winter and mid‐summer much larger (eightfold at 43 °).
FIGURE 2.7 The total potential sunlight (not accounting for clouds) at 23° latitude (6100 W m−2 yr−1) is almost double that at 53° latitude (3460 W m−2 yr−1). The latitudinal differences are much larger outside the summer, because in summer the low sun angles that extend through very long day lengths give similar totals to sites with higher‐angle sun for shorter days at low latitudes.
Source: Based on spreadsheet by Nicholas Coops.
The intensity of incoming light also depends on the angle of a surface relative to the incoming angle of the sunlight. Incoming sunlight is at a maximum on a perpendicular surface, and the intensity lessens as a surface is tilted away to more oblique angles. High sun angles in mid‐summer bring almost as much sunlight to a wide range of slopes, as high sun angles combine with the long path of the sun through the sky to mostly even‐out the differences among aspects (which direction a slope faces; Figure 2.8). North‐facing aspects receive little sunlight (in the Northern Hemisphere) because of fewer hours of direct sunlight, and lower angles of the sun relative to the slope.
These graphs of incoming radiation assume that all locations are fully exposed to the sun, but forested slopes are often shaded partially by surrounding topography. Shading by adjacent hillsides can be important, varying of course through a day and across a season in relation to the angle of the sun in the sky. A set of watersheds in northern Idaho, USA shows that the “blockage” of incoming light ranges from 0 to more than one‐third when averaged across a year (Figure 2.9). This landscape shading has large effects on soil temperatures, the accumulation and duration of snow cover, and of course on vegetation.
How important are these differences in incoming sunlight? The differences are important enough that the typical elevation for a given species may be a few hundred meters lower on N‐facing aspects (in the Northern Hemisphere) than on S‐facing aspects. The latitudinal range of species may reach hundreds of kilometers farther south when N‐facing slopes are available as habitat. Why does the incoming light make so much difference? It might seem that the simple answer would deal with the supply of light to drive photosynthesis, but two other factors are likely more important. The first is the seasonality of temperatures that favor growth. South‐facing aspects are warmer throughout the year, which might benefit some species in the spring and autumn. Incoming solar energy is a major driver of evaporation, and S‐facing aspects experience higher evaporative demands that may dry soils while soils on N‐facing aspects remain moist. The apparent dryness of S‐facing aspects is not a difference in precipitation inputs; the difference is in the drying effect of the extra radiation.
FIGURE 2.8 The daily amount of incoming sunlight depends on the aspect of a site. In the Northern Hemisphere at a latitude of 43°, a hillside that faces toward the north at an angle of 25° receives almost no direct‐beam sunlight in winter, when a 25° hillside facing to the south receives 9 MJ m−2. The effect of aspect is small in mid‐summer.
Source: Based on a spreadsheet by Nicholas Coops.
FIGURE 2.9 The amount of incoming radiation received by a site depends not only on latitude, slope angle and aspect, but also on whether nearby hillsides block sunlight (left). The shading effects in a mountainous landscape in northern Idaho reduces incoming sunlight by only a few percent on south‐facing slopes, but by an average of 30% on north‐facing slopes. The effect of slope combined with shading from surrounding hills resulted in a fourfold range of incoming solar radiation annually (right;
Source: Wei et al. 2018 / Elsevier).
Forests Receive Shortwave Sunlight, and Shine off Longwave Radiation
The sun is so hot that the wavelengths of light emitted are strong enough to activate the light sensors in our eyes. Cooler objects, like surfaces in forests, are so cool that the radiation they shine is too low in energy to activate our eyes. These radiation patterns are illustrated in Figure 2.10 for a clearcut forest in Oregon, where the incoming solar radiation hits the soil surface. Most of the shortwave solar radiation is absorbed in the system, though 13% reflects away (which is handy for us, or our eyes could not see to walk across the site). The site also receives some longwave radiation, mostly from the warm air. The soil surface shines longwave energy back the sky, and in fact the soil is hotter than the air at midday and the longwave emissions remove more energy than the sky returns as longwave emissions. When the incoming shortwave radiation from the sun stops in the evening, and the emission of radiation from the soil leads to cooling.
FIGURE 2.10 The energy budget for a forest clearcut in Oregon, USA on a summer day is driven by incoming solar (shortwave) radiation. About 13% of the light is reflected, with no effect on the forest. The rest of the solar energy is either absorbed by the forest (warming it), or evaporating water. The forest itself “shines” at long wavelengths, and the intensity of the emission depends on the daily trends in temperature. The warm air also emits longwave radiation to the forest, about 300 W m−2 through the day and night. The emission of longwave radiation from the forest increased by about 50% through the day, as absorption of shortwave solar radiation increased the thermal energy stored in the forest. The balance between incoming and outgoing fluxes of shortwave and longwave radiation determines how much energy is available to evaporate
