The best way to determine whether an eating pattern is at an appropriate number of calories is to monitor body weight and adjust calorie intake and expenditure in physical activity based on changes in weight over time. All foods and many beverages contain calories, and the total number of calories varies depending on the macronutrients in a food.
Concepts Extraterrestrial radiation Ra The radiation striking a surface perpendicular to the sun's rays at the top of the earth's atmosphere, called the solar constant, is about 0.
The local intensity of radiation is, however, determined by the angle between the direction of the sun's rays and the normal to the surface of the atmosphere. This angle will change during the day and will be different at different latitudes and in different seasons.
The solar radiation received at the top of the earth's atmosphere on a horizontal surface is called the extraterrestrial solar radiation, Ra.
If the sun is directly overhead, the angle of incidence is zero and the extraterrestrial radiation is 0. As seasons change, the position of the sun, the length of the day and, hence, Ra change as well.
Extraterrestrial radiation is thus a function of latitude, date and time of day. Daily values of Ra throughout the year for different latitudes are plotted in Figure The amount of radiation reaching a horizontal plane is known as the solar radiation, Rs.
Because the sun emits energy by means of electromagnetic waves characterized by short wavelengths, solar radiation is also referred to as shortwave radiation. Solar radiation is also known as global radiation, meaning that it is the sum of direct shortwave radiation from the sun and diffuse sky radiation from all upward angles.
Rs is the solar radiation that actually reaches the earth's surface in a given period, while Rso is the solar radiation that would reach the same surface during the same period but under cloudless conditions.
The relative shortwave radiation is a way to express the cloudiness of the atmosphere; the cloudier the sky the smaller the ratio. The ratio varies between about 0. In the absence of a direct measurement of Rn, the relative shortwave radiation is used in the computation of the net longwave radiation.
It is the ratio of the actual duration of sunshine, n, to the maximum possible duration of sunshine or daylight hours N. As with extraterrestrial radiation, the day length N depends on the position of the sun and is hence a function of latitude and date.
Daily values of N throughout the year for different latitudes are plotted in Figure Albedo a and net solar radiation Rns A considerable amount of solar radiation reaching the earth's surface is reflected. The fraction, a, of the solar radiation reflected by the surface is known as the albedo.
The albedo is highly variable for different surfaces and for the angle of incidence or slope of the ground surface. It may be as large as 0. A green vegetation cover has an albedo of about 0. For the green grass reference crop, a is assumed to have a value of 0.
The net solar radiation, Rns, is the fraction of the solar radiation Rs that is not reflected from the surface. Its value is 1-a Rs. Net longwave radiation Rnl The solar radiation absorbed by the earth is converted to heat energy. By several processes, including emission of radiation, the earth loses this energy.
The earth, which is at a much lower temperature than the sun, emits radiative energy with wavelengths longer than those from the sun. Therefore, the terrestrial radiation is referred to as longwave radiation. The emitted longwave radiation Rl, up is absorbed by the atmosphere or is lost into space.chapter 1, food safety rev aug 99 5 section i general information introduction purpose definitions responsibilities introduction a.
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