Higher Education

Unit 07

RADIATION AND THE HEAT BALANCE OF THE ATMOSPHERE

Unit Overview

This unit discusses the Earth's energy balance. The main sections of Unit 7 are:

The radiation balance
The heat balance
Global distribution of heat flows

The Sun is the primary source of energy for the Earth, and its energy is transmitted predominantly in the form of shortwave radiation. Because this radiation is scattered, reflected, and absorbed by the atmosphere, the Earth's surface is only able to absorb a fraction of the shortwave radiation that is available at the top of the atmosphere. The Earth absorbs the shortwave radiation and, in turn, emits longwave radiation back towards the atmosphere and space. The atmosphere, which contains gases such as water vapour and carbon dioxide, absorbs the Earth's longwave radiation and subsequently reemits longwave radiation in all directions. This is known as the "greenhouse effect," which is just one part of the energy exchange process between the surface and the atmosphere. Sensible heat and latent heat are also transferred. There is a radiation balance for the Earth as a whole, but this balance has positive and negative values when examined for different places on the Earth. Generally, the low latitudes have energy surpluses and the high latitudes have energy deficits.

Unit Objectives

To understand the Sun-generated flows of energy that affect the Earth and its atmosphere
To link the greenhouse effect to the Earth's habitability and climatic variation
To introduce the Earth's heat flows and their spatial patterns

Glossary of Key Terms

Albedo	The proportion of incoming solar radiation that is reflected by a surface; the whiter the colour of the surface (albedo derives from the Latin word albus, meaning white), the higher the albedo.
Conduction	The transport of heat energy from one molecule to the next.
Convection Spontaneous	Vertical air movement in the atmosphere.
Counter-radiation	Longwave radiation emitted by the Earth's surface that is absorbed by the atmosphere and re-radiated (also as longwave radiation) back down to the surface.
Diffuse radiation	The proportion of incoming solar energy (22 percent) that reaches the Earth's surface after first being scattered in the atmosphere by clouds, dust particles, and other airborne materials.
Direct radiation	The proportion of incoming solar energy that travels directly to the earth's surface; globally, this averages 31 percent.
Global warming	The notion, popular among scientists in the late 1980s and early 1990s, that human fossil-fuel consumption is causing atmospheric warming that will melt glaciers; raise sea levels, and inundate low-lying coastal areas. In the mid 1990s, however, competing theories of climate change are receiving considerable attention and gaining new proponents.
Greenhouse effect	The widely used analogy describing the blanket-like effect of the atmosphere in the heating of Earth's surface; shortwave insolation passes through the "glass" of the atmospheric "greenhouse," heats the surface, is converted to longwave radiation that cannot penetrate the "glass," and thereby results in trapping of heat that raises the temperature inside the "greenhouse."
Ground heat flow	The heat that is conducted into and out of the Earth's surface; also known as soil heat flow.
Longwave radiation	Radiation emitted by the Earth, which has much longer wavelengths - and involves much lower energy - than solar (shortwave, higher energy) radiation emitted by the Sun.
Net radiation	The amount of radiation left over when all the incoming and outgoing radiation flows have been tallied; totals about one-fourth of the shortwave radiation originally arriving at the top of the atmosphere.
Radiation	The transmission of energy in the form of electromagnetic waves; a wide range of energy occurs within the electromagnetic spectrum.
Sensible heat flow	The environmental heat we feel or sense on our skins.
Shortwave radiation	Radiation coming from the Sun, which has much shorter wavelengths - and involves much higher energy - than the terrestrial (longwave, lower energy). radiation emitted by the Earth.

Unit Outline

The radiation balance
- shortwave radiation emitted from Sun
  - direct radiation
  - diffuse radiation
  - albedo (reflectivity)
- Longwave radiation emitted from Earth
  - counter-radiation
  - greenhouse effect
- Net radiation
The Heat Balance
- Sensible heat flow
  - convection
- Ground heat flow
  - conduction
- Climates and heat balance
  - net radiation
  - latent heat flow
  - seasonal variation due to location
Global distribution of heat flows
- Distribution of latent heat loss (Fig. 7.7)
- Distribution of sensible heat loss (Fig. 7.8)

Review Questions

Describe generally the global patterns of latent heat loss, using Figure 7.7 from your textbook as a guide.
Define the terms direct radiation, diffuse radiation, and net radiation.