Sun is the major source of atmospheric temperature. In fact, the atmosphere receives a meagre amount of heat energy from the Sun as it receives most of its energy from the long-wave terrestrial radiation.
The heating and cooling of the atmosphere are accomplished through direct solar radiation and the transfer of energy from the earth through conduction, convection, and radiation.
Temperature belts of the world
The three major heat zones of the Earth are
- Temperate Zone
- Torrid Zone
- Frigid Zone
These are based on the distance they have from the Equator.
Torrid Zone (Tropical Zone)
This is the hottest zone of the Earth. The region that lies from the Tropic of Cancer (23.5°N), across Equator (0°) to the Tropic of Capricorn (23.5°S) is considered the torrid zone (Tropical Zone). The Sun’s ray falls directly at least once a year.
This is the habitable heat zone of the Earth. There are two temperate zones lie in between in both 23½° to 66½° the hemisphere. These regions have moderate, tolerable temperatures.
This is the coldest zone of the Earth. This area lies to the north of the Arctic circle (66.6°N) and the south of the Antarctic circle (66.5°S) and is permanently frozen. There is no sunlight for most of the months is of the year in this zone.
➤ Importance of the Heat Zones
This division of the Earth into different heat zones helps understand climate changes and study weather conditions across the world.
Try yourself:South Temperate zone lies between
South Temperate zone lies between 23 1/2 degree south and 66 1/2 degree south.
Factors affecting Temperature patterns on the globe
The following factors control the distribution of temperature on the earth’s surface-
- Effect of ocean and Seas
- Effect of local winds
- Effect of continentality
- Effect of slope aspect
- Temperature Are Higher at Or Near the Equator
- If Away (North & South Pole) from the Equator – Temperature is Lower.
For this is the surface of the earth’s part is curved. As a result, the sun‘s vertical ray strikes different earth surface parts at different angles. At the equator, the vertical rays hit the earth’s surface at an angle of 90◦ (angle of incidence) towards the poles.
➤ Transparency of Atmosphere
- Aerosols (smoke, soot), dust, water vapour, clouds, etc. affect transparency.
- If the wavelength (X) of the radiation is more than the obstructing particle's radius (such as a gas), then a scattering of radiation takes place.
- If the wavelength is less than the obstructing particle (such as a dust particle), total reflection occurs.
- Absorption of solar radiation occurs if the obstructing particles happen to be water vapour, ozone molecules, carbon dioxide molecules, or clouds.
- Most of the light received by earth is scattered light.
➤ Land-Sea Differential
- The albedo of land is much greater than the albedo of oceans and water bodies. Especially snow-covered areas reflect up to 70%-90% of insolation.
- The average penetration of sunlight is more in water – up to 20 meters than inland – where it is up to 1 meter only. Therefore, the land cools or becomes hot more rapidly compared to oceans. In oceans, continuous convection cycle helps in heat exchange between layers keeping diurnal and annual temperature ranges low.
➤ Earth’s Distance from Sun
- During its revolution around the sun, the earth is farthest from the sun (152 million km on 4th July). This position of the earth is called aphelion.
- On 3rd January, the earth is the nearest to the sun (147 million km). This position is called the perihelion.
- Therefore, the earth's annual insolation on 3rd January is slightly more than the amount received on 4th July.
- However, this variation in the solar output is masked by other factors like the distribution of land and sea and the atmospheric circulation.
- Hence, this variation in solar output does not greatly affect daily weather changes on the earth's surface.
Sunspots created on the outer surface due to periodic disturbance and explosions. The number of sunspots varies from year to year. Its cycle is completed in 11 years. The energy radiated from the sun increases when the number of sunspots increases and therefore, the amount of insolation received by the earth’s surface is also increasing.
- Altitude is the height above sea level.
- High altitude (at the mountain), low temperature
- Low altitude (on the land surface), high temperature
- At the higher altitudes, the amount of atmosphere decreases and as a result, there is less water vapour in the air. The atmosphere absorbs less heat and therefore, the temperature at higher altitude drops.
➤ Distance from the Sea
- The difference in land and water heating affects the temperature of places near the coast differently from those located inland.
- When the sea is cooler than the land in summer, it lowers the coastal place's temperature. However, during the winter the sea is warmer than the land and keeps coastal places warmer by moderating the winter temperature.
- Located in the interior of large continents or landmasses are under the continental influence. That is, the sea does not affect them as they are too far in temperatures. As the land heats up rapidly, inland locations tend to have hotter summers than areas near the coast in similar latitudes.
➤ Ocean Currents
- Ocean currents are large streams of water flowing in the oceans. These generated when winds blow over the water surface.
- There are two types of ocean currents.
- Cold currents that bring water from the polar regions
- Warm currents that bring warm water to the polar regions
- Ocean currents can raise or lower the temperature of the nearby coastal areas.
- The coastal area affected by warm currents will be kept warm during winter if the cold currents move along the coast; they will lower the area's temperatures.
➤ Types of Land Surface
- Dense forest – the vegetation prevents solar radiation from reaching the ground directly. The ground remains cool.
- In the city – the presence of concrete surfaces tends to keep the air temperature high. The concrete surface absorbs heat during the day and retains the heat at night.
Try yourself:Which of the following statements is/are correct regarding Sunspots?
The energy radiated from the sun increases when the number of sunspots increases and therefore, the amount of insolation received by the earth’s surface is also increasing.
- Aspect is the direction in which a slope faces in relation to the sun.
- In tropical areas, the aspect is not important because the sun is high in the sky during mid-day.
- In the temperate areas, the sun is the low angle in winter, affecting the temperature of slopes that face north to south. In the northern hemisphere, the south-facing slope receives greater solar radiation concentration and is usually warmer than the north-facing slope.
➤ Mean Annual Temperature Distribution
- Isotherm – An imaginary line joining places having equal temperatures.
- The horizontal or latitudinal distribution of temperature is shown with the help of a map with isotherms.
- The effects of altitude are not considered while drawing an isotherm. All the temperatures are reduced to sea levels.
General characteristics of Isotherms
- Generally follow the parallels: Isotherms have a close correspondence with the latitude parallels mainly because the same amount of insolation is received by all the points located on the same latitude.
- Sudden bends at ocean-continent boundaries: Due to differential heating of land and water, temperatures above the oceans and landmasses vary even on the same latitude. (we have seen how land-sea differential affects temperature distribution)
- Narrow spacing between isotherms indicates a rapid change in temperature (high thermal gradient).
- Wide spacing between isotherms indicates a small or slow change in temperatures (low thermal gradient).
General Temperature Distribution
- The highest temperatures occur over tropics and sub-tropics (high insolation). The lowest temperatures occur in polar and subpolar regions. In continents due to the effect of continentality.
- The diurnal and annual range of temperatures is highest in continents' interiors due to the effect of continentality (in continental interiors there will have no moderating effect of oceans).
- The diurnal and annual range of temperatures are least in oceans. [high specific heat of water and mixing of water keep the range low]
- Low-temperature gradients are observed over tropics (the sun is almost overhead the entire year) and high-temperature gradients over middle and higher latitudes (the sun’s apparent path varies significantly from season to season).
- Temperature gradients are usually low over the eastern margins of continents. (This is because of warm ocean currents)
- Temperature gradients are usually high over the western margins of continents. (This is because of cold ocean currents)
- The isotherms are irregular over the northern hemisphere due to an enhanced land-sea contrast. Because of the predominance of land over water in the north, the northern hemisphere is warmer. The thermal equator (ITCZ) generally lies to the north of the geographical equator.
- While passing through an area with warm ocean currents, the isotherms show a poleward shift. (North Atlantic Drift and Gulf Stream combined with westerlies in Northern Atlantic; Kurishino Current and North Pacific current combined with westerlies in Northern Pacific) (we will see about ocean currents in detail later.)
- Mountains also affect the horizontal distribution of temperature. For instance, the Rockies and the Andes stop the oceanic influence from going inwards into North and South America.
Inter-Tropical Convergence Zone
The Inter-Tropical Convergence Zone (ITCZ) is a broad trough of low pressure in equatorial latitudes. This is where the northeast and the southeast trade winds converge. This convergence zone lies more or less parallel to the equator but moves north or south with the sun's apparent movement.
➤ Seasonal Temperature Distribution
- The global distribution of temperature can well be understood by studying the temperature distribution in January and July.
- The temperature distribution is generally shown on the map with the help of isotherms. The Isotherms are lines joining places having an equal temperature.
- In general, the latitude on temperature is well pronounced on the map, as the isotherms are generally parallel to the latitude. The deviation from this general trend is more pronounced in January than in July, especially in the northern hemisphere.
- In the northern hemisphere, the land surface area is much larger than in the southern hemisphere. Hence, the effects of landmass and ocean currents are well pronounced.
Seasonal Temperature Distribution – January
- During January, it is winter in the northern hemisphere and summer in the southern hemisphere.
- The western margins of continents are warmer than their eastern counterparts since they can carry high temperature into the landmasses.
- The temperature gradient is close to the eastern margins of continents. The isotherms exhibit more regular behaviour in the southern hemisphere.
➤ Northern Hemisphere
- The isotherms deviate to the north over the ocean and to the south over the continent. This can be seen in the North Atlantic Ocean.
- The presence of warm ocean currents, Gulf Stream and North Atlantic drift, make the Northern Atlantic Ocean warmer and the isotherms show a poleward shift indicating that the oceans are warmer and can carry high temperatures poleward.
- An equatorward bend of the isotherms over the northern continents shows that the landmasses are overcooled and that polar cold winds can penetrate southwards, even in the interiors. It is much pronounced in the Siberian plain.
- Lowest temperatures are recorded over northern Siberia and Greenland.
➤ Southern Hemisphere
- The effect of the ocean is well pronounced in the southern hemisphere. Here the isotherms are parallel to the latitudes, and the temperature variation is more gradual than in the northern hemisphere.
- The high-temperature belt runs in the southern hemisphere, somewhere along 30°S latitude.
- The thermal equator lies to the south of the geographical equator (because the Intertropical Convergence Zone or ITCZ has shifted southwards with the sun's apparent southward movement).
Seasonal Temperature Distribution – July
- During July, it is summer in the northern hemisphere and winter in the southern hemisphere. The isothermal behaviour is the opposite of what it is in January.
- In July the isotherms generally run parallel to the latitudes. The equatorial oceans record warmer temperatures, more than 27°C. Over the land, more than 30°C is noticed in Asia's subtropical continental region, along the 30° N latitude.
➤ Northern Hemisphere
- The highest temperature range is more than 60° C over the north-eastern part of the Eurasian continent. This is due to continentality. The least range of temperature, 3°C, is found between 20° S and 15° N.
- Over the northern continents, a poleward bend of the isotherms indicates that the landmasses are overheated, and the hot tropical winds can go far into the northern interiors.
- The isotherms over the northern oceans show an equatorward shift indicating that the oceans are cooler and can carry the moderating effect into tropical interiors. The lowest temperatures are experienced over Greenland.
- The highest temperature belt runs through northern Africa, West Asia, north-west India arid the southeastern USA. The temperature gradient is irregular and follows a zig-zag path over the northern hemisphere.
➤ Southern Hemisphere
- The gradient becomes regular over the southern hemisphere but shows a slight bend towards the equator at the continents' edges. Thermal equator now lies to the north of the geographical equator.
Try yourself:What does ‘ITCZ’ mean?
Vertical Distribution of Temperature
- The normal, lapse rate is uniform at a given level at all altitudes within the troposphere.
- At the Tropopause, the lapse rate stops at zero, i.e. there is no change in temperature there.
- In the lower stratosphere, the lapse rate remains constant for some height, while higher temperatures exist over the poles because this layer is closer to earth at the poles.
➤ Temperature Anomaly
- The difference between the mean temperature of a place and its parallel (latitude) temperature is called the temperature anomaly or thermal anomaly.
- The largest anomalies occur in the northern hemisphere and the smallest in the southern hemisphere.
Mean Thermal Equator
The thermal equator is a global isotherm having the highest mean annual temperature at each longitude around the globe. The thermal equator does not coincide with the geographical equator.
The highest absolute temperatures are recorded in the Tropics, but the highest mean annual temperatures are recorded at the equator. But because local temperatures are sensitive to the geography of a region, and mountain ranges and ocean currents ensure that smooth temperature gradients (such as might be found if the Earth were uniform in composition and devoid of surface irregularities) are impossible, the location of the thermal equator is not identical to that of the geographic Equator.
We know that the Earth reaches perihelion (the minimum distance from the Sun in its orbit) in early January and is at aphelion (maximum distance) in early July. During the winter season of the respective hemispheres, the sun's rays' angle of incidence is low in the tropics. Therefore, the average annual temperature of the tropical regions is lower than the observed near the equator, as the change in the angle of incidence is minimum at the equator.
The thermal equator shifts towards the north and south with the north-south shift in the position of vertical rays of the sun. However, the annual average position of the Thermal equator is 5° N latitude. The reason is that the highest mean annual temperature shifts towards northwards during the summer solstice to a much greater extent than it does towards the south at the winter solstice.