Reference Work Entry

Encyclopedia of World Climatology

Part of the series Encyclopedia of Earth Sciences Series pp 711-716

Temperature Distribution

  • L. M. Trapasso

Each day Earth receives energy in the form of incoming solar radiation from the sun. This shortwave solar radiation ranges mostly from ultraviolet (0.2 μm wavelength) to the near-infrared (2.0 μm wavelength), but reaches its maximum at around 0.5 μm wavelength (blue-green visible light). This insolation is absorbed by Earth’s surface and is converted to heat (longwave radiation). Earth’s (terrestrial) longwave radiation reaches its peak intensity at the 10 μm wavelength (thermal infrared) and is responsible for heating the lower atmosphere.

Temperature is represented by a human-invented quantitative measure of heat energy emitted by or contained within a surface or material. As such, these numerical temperature values can be used to differentiate one climatic region from another. Spatial variations in temperatures (i.e. temperature distributions) occur both vertically and horizontally within the Earth’s atmospheric envelope. These temperature distributions, their causes and variations, are discussed below.

Vertical distributions

The atmosphere can be divided into four distinct layers based on characteristic temperature distributions (see Figure T7). The layer closest to the Earth’s surface is called the troposphere. Almost all atmospheric water vapor and turbulent mixing exist within this layer, thus it is here that virtually all phenomena related to weather or climate take place. Temperatures tend to decrease with increasing altitude above Earth’s surface (the source of terrestrial heat radiation). Cold temperatures observed at high elevations and snowcapped mountains are physical manifestations of this tropospheric temperature trend. On the average, temperatures in the troposphere will decrease at the normal or environmental lapse rate of 6.5°C/1000 m of rise.
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Figure T7

Vertical temperature structure of the atmosphere.

The troposphere ends at a boundary layer called the tropopause. The tropopause is at its highest elevation over the tropics (approximately 16 km) but decreases in elevation poleward (9 km or less at the poles; Lutgens and Tarbuck, 2001). Temperatures in the tropopause remain constant with height and this layer is isothermal (same temperature) in nature.

Temperatures tend to increase with height within the stratosphere. This increasing temperature trend is caused by heat released during the interaction between incoming ultraviolet radiation and the ozone (O3) layer (ozonosphere) nested within the stratosphere. It is the formation, the destruction (into molecular and atomic oxygen, O2 and O, respectively), and the reformation (O2+O→O3) of ozone that shields the Earth from the harmful effects of ultraviolet radiation and, at the same time, adds heat energy to the stratosphere. The upper limit of the stratosphere is bounded by another isothermal layer called the stratopause.

Above the stratopause lies the next layer of the atmosphere called the mesosphere. The temperatures in this atmospheric shell decrease with an increase in altitude. The mesosphere (middle sphere) ascends to an elevation of around 80 km where it ends at the last isothermal layer called the mesopause.

Extending from the mesopause to the outer limits of the atmosphere lies the final layer, the thermosphere, which accounts for a minute fraction of the atmosphere’s total mass. The particles (mostly ions) in this uppermost layer are energized by incoming solar radiation and move at very fast speeds, thus causing heat sensors to register an increasing temperature with height.

Factors influencing the horizontal distribution

Though it has been shown that temperatures vary vertically from the surface of the planet to the outer limits of the atmosphere, horizontal temperature distributions and the mechanisms that create them are of greater importance to climatologists and meteorologists. The causative factors and resulting temperature distributions on the surface of Earth are discussed in further detail.

Latitude and sun angle

Latitude is the single most important factor determining planetary temperature distributions. In general, as one traverses from the equator (0° latitude) to either pole (90° latitude) temperatures decrease. This equator-to-pole temperature gradient is directly related to the angle at which the sun’s rays strike Earth’s surface.

Solar radiation, having traveled some 150 million km, reaches Earth’s spheroid surface in essentially parallel lines (see Figure T8). As such, there can be only one latitude where the sun’s rays intersect the surface at a direct (90°) angle. A displacement north or south from this line of direct (maximum) radiation must result in an incident sun angle less than 90°; thus the intensity of the radiation decreases (Figure T9).
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Figure T8

The Earth in the December solstice position. Parallel sun rays strike the Earth at a variety of sun angles. Rays striking the Earth at low angles, must traverse more of the atmosphere than rays striking at a high angle, and are thus subject to greater depletion by reflection and absorption (Lutgens and Tarbuck, 2001).

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Figure T9

The sun rays intersecting Earth’s surface at 60° (right) will cover twice the ground surface but with only half the intensity of a direct 90° angle (left).

The relationship between the intensity of solar radiation and sun angle is described by Lambert’s Cosine Law: I=I 0 (cos γ), where I=radiant intensity of radiation reaching some point on the surface, I 0=radiant intensity at maximum (where sun angle is 90° overhead) and γ=angle from the vertical to the direction of the radiation (zenith angle; Rosenberg, 1974). For example, when γ equals 60° from vertical, the intensity of solar radiation is half of its maximum (cos 60°=0.5000; Figure T9).

The thickness of the atmosphere through which insolation must pass is also affected by sun angle. Low-latitude high-sun angles pass through a shorter distance of atmosphere as opposed to high-latitude oblique sun angles, which must traverse a much greater distance (see Figure T8). By traveling through a greater distance of atmospheric gases, the intensity of the solar radiation is partially extinguished by the gas molecules. This relationship is best described by Beer’s Law:
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where I 0 is the initial radiant intensity and I is the radiant intensity after passing through a depth x of a medium (i.e. the atmosphere) with extinction coefficient a. Extinction of radiant intensity can occur by absorption and scattering of the solar radiation (Rosenberg, 1974). Again, the low sun angles in polar regions are less intense than the tropical high sun angles.

Seasons The migration of the maximum sun angle north and south of the equator is responsible for seasonal variations on the Earth. On 21–22 December the most direct (90°) sun angle contacts 23 1/2°S latitude (the Tropic of Capricorn). In this orbital position — the December Solstice — the northern hemisphere is tilted its farthest back (23 1/2°) from the sun and experiences the winter season. This represents the shortest day of the year in the northern hemisphere, and the North Pole is in total darkness (see Figure T8). In the southern hemisphere this is the longest day of the year, the first day of summer and the sun is visible above horizon 24 hours a day at the South Pole. During 21–22 June, radiation strikes directly upon 23 1/2°N latitude (Tropic of Cancer). The June Solstice marks the first day of summer in the northern hemisphere and winter in the southern hemisphere.

The vernal and autumnal equinoxes (21°22 March and September, respectively) are the days when the sun’s rays hit directly on the equator. Earth experiences 12 hours of daylight and 12 hours of darkness on these equinox dates.

During the course of a year the direct and most intense rays of the sun will migrate from 23 1/2°N to 23 1/2°S and back again. For this reason the tropics are always warm, energy-rich areas of the world, whereas the poles are always cold, energy-poor regions of the world. Again, the angle of incoming solar radiation will cause an equator to pole (latitude-dependent) temperature gradient found on the Earth.

Air mass circulation

As stated above, incident sun angles cause the tropics to be energy-rich, and the poles energy-deficient. This imbalance of energy has not, however, caused the tropics to become warmer nor the poles to become colder through time. The tropics and the poles maintain their climatic characteristics by exchanging energy, mass, and momentum through the middle latitudes (23 1/2° to 66 1/2° N or S). The weather and climates in the middle latitudes are variable in nature due to the invasion of large air masses poleward from the tropics and equatorward from Arctic and Antarctic regions.

An air mass is a large body of air measuring hundreds to thousands of kilometers in length and breadth, and may extend from the surface to the tropopause. Air masses have definable characteristics of temperature and humidity derived from their source regions; that is, the region from which the air mass originates.

The circulation of these air masses, and more importantly the frequency with which an air mass type dominates an area, will affect the temperature distribution and the climate of that area (Oliver, 1970). For example, if the midwestern United States experiences invasions of continental arctic (CA) air from northern Canada during a winter season, it may become a record cold winter. In contrast, if that same region during the winter is frequented by maritime tropical (mT) air from the Gulf of Mexico, a mild season would result. Air mass movements are variable and their variations are reflected in the temperature distributions.

Cloud cover

The effects of cloud cover on temperature are most evident in the diurnal temperature cycle (Figure T10). Cloud cover during daylight hours prevents some incoming solar radiation from reaching Earth’s surface by reflecting it back to space. Thus temperatures may be cooler on cloudy days. Cloud cover at night causes overnight low temperatures to be warmer. The clouds act like a blanket, keeping longwave terrestrial (heat) radiation close to Earth’s surface. This heat radiation would otherwise be lost to space by escaping through a cloudless sky.
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Figure T10

The daily temperature cycle on a clear day versus a cloudy day. Cloud cover will depress the daily maximum while raising the daily minimum temperature (after Lutgens and Tarbuck, 2001).

Excessive cloud cover may cause variances in annual temperature cycles as well. Monsoonal regions are particularly susceptible. During the wet monsoon, long periods of heavy cloud cover will cause temperature values to be depressed. Temperature data for Calcutta, India (Table T1), shows April, May, and June to have the highest temperatures, whereas July and August (considered months with the highest temperatures) are cooler beneath the monsoonal cloud cover (beginning in June and extending through August).

On a global scale, areas of persistent cloud cover or clear skies will cause alterations in temperature distributions. The equatorial region of the Earth is one with abundant cloud cover. The perennial high sun angles of the tropics warm the moist equatorial air, causing it to become buoyant and rise (equatorial low pressure) and cool aloft to form ample cloud cover. By contrast, subtropical regions (30–35° N or S latitudes) are regions of stable, descending air (subtropical high pressure), which is conducive to the formation and maintenance of clear skies. Subtropical high-pressure systems and their associated clear skies are responsible for many of the world’s large deserts (e.g. North Africa and Saudi Arabia).

Though the highest sun angles occur in the tropics, the clear skies and abundant solar radiation of the subtropics allow these regions to yield higher temperatures. Many all-time record high temperatures were observed at stations located between 30° and 40° latitude. Some examples are Azizia, Libya (57.8°C); Death Valley, California (56.7°C); Seville, Spain (50°C); and Tirat Tsvi, Israel (53.4°C) (National Climatic Data Center). From the microscale to the global scale, and within diurnal as well as annual temperature cycles, cloud cover remains a factor that affects temperature distributions.

Distribution of land and sea

The surface of Earth is covered by a wide range of materials. The character of these surfaces will affect the way they accept solar radiation and emit terrestrial radiation. The largest division of surface materials lies between land and water. These two substances react differently to incoming and outgoing radiation. In general, land heats and cools considerably faster than water. This difference in the rate of their temperature gain or loss is caused primarily by their different specific heat values. Specific heat is the amount of heat necessary to raise the temperature of a material 1°C. Water has a specific heat of 1 calorie per gram °C, whereas most earthen materials range between 0.3 and 0.5 calories per gram °C. Thus land masses can heat and cool two to three times faster than water bodies.

Water is also transparent (or translucent) and accepts sunlight into its upper layers. In addition, water has the ability to circulate the heat energy. Unlike water, an opaque and rigid
Table T1

Mean monthly temperatures (°C), Calcutta, India

Month

J

F

M

A

M

J

J

A

S

O

N

D

 

18

21

27

30

31

30

28

27

28

27

22

18

land surface reacts more quickly to heat energy exchanges with the atmosphere.

Surface temperature characteristics vary with distance from large bodies of water (e.g. large lakes or oceans). Proximity to a large body of water tends to moderate extreme temperatures. This effect is clearly demonstrated in annual temperature range value (i.e. the difference between the highest mean monthly temperature in summer and the lowest mean monthly temperature in winter). For example, choosing three stations of approximately equal latitude (32°N), we find Bermuda (surrounded by water) has a temperature range of 9–10°C. By comparison, Charleston, South Carolina (a coastal station), has an annual temperature range of 17°C. Further inland, Pine Bluff, Arkansas (a landlocked station), has an annual temperature range of 23–24°C (Tanner, 1964). Therefore, the continentality of a location (the degree to which the interior of a continent affects the climate) will have an influence on temperature distributions.

Albedo

Land surfaces are composed of many different materials, each with its own set of characteristics that may affect Earth’s energy balance and temperature distributions. Albedo is one such characteristic and is defined as the amount of solar radiation reflected by a surface to the amount incident upon it; it is commonly expressed as a percentage (Huschke, 1959). Surfaces with high albedo (e.g. snow and ice) reflect 80–85% of the incoming solar radiation and express lower temperatures than surfaces of low albedo (e.g. green forest, albedo =3–10%).

Cloud cover has the most variable albedo, which changes with the type and thickness of clouds (ranges from negligible values to 80%). In summary, the changing colors, textures, and substances of surface materials will determine albedo values and thus affect surface temperature distributions.

Ocean drifts and currents

Ocean water is transported by two major mechanisms: (1) frictional drag of prevailing winds on the ocean surface; and (2) differences in water density (density currents) with varying heat and salt concentrations. A slow, inconspicuous movement (3–4 km/h) over broad areas are called drifts, as opposed to currents, which are narrower, extend to greater depths and move faster (10 km/h).

If a drift or current originates in tropical (low-latitude) regions and migrates poleward, it will invade colder ocean waters and by comparison to its surroundings would appear as a warm current. Conversely, drifts and currents that stem from the higher latitudes and advect equatorward are perceived as cold (or cool) currents.

Where they exist, warm and cold currents will influence surface temperature distributions, especially along coastal regions. For instance, if we examine the mean annual temperatures of the two South American coastal locations at the same latitude, the effect of ocean currents is clearly visible. Lima, Peru, is under the dominance of the cold Peruvian current moving equatorward and has a mean annual temperature of 19°C, whereas Salvador, Brazil, under the influence of the poleward-flowing, warm Brazilian current, has a mean annual temperature of 26°C (National Climatic Data Center). Though all the atmosphere-ocean interrelationships are not fully understood, the consequences of ocean movements and climate (and more specifically temperature distributions) cannot be ignored.

Urban-rural temperatures

Surface temperature distributions tend to take on distinctive patterns in and around urban areas, designating what is called an urban heat island. In general, temperatures tend to increase as one traverses from the rural surroundings to the center of an urban area.

Observed horizontal distribution

The actual horizontal distribution of temperatures are shown in Figures T11 and T12. These maps depict mean global isotherms (lines of equal temperature value) for January and July. The patterns of the isotherms tend to confirm some of the influencing factors previously mentioned.
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Figure T11

Map showing global distributions of mean January temperatures. The isotherm line patterns reflect some of the controls on surface temperatures.

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Figure T12

Map showing global distributions of mean July temperatures. The isotherm line patterns reflect some of the controls on surface temperatures.

Equator to pole gradient

The first noteworthy pattern can be seen on both figures (regardless of season). The east-west arrangement of isotherms mimics the orientation of latitude lines and reflects a changing sun angle. Here, the effects of Lambert’s Cosine Law and Beer’s Law work together to create an obvious equator-to-pole temperature gradient. The isotherms will deviate from their east-west alignment when passing from sea to land and back again. These disruptions in the continuity of the isotherms demonstrate the way land and sea accept solar radiation and emit terrestrial radiation. During the same season, and at comparable latitudes, land and adjacent sea may not maintain the same temperature.

Maximums and minimums

The maximum temperatures are observed in the high sun hemisphere (hemisphere that is experiencing its summer season). This shifting of maximum temperatures from the northern hemisphere (in July) to the southern hemisphere (in January) follows the migration of the direct (90°) incident sun angle from the June solstice position (23 1/2°N latitude) to the December solstice position (23 1/2°S latitude). Further note that the maximum temperatures are found in the subtropical regions (approximately 30° N or S latitude) where clear skies dominate, and not in the tropics where a cloud cover prevails much of the time Finally, maximum temperature (e.g. over North Africa) and the minimum temperatures (e.g. over Antarctica and northern Siberia) are found over land bodies. Here again, a low specific heat value and other factors mentioned earlier allow land bodies to respond more rapidly to seasonal changes. This is further exemplified in Figure T13, which displays average annual temperature ranges (the difference in temperature between January and July averages). In this figure, small temperature ranges are found over the oceans where temperature changes are less dramatic and large temperature ranges (i.e. closed isotherms) are found over continental regions (effects of continentality).
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Figure T13

Map depicting global distributions of temperature range. The difference between mean January and mean July temperatures constitutes useful information to the climatologist.

Ranges and seasonality

The temperature range isoline map (Figure T13) is useful in demonstrating more than just the continentality of a particular region. Temperature ranges can further indicate the degree to which a region is affected by the seasonal changes. The small temperature range isolines that straddle the equator are showing that the tropics experience no appreciable seasonal changes. The highest sun angles within the tropics year-round; therefore the climate remains essentially the same. Near the poles, low sun angles and periods of total darkness account for seasonal variation. However, the middle-latitude temperature ranges are much higher (seasonal changes are more prominent). This is an area where sun angles, air masses, ocean currents, surface albedo, and cloud cover vary through time and space. With alterations in these and other factors come changes in temperature across the surface.

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