Section 2.3:
Mechanisms of Heat Transfer

Learning Objective

List and describe the three mechanisms of heat transfer.

Section Content

The flow of energy can occur in three ways: conduction, convection, and radiation (Figure 2.11). Although we will present them separately, all three mechanisms of heat transfer can operate simultaneously and, working in tandem, these processes can transfer heat between the Sun and Earth and between Earth’s surface, its atmosphere, and outer space.

Figure 2.11
Three mechanisms of heat transfer: conduction, convection, and radiation

Tutorial Video - Three Mechanisms of Heat Transfer (Click to watch the video)


Anyone who attempts to pick up a metal spoon left in a boiling pot of soup realizes that heat is transmitted along the entire length of the spoon. The transfer of heat in this manner is called conduction. The hot soup causes the molecules at the bowl end of the spoon to vibrate more rapidly. These molecules collide more vigorously with their neighbors and so on up the handle of the spoon. Thus, conduction is the transfer of heat through molecular collisions from one molecule to another. The ability of substances to conduct heat varies considerably. Metals are good conductors, as those of us who have touched a hot metal spoon quickly learned. Air, in contrast, is a very poor conductor of heat. Consequently, conduction is important only between Earth’s surface and the air immediately in contact with the surface. Conduction is the least significant means of heat transfer for the atmosphere as a whole, and we can disregard it when considering most meteorological phenomena.

Objects that are poor conductors, such as air, are called insulators. Most objects that are good insulators, such as cork, plastic foam, or goose down, contain many small air spaces. The poor conductivity of the trapped air gives these materials their insulating value. Snow, like other good insulators, contains numerous air spaces that impair the flow of heat. This is why wild animals may burrow into a snowbank to escape the “cold.” The snow, like a down-filled comforter, does not supply heat; it simply retards the loss of the animal’s own body heat.


Much of the heat transport in Earth’s atmosphere and oceans occurs by convection. Convection is heat transfer that involves the actual movement or circulation of a substance. It takes place in fluids (liquids such as water and gases such as air) where the material can flow.

The pan of water being heated over a campfire in Figure 2.11 illustrates the nature of a simple convective circulation. The fire warms the bottom of the pan, which conducts heat to the water inside. Because water is a relatively poor conductor of heat, only the water in close proximity to the bottom of the pan is heated by conduction. Heating causes water to expand and become less dense. Thus, the hot, buoyant water near the bottom of the pan rises, while the cooler, denser water above sinks. As long as the water is heated from the bottom and cools near the top, it will continue to “turn over,” producing a convective circulation.

In a similar manner, some of the air in the lowest layer of the atmosphere that is heated by radiation and conduction is then transported by convection to higher layers of the atmosphere. For example, on a hot, sunny day, a dark plowed field becomes warmer than the surrounding croplands, and the air above the plowed field will be heated more than the air above the crops. As warm, less-dense air above the plowed field buoys upward, it is replaced by the cooler air above the croplands (Figure 2.12). In this way, a convective flow is established. The warm parcels of rising air are called thermals, and they are what hang-glider pilots use to keep their crafts soaring. Convection of this type not only transfers heat but also transports moisture (water vapor) aloft. The result is an increase in cloudiness that frequently can be observed on warm summer afternoons.

Figure 2.12
Rising warmer air and descending cooler air are examples of convective circulation

A. Heating of Earth’s surface produces thermals of rising air that transport heat and moisture aloft. B. The rising air cools, and if it reaches the condensation level, clouds form.

On a much larger scale is the global convective circulation of the atmosphere, which is driven by the unequal heating of Earth’s surface. These complex movements are responsible for the redistribution of heat between hot equatorial regions and frigid polar latitudes; we will discuss them in detail in Chapter 7.

Atmospheric circulation consists of vertical as well as horizontal components, so energy is transferred both vertically and horizontally. Meteorologists often use the term convection to describe the part of atmospheric circulation that involves upward and downward motion of air. By contrast, the term advection is used to denote the horizontal component of airflow. A common example of advection is wind, a phenomenon we will examine closely in later chapters. Residents of the midlatitudes often experience the effects of heat transfer by advection. For example, when frigid Canadian air invades the U.S. Midwest in January, it brings bitterly cold winter weather, whereas advection of latent heat from the Gulf of Mexico is a primary source of energy for spring thunderstorms.


The third mechanism of heat transfer is radiation. Unlike conduction and convection, radiation is the only mechanism that can transfer thermal energy through the vacuum of space and thus is responsible for solar energy reaching Earth.

Solar Radiation

Mini-Lecture Video - Solar Radiation (Click to watch the video)

The Sun is the ultimate source of energy that drives the weather machine. We know the Sun emits light of varying energy—including visible light, infrared radiation, and ultraviolet radiation. Although these forms of energy constitute a major portion of the total energy that radiates from the Sun, they are only a part of a large array of energy called radiation, or electromagnetic radiation. This array or spectrum of electromagnetic energy is shown in Figure 2.13.

Figure 2.13
The electromagnetic spectrum

The names and wavelengths of various types of electromagnetic radiation are shown. A nanometer (nm) is one thousandth of a micrometer.

Video - Tour of the Electromagnetic Spectrum (Click to watch the video)

All types of radiation from the Sun travel through the vacuum of space at 300,000 kilometers (186,000 miles) per second, a value known as the speed of light. To help visualize radiant energy, imagine ripples made in a calm pond when a pebble is tossed in. Like the waves produced in the pond, waves of electromagnetic radiation come in various sizes, or wavelengths—the distance from one crest to the next (Figure 2.13). Radio waves have the longest wavelengths, up to thousands of meters in length. Gamma waves are the shortest, at less than one-billionth of a centimeter. Shortwave radiation is usually measured in micrometers (abbreviated μm), which are one-millionth of a meter.

Radiation is often identified by the effect that it produces when it interacts with an object. The retinas of our eyes, for instance, are sensitive to a range of wavelengths called visible light. We often refer to visible light as white light because it appears white in color. It is easy to show, however, that white light is an array of colors, each color corresponding to a specific range of wavelengths. When it is passed through a prism, white light can be divided into the colors of the rainbow, from violet (with the shortest wavelength, 0.4 micrometer [μm]) to red (with the longest wavelength, 0.7 micrometer). See Figure 2.13.

Located adjacent to the color red, and having a longer wavelength, is infrared radiation (IR), which cannot be seen by the human eye but is detected as heat. On the opposite side of the visible range, located next to violet, the energy emitted is called ultraviolet (UV) radiation and consists of shorter wavelengths that may cause skin to become sunburned.

You might have wondered . . . 

On a cold morning, why does a bathroom’s tile floor feel much colder than a bedroom’s carpet, even though both materials are the same temperature?

The difference you feel is due mainly to the fact that the tile is a much better conductor of heat. Hence, energy is more rapidly conducted from your bare feet to the tile floor than from your feet to the carpet. Even at room temperature (20°C [68°F]), objects that are good conductors can feel chilly to the touch. (Remember that body temperature is about 37°C [98.6°F].)

Although we divide radiant energy into categories based on our ability to perceive them, all wavelengths of radiation behave similarly. When an object absorbs any form of electromagnetic energy, the waves excite subatomic particles (electrons). This results in an increase in molecular motion and a corresponding increase in temperature. Thus, electromagnetic waves from the Sun travel through space and, upon being absorbed, increase the molecular motion of other molecules—including those that make up the atmosphere, Earth’s surface, and human bodies.

One important difference among the various wavelengths of radiant energy is that shorter wavelengths are more energetic. This accounts for the fact that exposure to relatively short (high-energy) ultraviolet waves can cause significant sunburn, while similar exposure to longer-wavelength radiation cannot. Extended exposure to UV radiation can result in skin cancer and cataracts (see Severe & Hazardous Weather Box 2.3).

Severe & Hazardous Weather 2.3

The Ultraviolet Index

On warm days, when the sky is cloudless and bright, people enjoy spending a great deal of time outdoors “soaking up” the sunshine (Figure 2.C). For many, the goal is to develop a dark tan that sunbathers often describe as looking “healthy.” Ironically, there is strong evidence that too much sunshine (specifically, too much ultraviolet radiation) can lead to premature skin aging and serious health problems such as skin cancer and cataracts.

Figure 2.C
Exposure to too much ultraviolet radiation can cause serious health problems.

Since June 1994, the National Weather Service (NWS) has issued the ultraviolet (UV) index forecasts to warn the public of potential health risks of exposure to sunlight. The UV index is determined by taking into account the predicted cloud cover and reflectivity of the surface, as well as the Sun angle and atmospheric depth for each forecast location. Because atmospheric ozone strongly absorbs ultraviolet radiation, the extent of the ozone layer is also considered. The UV index values lie on a scale from 0 to 11+, with larger values representing greatest risk.

The U.S. Environmental Protection Agency (EPA) has established five exposure categories based on UV index values: Low, Moderate, High, Very High, and Extreme, as described in Table 2.B. Table 2.B also indicates the range of minutes it will take the most susceptible skin types (pale or milky white) to burn. The EPA recommends applying sunscreen with a sun-protection factor (SPF) of 30 or higher to all exposed skin (and reapplied every 2 hours). This is especially important after swimming or while sunbathing, even on cloudy days with the UV index in the Low category. The public is advised to minimize outdoor activities when the UV index is Very High or Extreme.

Table 2.B
The UV Index: Minutes to Burn for the Most Susceptible Skin Type

Weather Safety

The EPA also recommends using the “shadow rule” to determine how much UV exposure you are getting:

It is important to note that the Sun emits all forms of radiation, but in varying quantities. Over 95 percent of all solar radiation is emitted in a narrow band between 0.1 and 2.5 micrometers, much of which is concentrated in the visible and near-infrared parts of the electromagnetic spectrum (Figure 2.14). Visible light represents over 43 percent of the total energy, infrared accounts for 49 percent, and ultraviolet, 7 percent. Less than 1 percent of solar radiation is emitted as X-rays, gamma rays, microwaves, and radio waves.

Figure 2.14
Comparison of the intensity of solar radiation and radiation emitted by Earth

Because of the Sun’s high surface temperature, most of its energy is radiated at energetic wavelengths shorter than 2.5 micrometers (mm). The greatest intensity of solar radiation is in the visible range of the electromagnetic spectrum. Earth, in contrast, radiates most of its energy in wavelengths longer than 2.5 micrometers, primarily in the far end (less-energetic) of the infrared band. Thus, we call the Sun’s radiation shortwave and Earth’s radiation longwave.

Laws of Radiation

To better appreciate how the Sun’s radiant energy interacts with Earth’s atmosphere and surface, it is helpful to have a general understanding of the basic radiation laws. Although the mathematics of these laws is beyond the scope of this text, the fundamental concepts are straightforward:

Although the Sun is the ultimate source of radiant energy, all objects continually radiate energy over a range of wavelengths. Hot objects, such as the Sun, emit mostly shortwave (high-energy) radiation, but cooler objects, such as Earth, emit longwave (low-energy) radiation. Objects that are good absorbers of radiation, such as Earth’s surface, are also good emitters. By contrast, most atmospheric gases are good absorbers (emitters) of radiation only in certain wavelengths but are poor absorbers (emitters) in other wavelengths.

Section Glossary

Section Summary

Section Study Questions

Try to answer the following questions on your own, then click the question to see the correct answer.

Describe the three basic mechanisms of energy transfer. Which mechanism is least important meteorologically?

Conduction is the transfer of heat through matter by molecule-to-molecule contact, whereas convection refers to heat transfer by the movement of a mass or substance. Radiation, the method of heat transfer between the Sun and Earth, is the transfer of heat through space by electromagnetic waves. Meteorologically, conduction is the least important mechanism of heat transfer.

Why do we describe solar radiation as shortwave radiation and radiation coming from Earth as longwave radiation?

Peak terrestrial radiation (10 micrometers) is roughly 20 times longer than the peak solar radiation (0.5 micrometers).

Describe the relationship between the temperature of a radiating body and the wavelengths it emits.

The higher the temperature of a radiating body, the shorter the wavelength of maximum radiation. The entire spectrum of emitted wavelengths is also shifted toward shorter values as the temperature increases.