2.4 Earth’s Energy Balance (Shortwave and Longwave Radiation)

For the Earth to remain at a constant temperature the incoming radiation must equal the outgoing radiation.  If incoming exceeds outgoing, the Earth will warm.  If outgoing exceeds incoming, the Earth will cool.  This module examines Earth’s energy balance from the perspective of keeping the Earth at constant temperature in that incoming equals outgoing.

 

 

Video: NASA: Why Does the Sun Matter for Earth’s Energy Budget?
(1:39 min.)

This NASA video introduces the concept of Earth’s energy balance.

 

 

2.4.3 Sensible and Latent Heat

Sensible heat is the transfer of heat from one substance to another that results in a change in temperature.  For the Earth and the atmosphere, the substances are land, water, and air.  If you go outside on a hot day, the increase in the temperature you feel is your body gaining sensible heat.  Sensible heat transfers by conduction.

Associated with this heat transfer is another term called specific heat.  Specific heat is the amount of energy required to produce a temperature change.  Different substances have different specific heats.  For example, this diagram illustrates the differences between the specific heat of water and soil.

 

 

Water has a higher specific heat than soil; more energy is required to raise its temperature.  This difference in specific heat between water and soil is important!  If the Sun is shining equally on an ocean (water) and nearby land (soil), the land will heat up faster and to a higher temperature than the ocean.  The higher specific heat causes water bodies to generally cool slower than land during the night too.

Latent Heat

Latent heat is the energy required to change the phase (gas, liquid or solid) of a substance.  Water is the substance of most interest in the atmosphere.  One gram of water requires 1 calorie to raise its temperature 1oC.  For 1 gram of water to go from 0oC to 100oC requires 100 calories.  The phase change for 1 gram of water to freeze (or melt) requires 80 calories.  The phase change for 1 gram of water to evaporate (or condense) requires a whopping 540 calories.

 

Additionally, water can skip the intermediate step of being a liquid and go straight from a solid to a gas (water vapor), which is called sublimation.  The reverse of going from water vapor to ice is deposition.  The latent heat associated with these changes is either taken from the atmosphere or released to the atmosphere.  For rain to occur, water vapor must condense.  The latent heat associated with condensation releases a lot of heat into the atmosphere.  Most people do not think of rain as heating the atmosphere, but from a thermodynamic perspective, this is occurring.  The latent heat of condensation will be referenced repeatedly in this course, because it dramatically affects atmospheric stability.

 

2.4.4 Greenhouse Effect

The name “greenhouse effect” was dubbed in the early 1800s when it was thought that greenhouses stayed warmer because the panes of glass allowed solar radiation to enter, but prevented radiation emitted from plants and other objects inside the greenhouse from escaping. Actually, the reason why greenhouses stay warmer inside has to do with the fact that the air inside cannot mix with cooler air outside. The warm air in a greenhouse essentially gets trapped inside the panes of glass.

 

 

The atmospheric “greenhouse effect” is all about the absorption and emission of radiation by atmospheric gases.  The physical processes in a greenhouse are much different than the atmospheric “greenhouse effect”.  The variable gases associated with capturing heat are now referred to as greenhouse gases (GHGs), which indicates that the greenhouse terminology will be used for decades.

Video: 1.1.2 Earth’s Energy Budget (7:00 min.)

This video from the University of Oklahoma covers several concepts in this module, including the greenhouse effect.

 

 

References:

incoming equals outgoing.png – Public domain diagrams from NASA at https://earthobservatory.nasa.gov/features/EnergyBalance/page1.php

incoming energy details.jpg – From NOAA, NWS public domain at https://www.weather.gov/jetstream/energy Modified image in this module to only display the incoming portion.

outgoing energy details.jpg – From NOAA, NWS public domain at https://www.weather.gov/jetstream/energy Modified image in this module to only display the outgoing portion.

License

Icon for the Creative Commons Attribution 4.0 International License

Exploring Weather and Climate Copyright © by Glen Sampson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.