PAUL HAY Capital Projects
Sensible Heat & Climate
Author: Paul Hay
e-mail: paul.hay@phcjam.com
profile: www.linkedin.com/in/phcjam
1.0 INTRODUCTION TO HEAT TRANSFER
1.1_ The first law of thermodynamics states that energy cannot be created or destroyed, only transferred from one place to another, in one form or another.
1.2 Heat is energy transferred from one body to another as the result of a temperature difference between them:
1.2.1 Sensible Heat is heat energy that can be measured by a thermometer;
1.2.2 Latent Heat is stored energy that cannot be directly measured by a thermometer.
1.3 Heat transfer takes place in four different modes:
1.3.1 Radiation is the transfer of heat by electromagnetic waves which does not affect the medium through which it passes;
1.3.2 Convection is the transfer of heat through a fluid medium;
1.3.3 Conduction is the transfer of heat from one solid body to another through physical contact;
1.3.4 Phase Change is the transfer of heat which takes place when the medium undergoes a change of state.
2.0 SOLAR RADIATION INCIDENT ON THE EARTH VARIES
2.1 Solar Radiation is the short-wave heat input emitted from the sun;
2.2 Extra-terrestrial Radiation is solar radiation outside the Earth's atmosphere;
2.3 The intensity of extra-terrestrial radiation is constant over time;
2.4 The intensity of solar radiation incident on the Earth's surface varies during the course of a day because of atmospheric attenuation and rotation of the Earth.
3.0 THE EARTH ROTATES ABOUT A SKEWED AXIS
3.1 The Earth's atmosphere is classified by temperature distribution into five layers:
3.1.1 The upper atmosphere comprises two cold layers having 90% of the Earth=s ozone layer.
3.1.2 A large portion of ultra-violet rays is removed by the ozone layer, thus changing the solar spectrum.
3.1.3 The lower atmosphere comprises three warm layers which represents 86% of the atmosphere by mass:
3.1.1 Clouds reflect 28% of solar radiation out of the lower atmosphere;
3.1.2 The atmosphere absorbs 25% of solar radiation;
3.1.3 Remaining Solar Radiation is separated into beam and diffuse components as it passes through the lower atmosphere.
3.2 Rotation of the Earth causes the angle of incidence with the surface to vary throughout the day.
3.3 Rotation of the Earth also causes attenuation to increase at sunrise and sunset because the path through the atmosphere is increased;
3.4 The Earth's surface absorbs 47% of the solar radiation;
3.5 The Cosine Law states that the intensity of an electromagnetic wave (such as solar radiation) on any surface is proportional to the cosine of the angle of incidence to the normal of the surface:
3.5.1 Higher latitudes receive less intense solar radiation;
3.5.2 Solar radiation is least intense at sunrise and sunset;
3.5.3 Solar radiation is more intense at noon; and
3.5.4 Solar radiation is most intense when the Sun's rays are perpendicular to the Earth's surface.
3.6 Ground cover shades or reflects a portion of the available solar radiation.
3.7 Thermal storage is primarily influenced by the oceans, which cover 75% of the Earth's surface.
4.0 TERRESTRIAL RADIATION IS RESPONSIBLE FOR COOLING
4.1 Terrestrial Radiation is the long wave heat output emitted from the Earth.
4.2 Atmospheric Turbidity inhibits transmission of terrestrial radiation.
4.3 Accumulation of carbon dioxide in the atmosphere also inhibits terrestrial radiation and is referred to as the AGreen House Effect@.
5.0 TEMPERATURE FLUCTUATES ACCORDING TO INCIDENT SOLAR RADIATION, TERRESTRIAL RADIATION AND THERMAL STORAGE BY EARTH
5.1 The temperature of the Earth's surface rises as solar radiation is absorbed:
5.1.1 Higher latitudes are colder when sunlight hours are reduced, because the intensity of solar radiation is less;
5.1.2 Temperature increase is less over bodies of water because the heat capacity of water is 2-3 times higher than that of dry soil.
5.1.3 Heat Capacity is the ability of a given volume of material to store or absorb heat while undergoing a temperature change.
V = ρ x Cv 5.1
where.
V = Heat Capacity, W/m3*K
ρ = Density, Kg/m3
Cv = Specific Heat, W/Kg*K
5.2 The Earth's surface transmits heat to adjoining materials:
5.2.1 A portion of heat at the surface of dry soil is conducted to deeper layers of soil;
5.2.2 A portion of heat at the surface of bodies of water is convected to greater depths;
5.2.3 Adjacent air is heated by conduction; and
5.2.4 Air currents transfer heat by convection.
5.3 Diurnal temperature results from daily storage of absorbed solar radiation and cooling by terrestrial radiation:
5.3.1 The amplitude of temperature fluctuations is greater over continental land masses;
5.3.2 The time lag of diurnal temperature is more delayed over continental land masses.
6.0 WIND RESULTS FROM DIFFERENTIAL HEATING ON THE EARTH'S SURFACE AND ROTATION OF THE EARTH
6.1 Differential heating gives rise to convective air-currents:
6.1.1 Land and Sea Breezes result from differential heating of land and sea;
6.1.2 Between latitude 30EN and 30ES, two opposite eddy currents develop, known as Hadley Cells.
6.2 The Earth's rotation sets up a Coriolis Force to turn the eddy currents.
7.0 SEASONS RESULT FROM ROTATION OF THE EARTH ABOUT THE SUN
7.1 The Earth is divided into 6 declinations because its axis of rotation is not perpendicular to the plane in which it rotates about the sun.
7.2 Seasons are produced by the revolution of the Earth about the Sun:
7.2.1 The sub-tropics (between 23.5EN and 23.5ES) generally receive the greatest solar insolation because the surface is inclined towards the sun;
7.2.2 Daylight hours and solar insolation are more variable outside the tropics, so there is greater fluctuation of temperature.
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