EG1204: Earth Systems: an introduction

Meteorology and Climate

Lecture 2Energy, 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 object g = 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 mv2

• 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 -273.15 ºC• Absolute zero has a value of 0 on a

temperature scale called the Kelvin scale - after Lord Kelvin (1824-1907)

• 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