GEOG 1F91 Lab #1 Net Radiation Fall 2023

GEOG 1F91 Principles of Physical Geography Lab Assignment #1 – Net Radiation at the Surface Fall 2023 TEXT REFERENCE: Chapter 4 – The Global Energy System ADDITIONAL RESOURCES - available under “Helpful Stuff” on Brightspace: - The International System of Units - Physical Dimensions and Units - Measurement Precision and the Use of Significant Figures OBJECTIVE: This exercise serves to introduce the concept of net radiation, and the controls on it. Through field work and calculation, it should become apparent that it is not just the amount of sunshine that determines how much net radiation a surface actually receives. This concept will be illustrated by means of two demonstrations during the lab period. WHY IS THIS IMPORTANT? The primary source of energy for the atmospheric processes we’ll be looking at over the next few weeks is incoming solar radiation, or insolation. The nature of the surface it strikes and the atmospheric conditions at the time are very important in controlling what happens after that. This lab endeavours to illustrate this concept so as to help you understand the resulting atmospheric processes. INTRODUCTION No system will operate without some sort of energy supply. For the earth-atmosphere system, the principal energy supply is the sun. It provides more than 99.8% of the energy needed for life and life- support processes on the earth as well as the energy necessary to maintain the motions of the atmosphere and oceans; it is thus the principal factor in determining the nature and distribution of climates. While the energy balance of the earth-atmosphere system is relatively constant over time, i.e., the earth's surface is neither heating up nor cooling down (Text Figure 4.3 & 4.16 & 4.21), there are significant variations in time and space (for example, see Text Fig. 4.22 & 4.25). This is currently a point of conjecture with the growing concern over the possible consequences of both the greenhouse effect and global warming (Text, Climate Change – Is Anthropocentric climate change really occurring, pp. 241-244). Because of its fundamental importance, it is necessary to be able to determine the amount of energy available at the surface of the earth at any particular place or time. Measurements have been taken on the Brock University campus over two different surface materials (grass and concrete) and these measurements will be used to compute the energy available at the surface. We will consider other surface effects, and seasonal effects by means of some calculated examples. 1

Since we are concerned with the rate of energy flow, the unit to be used is the watt (just think about the brightness of a light bulb – say a 40 watt versus a 100-watt bulb, and since we must specify the area of the surface on which this energy is acting we will choose 1 m 2 ; our basic working unit thus becomes one watt per metre squared. We will therefore express energy receipts and losses as watt per m 2 (W/m 2 , or W m -2 ). The energy budget, or the balance between energy receipts and losses (hence net radiation) at the surface can be expressed as: R n = Q -  Q - M out + M in where R n = the net radiation Q = the total incoming solar radiation or insolation  Q = the solar radiation reflected from the surface, where  = albedo (reflectivity in %) M out = the terrestrial radiation emitted from the surface M in = the terrestrial radiation incoming to the surface from clouds, water vapour and CO 2 in the atmosphere, also called counter radiation. As you can see from the formula, there are both positive terms (Q and M in ) and negative terms (  Q and M out ). The positive terms add energy to the surface while the negative terms remove energy from the surface. The net radiation is the balance between these positive and negative terms. For example, the net radiation is usually positive during the day when the incoming solar radiation, or insolation (Q in the equation), is so large as to overwhelm the negative terms in the equation. This positive value of R n simply means the surface is warming up as there is more energy coming in than going out. Conversely, at night when there is no sun, Q, and, of course,  Q, both equal zero. The negative term M out dominates the equation resulting in a negative value of R n , meaning the surface is cooling down as there is more energy going out than coming in. The incoming solar radiation (Q) can be measured with a pyranometer (Figure 1), and we will do this outside. While incoming solar radiation, or insolation, is measured with the device pointing upward, by flipping it over so it points downward, we can get a measure of the solar radiation reflected from the surface (  Q). Figure 1 – Pyranometer Diagram 2

The terrestrial radiation (represented in the equation by the terms M in and M out ) can be calculated via the Stefan-Boltzmann Law, given as: M =  T 4 W m -2 where  = Stefan-Boltzmann constant = 5.67 x 10 -8 W m -2 K -4 T = temperature of the emitting surface in K (K = °C + 273) As can be seen from the formula, all we need is a surface temperature. This is easily obtained using a thermal infrared thermometer. For terrestrial radiation emitted by the surface (M out ), the thermometer is simply pointed directly downwards toward the ground surface. The terrestrial radiation incoming to the surface, also called counter radiation (M in ), originating from the base of clouds, as well as water vapour and carbon dioxide (CO 2 ) in the atmosphere, can also be measured with the thermal infrared thermometer. By simply pointing it straight upward, we can get a measure of the equivalent temperature at which the sky is radiating. Once we have the temperatures of the emitting surfaces, either the ground surface or the sky itself, the terrestrial radiation (either M in or M out ) can be computed using the Stefan-Boltzmann Law. 3

EXAMPLES OF NET RADIATION COMPUTATIONS Example A: Month, time and weather: June, midday, no cloud Solar radiation measured at surface (Q): 850.0 W m -2 Type of Surface: Brock’s asphalt parking lots Surface Temperature: 35 °C Sky radiation from CO 2 and water vapour radiated at an equivalent temperature of: 10 °C 1. Solar radiation (Q): given as 850.0 W m -2 2. Solar radiation reflected (  Q): From Table 1.1 (see pg. 7), the albedo of asphalt pavement is 3%, or in decimal form, 0.03. Solar radiation (Q) is as given in Step 1 (Q = 850.0 W m -2 ). Thus:  Q = 0.03 x 850.0 W m -2 = 25.5 W m -2 3. Outward terrestrial radiation (M out ): The intensity of radiation from a surface is given by the Stefan-Boltzmann Law: M =  T 4 W m -2 where  = Stefan-Boltzmann constant = 5.67 x 10 -8 W m -2 K -4 T = temperature of the emitting surface in °K (K = °C + 273) Thus: M out = (5.67 x 10 -8 W m -2 K -4 ) x ((35 °C + 273) K) 4 = (5.67 x 10 -8 ) x (308) 4 W m -2 = 510.3 W m -2 4. Inward terrestrial, or counter-radiation (M in ): (again, using the Stefan-Boltzmann Law: M =  T 4 W m -2 ) M in = (5.67 x 10 -8 W m -2 K -4 ) x ((10 °C + 273) K) 4 = (5.67 x 10 -8 ) x (283) 4 W m -2 = 363.7 W m-2 5. Combination of the various factors: We can now insert the above values into the original energy budget formula: R n = Q -  Q - M out + M in Thus: R n = 850.0 - 25.5 - 510.3 + 363.7 = 677.9 W m -2 for Example A. 4