EG1204: Earth Systems: an introduction Meteorology and Climate Lecture 2 Energy, radiation and temperature
Topics we will cover Energy - basic laws and theory Temperature scales Specific heat and latent heat Energy transfer in the atmosphere The Earth’s energy balance The greenhouse effect
Energy: basic laws and theory The energy stored in an object determines how much work it can do (e.g. water in a dam). This is potential energy PE = potential energy m = mass of the objectg = acceleration of gravity h = object’s height above the ground PE = mgh
Energy: basic laws and theory A volume of air aloft has more potential energy than the same volume of air above the surface The air aloft has the potential to sink and warm through a greater depth of the atmosphere Any moving object possesses energy of motion or kinetic energy
Energy: basic laws and theory The kinetic energy of an object is: KE = ½ mv 2 The faster something moves, the greater its kinetic energy. A strong wind has more kinetic energy than a light breeze m = mass and v = velocity
Energy: basic laws and theory Temperature is a measure of the average speed of the atoms and molecules, where higher temperatures correspond to faster average speeds If a volume of air within a balloon were heated the molecules would move faster and slightly further apart - making the air less dense Cooling air slows molecules down and so they crowd together becoming more dense
Energy: basic laws and theory Heat is energy in the process of being transferred from one object to another because of the temperature difference between them
Temperature scales Hypothetically, the lowest temperature attainable is absolute zero Absolute zero is ºC Absolute zero has a value of 0 on a temperature scale called the Kelvin scale - after Lord Kelvin ( ) The Kelvin scale has no negative numbers
Temperature scales The Celsius scale was introduced in the 18th century. The value of 0 is assigned to the freezing point of water and the value 100 when water boils at sea-level An increasing temperature of 1 ºC equals an increase of 1.8 ºF
Specific heat and latent heat Liquids such as water require a relatively large amount of heat energy to bring about just a small temperature change The heat capacity of a substance is the ratio of the amount of heat energy absorbed by that substance to its corresponding temperature rise
Specific heat and latent heat The heat capacity of a substance per unit mass is called specific heat Specific heat is the amount of heat needed to raise the temperature of one gram (g) of a substance by one degree Celsius 1g of liquid water on a stove would need 1 calorie (cal) to raise its temperature by 1 ºC
Specific heat and latent heat When water changes its state (solid to liquid, liquid to gas etc) heat energy will be exchanged The heat energy required to change a substance from one state to another is called latent heat Evaporation is a cooling process Condensation is a warming process
EMR and the Sun-atmosphere system About 50% of incoming solar radiation is lost by the atmosphere: scattered (30%) and absorbed (20%) Scattering involves the absorption and re-emission of energy by particles Absorption (unlike scattering) involves energy exchange
EMR and the Sun-atmosphere system The human eye cannot see infrared radiation Infrared radiation is absorbed by water vapour and carbon dioxide in the troposphere The atmosphere’s relative transparency to incoming solar (SW) radiation, and ability to absorb/re-emit outgoing infrared (LW) radiation is the natural greenhouse effect
Greenhouse effect The natural greenhouse effect maintains a stable climate for life on earth Outgoing radiation (longwave) is absorbed by molecules such as water vapour, methane and carbon dioxide Energy is then re-emitted in all directions - forming a blanket