Chapter 3 Energy Balance and Temperature

Atmospheric gases, particulates, and droplets all reduce the intensity of solar radiation (insolation) by absorption, a process in which radiation is captured by a molecule. It is important to note that absorption represents an energy transfer to the absorber. This transfer has two effects: the absorber gains energy and warms, while the amount of energy delivered to the surface is reduced.

The reflection of energy is a process whereby radiation making contact with some material is simply redirected away from the surface without being absorbed. The percentage of visible light reflected by an object or substance is called its albedo. When light strikes a mirror, it is reflected back as a beam of equal intensity, in a manner known as specular reflection. When a beam is reflected from an object as a larger number of weaker rays traveling in different directions, it is called diffuse reflection, or scattering.

In addition to large solid surfaces, gas molecules, particulates, and small droplets scatter radiation. Although much is scattered back to space, much is also redirected forward to the surface. The scattered energy reaching Earth’s surface is thus diffuse radiation, which is in contrast to unscattered direct radiation.

Scattering agents smaller than about one-tenth the wavelength of incoming radiation disperse radiation through Rayleigh scattering, which is particularly effective for those colors with the shortest wavelengths. Thus, blue light is more effectively scattered by air molecules than is longer-wavelength red light.

Microscopic aerosol particles are considerably larger than air molecules and scatter sunlight by a process known as Mie scattering, which does not have nearly the tendency to scatter shorter wavelength radiation that Rayleigh scattering does. Mie scattering causes sunrises and sunsets to be redder than they would due to Rayleigh scattering alone, so episodes of heavy air pollution often result in spectacular sunsets.

The sky appears blue because gases and particles in the atmosphere scatter some of the incoming solar radiation in all directions. Air molecules scatter shorter wavelengths most effectively. Thus, we perceive blue light, the shortest wavelength of the visible portion of the spectrum.

Sunrises and sunsets appear red because sunlight travels a longer path through the atmosphere. This causes a high amount of scattering to remove shorter wavelengths from the incoming beam radiation. The result is sunlight consisting almost entirely of longer (e.g., red) wavelengths.

The water droplets in clouds are considerably larger than suspended particulates reflecting all wavelengths of incoming radiation about equally, which is why clouds appear white or gray. Because of the absence of preference for any particular wavelength, scattering by clouds is sometimes called nonselective scattering.

Incoming solar radiation available is subject to a number of processes as it passes through the atmosphere. The clouds and gases of the atmosphere reflect 19 and 6 units, respectively, of insolation back to space. The atmosphere absorbs another 25 units. Only half of the insolation available at the top of the atmosphere actually reaches the surface, of which another 5 units are reflected back to space. The net solar radiation absorbed by the surface is 45 units.

The difference between absorbed and emitted longwave radiation is referred to as the net longwave radiation. Shortwave and longwave radiation are not separate entities as far as the heating of the atmosphere and surface are concerned. When either is absorbed, the absorber is warmed. It is therefore natural to combine longwave and shortwave into net allwave radiation, or simply net radiation, defined as the difference between absorbed and emitted radiation, or equivalently, the net energy gained or lost by radiation.

Net radiation is the end result of the absorption of insolation and the absorption and radiation of longwave radiation. The surface has a net radiation surplus of 29 units, while the atmosphere has a deficit of 29 units.

Convection is a heat transfer mechanism involving the mixing of a fluid. In free convection, local heating can cause a parcel of air to rise and be replaced by adjacent air.

Forced convection (also called mechanical turbulence) occurs when a fluid breaks into disorganized swirling motions as it undergoes a large-scale flow. Air is forced to mix vertically because of its low viscosity and the deflection of wind by surface features.

When energy is added to a substance, an increase in temperature occurs that we physically sense (sensible heat). The magnitude of temperature increase is related to two factors, the first of which is specific heat, defined as the amount of energy needed to produce a given temperature change per unit mass of the substance. The temperature increase resulting from a surplus of energy receipt also depends on the mass of a substance.

Latent heat is the energy required to change the phase of a substance (solid, liquid, or gas). In meteorology we are concerned with the heat involved in the phase changes of water. In the case of melting ice, the energy is called the latent heat of fusion. For the change of phase from liquid to gas, the energy is called the latent heat of evaporation.

Both the surface and atmosphere lose exactly as much energy as they gain. The surface has a surplus of 29 units of net radiation, which is offset by the transfer of sensible and latent heat to the atmosphere. The atmosphere offsets its 29 units of radiation deficit by the receipt of sensible and latent heat from the surface.

The interactions that warm the atmosphere are often collectively referred to as the greenhouse effect, but the analogy to a greenhouse is not strictly accurate. The greenhouse gases of the atmosphere do not impede the transfer of latent and sensible heat. Thus, it would be more accurate if the term “greenhouse effect” were replaced by “atmospheric effect.” The greenhouse effect keeps Earth warmer by absorbing most of the longwave radiation emitted by the surface, thereby warming the lower atmosphere, which in turn emits radiation downward.

One of the most immediate and obvious outcomes of radiation gain or loss is a change in the air temperature. The map depicts differences between mean temperatures in January and July through the use of isotherms, which are lines that connect points of equal temperature.

Certain geographical factors combine to influence temperature patterns across the globe. These factors include latitude, altitude, atmospheric circulation patterns, local conditions, continentality, (the effect of an inland location that favors greater temperature extremes) and ocean current characteristics along coastal locations.

The daily mean is defined as the average of the maximum and minimum temperature for a day. The daily temperature range is obtained by subtracting the minimum temperature from the maximum. The monthly mean temperature is found by summing the daily means and dividing by the number of days in the month. The annual mean temperature is obtained by summing the monthly means for a year and dividing by 12. The annual range is obtained as the difference between the highest and lowest monthly mean temperatures.

If low temperatures are accompanied by windy conditions, a person’s body loses heat much more rapidly than it would under calm conditions due to an increase in sensible heat loss. It is common for weather reports to state both the actual temperature and how cold that temperature actually feels, the wind chill temperature index.

Thermodynamic diagrams (such as the Stuve above) depict the vertical profiles of temperature and humidity with height above the surface enabling forecasters to determine the height and thickness of existing clouds and the ease with which the air can be mixed vertically. The data on the charts are obtained from radiosondes that are carried aloft by weather balloons twice a day at weather stations across the globe.

The next chapter examines atmospheric pressure and wind.