SECTION 2 CLIMATOLOGY
CHAPTER 1. Latitudes and Longitudes
- Latitudes and Longitudes are imaginary lines used to determine the location of a place on earth.
- The shape of the earth is ‘Geoid’. And the location of a place on the earth can be mentioned in terms of latitudes and longitudes.
- Example: The location of New Delhi is 28° N, 77° E.
- Latitude is the angular distance of a point on the earth’s surface, measured in degrees from the center of the earth.
- As the earth is slightly flattened at the poles, the linear distance of a degree of latitude at the pole is a little longer than that at the equator.
- For example at the equator (0°) it is 68.704 miles, at 45° it is 69.054 miles and at the poles it is 69.407 miles. The average is taken as 69 miles (111km).
- 1 mile = 1.607 km.
Important parallels of latitudes
- Besides the equator (0°), the north pole (90°N) and the south pole (90° S), there are four important parallels of latitudes–
- Tropic of Cancer (23½° N) in the northern hemisphere.
- Tropic of Capricorn (23½° S) in the southern hemisphere.
- Arctic circle at 66½° north of the equator.
- Antarctic circle at 66½° south of the equator.
Latitudinal Heat zones of the earth
- The mid-day sun is exactly overhead at least once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. This area, therefore, receives the maximum heat and is called the torrid zone.
- The mid-day sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn. The angle of the sun’s rays goes on decreasing towards the poles. As such, the areas bounded by the Tropic of Cancer and the Arctic circle in the northern hemisphere, and the Tropic of Capricorn and the Antarctic circle in the southern hemisphere, have moderate temperatures. These are, therefore, called temperate zones.
- Areas lying between the Arctic circle and the north pole in the northern hemisphere and the Antarctic circle and the south pole in the southern hemisphere, are very cold. It is because here the sun does not raise much above the horizon. Therefore, its rays are always slanting. These are, therefore, called frigid zones.
- Longitude is an angular distance, measured in degrees along the equator east or west of the Prime (or First) Meridian.
- On the globe longitude is shown as a series of semi-circles that run from pole to pole passing through the equator. Such lines are also called
- Unlike the equator which is centrally placed between the poles, any meridian could have been taken to begin the numbering of longitude. It was finally decided in 1884, by international agreement, to choose as the zero meridian the one which passesthrough the Royal Astronomical Observatory at Greenwich, near London.
- This is the Prime Meridian (0°) from which all other meridians radiate eastwards and westwards up to 180°.
- As the parallels of latitude become shorter poleward, so the meridians of longitude, which converge at the poles, enclose a narrower space.
- They have one very important function, they determine local time in relation to G.M.T. or Greenwich Mean Time, which is sometimes referred to as World Time.
Longitude and Time
- Since the earth makes one complete revolution of 360° in one day or 24 hours, it passes through 15° in one hour or 1° in 4 minutes.
- The earth rotates from west to east, so every 15° we go eastwards, local time is advanced by 1 hour. Conversely, if we go westwards, local time is retarded by 1 hour.
- We may thus conclude that places east of Greenwich see the sun earlier and gain time, whereas places west of Greenwich see the sun later and lose time.
- If we know G.M.T., to find local time, we merely have to add or subtract the difference in the number of hours from the given longitude.
Standard Time and Time Zones
- If each town were to keep the time of its own meridian, there would be much difference in local time between one town and the other.
- Travelers going from one end of the country to the other would have to keep changing their watches if they wanted to keep their appointments. This is impractical and very inconvenient.
- To avoid all these difficulties, a system of standard time is observed by all countries.
- Most countries adopt their standard time from the central meridian of their countries.
- In larger countries such as Canada, U.S.A., China, and U.S.S.R, it would be inconvenient to have single time zone. So these countries have multiple time zones.
- Both Canada and U.S.A. have five time zones—the Atlantic, Eastern, Central, Mountain and Pacific Time Zones. The difference between the local time of the Atlantic and Pacific coasts is nearly five hours.
- S.S.R had eleven time zones before its disintegration. Russia now has nine time zones.
The International Date Line
- A traveler going eastwards gains time from Greenwich until he reaches the meridian 180°E, when he will be 12 hours ahead of G.M.T.
- Similarly in going westwards, he loses 12 hours when he reaches 180°W. There is thus a total difference of 24 hours or a whole day between the two sides of the 180° meridian.
- This is the International Date Line where the date changes by exactly one day when it is crossed. A traveler crossing the date line from east to west loses a day (because of the loss in time he has made); and while crossing the dateline from west to east he gains a day (because of the gain in time he encountered).
- The International Date Line in the mid-Pacific curves from the normal 180° meridian at the Bering Strait, Fiji, Tonga and other islands to prevent confusion of day and date in some of the island groups that are cut through by the meridian.
- Some of them keep Asiatic or New Zealand standard time, others follow the American date and time.
Why is the international dateline drawn in a zigzag manner?
- The International Date Line (IDL) passes through the Pacific Ocean. It is an imaginary line, like longitudes and latitudes.
- The time difference on either side of this line is 24 hours. So, the date changes as soon as one crosses this line.
- Some groups of Islands (Polynesia, Melanesia, Micronesia) fall on either of the dateline. So if the dateline was straight, then two regions of the same Island Country or Island group would fall under different date zones. Thus to avoid any confusion of date, this line is drawn through where the sea lies and not land. Hence, the IDL is drawn in a zig-zag manner.
Indian Standard Time
- The Indian Government has accepted the meridian of 82*5° east for the standard time which is 30 mins, ahead of Greenwich Mean Time.
CHAPTER 2 Motions of the earth: Rotation and Revolution
Primarily two motions: Rotation and Revolution.
Rotation of Earth
- Earth rotates along its axis from west to east.
- It takes approximately 24 hrs to complete on rotation.
- Days and nightsoccur due to rotation of the earth.
- The circle that divides the day from night on the globe is called the circle of illumination.
- Earth rotates on a tilted axis. Earth’s rotational axis makes an angle of 5° with the normal i.e. it makes an angle of 66.5° with the orbital plane. Orbital plane is the plane of earth’s orbit around the Sun.
Why are days always longer than nights at the equator?
- If there was no atmosphere, there would be no refraction and the daytime and nighttime would be near equal at the equator, at least during equinoxes.
- But due to atmosphere, the sun’s rays gets refracted (bending of light). Refraction is particularly stronger during the morning and the evening time when the sun’s rays are slant.
- Even though the actual sun is below the horizon, its apparent image would appear above the horizon due to refraction. This makes the days longer than nights at the equator.
- The second motion of the earth around the sun in its orbit is called revolution. It takes 365¼ days (one year) to revolve around the sun.
- Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February. Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.
- On 21st June, the northern hemisphere is tilted towards the sun. The rays of the sun fall directly on the Tropic of Cancer. As a result, these areas receive more heat.
- The areas near the poles receive less heat as the rays of the sun are slanting.
- The north pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months.
- Since a large portion of the northern hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21st June.
- At this time in the southern hemisphere all these conditions are reversed. It is winter season there. The nights are longer than the days. This position of the earth is called the summer solstice.
- On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the south pole tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23½° s), a larger portion of the southern hemisphere gets light. Therefore, it is summer in the southern hemisphere with longer days and shorter nights. The reverse happens in the northern hemisphere. This position of the earth is called the winter solstice.
- On 21st March and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.
- On 23rd September, it is autumn season [season after summer and before the beginning of winter] in the northern hemisphere and spring season[season after winter and before the beginning of summer] in the southern hemisphere. The opposite is the case on 21st March, when it is spring in the northern hemisphere and autumn in the southern hemisphere.
- Thus, you find that there are days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.
- Rotation === Days and Nights.
- Revolution === Seasons.
CHAPTER 3 ATMOSPHERE
- Our planet earth is enveloped by a deep blanket of gases extending several thousands of kilometres above its surface. This gaseous cover of the earth is known as the atmosphere.
- Like land (lithosphere) and water (hydrosphere), the atmosphere is an integral part of the earth.
- Compared to the earth’s radius, the atmosphere appears to be only a very thin layer of gases. However, because of the force of gravity, it is inseparable from the earth.
- Atmospheric pressure: The air exerts pressure on earth’s surface by virtue of its weight. This pressure is called atmospheric pressure. Atmospheric pressure is the most important climatic element. The atmospheric pressure at sea level is 1034 gm per square centimeter.
Role of Earth’s Atmosphere
- The atmosphere contains various gases like oxygen, carbon dioxide, nitrogen etc.
- Plants require carbon dioxide to survive while animals and many other organisms need oxygen for their survival. The atmosphere supplies these life giving gases.
- All life forms need a particular range of temperature and a specific range of frequencies of solar radiation to carry out their biophysical processes. The atmosphere absorbs certain frequencies and lets through some other frequencies of solar radiation. In other words, the atmosphere regulates the entry of solar radiation.
- The atmosphere also keeps the temperature over the earth’s surface within certain limits. In the absence of the atmosphereextremes of temperature would exist between day and night over the earth’s surface.
- Harmful ultraviolet radiation would find its way through, if the atmosphere (ozone in stratosphere to be specific) were absent.
- The atmosphere also takes care of extra-terrestrial objects like meteors which get burnt up while passing through the atmosphere (mesosphere to be precise) due to friction.
- Weather is another important phenomenon which dictates the direction of a number of natural and man-made processes like plant growth, agriculture, soil-formation, human settlements, etc. Various climatic factors join together to create weather.
Composition of Atmosphere
- The atmosphere is a mixture of many gases. In addition, it contains huge numbers of solid and liquid particles, collectively called ‘aerosols’.
- Some of the gases may be regarded as permanent atmospheric components which remain in fixed proportion to the total gas volume.
- Other constituents vary in quantity from place to place and from time to time. If the suspended particles, water vapour and other variable gases were excluded from the atmosphere, then the dry air is very stable all over the earth up to an altitude of about 80 kilometres.
- The proportion of gases changes in the higher layers of the atmosphere in such a way that oxygen will be almost in negligible quantity at the height of 120 km. Similarly, carbon dioxide and water vapour are found only up to 90 km from the surface of the earth.
- Nitrogen and oxygen make up nearly 99% of the clean, dry air. The remaining gases are mostly inert and constitute about 1% of the atmosphere.
- Besides these gases, large quantities of water vapour and dust particles are also present in the atmosphere. These solid and liquid particles are of great climatic significance.
- Different constituents of the atmosphere, with their individual characteristics, are discussed below.
- Oxygen, although constituting only 21% of total volume of atmosphere, is the most important component among gases. All living organisms inhale oxygen. Besides, oxygen can combine with other elements to form important compounds, such as, oxides. Also, combustion is not possible without oxygen.
- Nitrogen accounts for 78% of total atmospheric volume. It is a relatively inert gas, and is an important constituent of all organic compounds. The main function of nitrogen is to control combustion by diluting oxygen. It also indirectly helps in oxidation of different kinds.
- The third important gas is Carbon Dioxide which constitutes only about 03% of the dry air and is a product of combustion. Green plants, through photosynthesis, absorb carbon dioxide from the atmosphere and use it to manufacture food and keep other bio-physical processes going.
- Being an efficient absorber of heat, carbon dioxide is considered to be of great climatic significance. Carbon dioxide is considered to be a very important factor in the heat energy budget.
- With increased burning of fossil fuels – oil, coal and natural gas – the carbon dioxide percentage in the atmosphere has been increasing at an alarming rate.
- More carbon dioxide in the atmosphere means more heat absorption. This could significantly raise the temperature at lower levels of the atmosphere thus inducing drastic climatic changes.
- Ozone (03) is another important gas in the atmosphere, which is actually a type of oxygen molecule consisting of three, instead of two, atoms. It forms less than 00005% by volume of the atmosphere and is unevenly distributed. It is between 20 km and 25 km altitude that the greatest concentrations of ozone are found. It is formed at higher altitudes and transported downwards.
- Ozone plays a crucial role in blocking the harmful ultraviolet radiation from the sun.
- Other gases found in almost negligible quantities in the atmosphere are argon, neon, helium, hydrogen, xenon, krypton, methane etc.
- Water Vapour is one of the most variable gaseous substances present in atmosphere – constituting between 02% and 4% of the total volume (in cold dry and humid tropical climates respectively). 90% of moisture content in the atmosphere exists within 6 km of the surface of the earth. Like carbon dioxide, water vapour plays a significant role in the insulating action, of the atmosphere.
- It absorbs not only thelong-wave terrestrial radiation (infrared or heat emitted by earth during nights), but also a part of the incoming solar radiation.
- Water vapour is the source of precipitation and clouds. On condensation, it releases latent heat of condensation —the ultimate driving force behind all storms.
The moisture – carrying capacity of air is directly proportional to the air temperature.
- The Solid Particles present in the atmosphere consist of sand particles (from weathered rocks and also derived from volcanic ash), pollen grains, small organisms, soot, ocean salts; the upper layers of the atmosphere may even have fragments of meteors which got burnt up in the atmosphere. These solid particles perform the function of absorbing, reflecting and scattering the radiation.
- The solid particles are, consequently, responsible for the orange and red colours at sunset and sunrise and for the length of dawn (the first appearance of light in the sky before sunrise) and twilight (the soft glowing light from the sky when the sun is below the horizon, caused by the reflection of the sun’s rays by the atmosphere. Dusk: the darker stage of twilight.). The blue colour of the sky is also due to selective scattering by dust particles.
- Some of the dust particles are hygroscopic (i.e. readily absorbing moisture from air) in character, and as such, act as nuclei of condensation. Thus, dust particles are an important contributory factor in the formation of clouds, fog and hailstones.
Structure of Atmosphere
- The atmosphere can be studied as a layered entity – each layer having its own peculiar characteristics. These layers are systematically discussed below.
- It is the atmospheric layer between the earth’s surface and an altitude of 8 km at the poles and 18 km at the equator.
- The thickness is greater at the equator, because the heated air rises to greater heights.
- The troposphere ends with the Tropopause.
- The temperature in this layer, as one goes upwards, falls at the rate of 5°C per kilometer, and reaches -45°C at the poles and -80°C over the equator at Tropopause (greater fall in temperature above equator is because of the greater thickness of troposphere – 18 km).
- The fall in temperature is called ‘lapse rate’.
- The troposphere is marked by temperature inversion, turbulence and eddies.
- It is also meteorologically the most significant zone in the entire atmosphere (Almost all the weather phenomena like rainfall, fog and hailstorm etc. are confined to this layer).
- It is also called the convective region, since all convection stops at Tropopause.
- The troposphere is the theatre for weather because all cyclones, anticyclones, storms and precipitation occur here, as all water vapours and solid particles lie within this.
- The troposphere is influenced by seasons and jet streams.
- Top most layer of troposphere.
- It acts as a boundary between troposphere and stratosphere.
- This layer is marked by constant temperatures.
- It lies beyond troposphere, up to an altitude of 50 km from the earth’s surface.
- The temperature in this layer remains constant for some distance but then rises to reach a level of 0°C at 50 km altitude.
- This rise is due to the presence of ozone (harmful ultraviolet radiation is absorbed by ozone).
- This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes. So aeroplanes fly in lower stratosphere, sometimes in upper troposphere where weather is calm.
- Sometimes, cirrus clouds are present at lower levels in this layer.
- It lies at an altitude between 30 km and 60 km from the earth’s surface and spans the stratosphere and lower mesosphere.
- Because of the presence of ozone molecules, this layer reflects the harmful ultraviolet radiation.
- The ozonosphere is also called chemosphere because, a lot of chemical activity goes on here.
- The temperature rises at a rate of 5°C per kilometer through the ozonosphere.
- This is an intermediate layer beyond the ozone layer and continues upto an altitude of 80 km from the earth’s surface.
- The temperature gradually falls to -100°C at 80 km altitude.
- Meteorites burn up in this layer on entering from the space.
- In thermosphere temperature rises very rapidly with increasing height.
- Ionosphere is a part of this layer. It extends between 80-400 km.
- This layer helps in radio transmission. In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
- Person would not feel warm because of the thermosphere’s extremely low pressure.
- The International Space Station and satellites orbit in this layer. (Though temperature is high, the atmosphere is extremely rarified – gas molecules are spaced hundreds of kilometers apart. Hence a person or an object in this layer doesn’t feel the heat)
- Aurora’s are observed in lower parts of this layer.
- This layer is located between 80 km and 400 km and is an electrically charged layer.
- This layer is characterized by ionization of atoms.
- Because of the electric charge, radio waves transmitted from the earth are reflected back to the earth by this layer.
- Temperature again starts increasing with height because of radiation from the sun.
- This is the uppermost layer of the atmosphere extending beyond the ionosphere above a height of about 400 km.
- The air is extremely rarefied and the temperature gradually increases through the layer.
- Light gases like helium and hydrogen float into the space from here.
- Temperature gradually increases through the layer. (As it is exposed to direct sunlight)
CHAPTER 4 Temperature Distribution on Earth
Sun is the ultimate source of heat. And the differential heat received from sun by different regions on earth is the ultimate reason behind all climatic features. So understanding the patterns of distribution of temperature in different seasons is important for understanding various climatic features like wind systems, pressure systems, precipitation etc..
- Earth intercepts only one in two billion parts of solar radiation. This intercepted radiation is called Insolation.
- Insolation == Proportion of Solar energy received or intercepted by earth.
- Some heat within the core and mantle is transferred to the surface and ocean bottoms through volcanoes, springs and geysers. But this heat received at the surface form interiors of the earth is negligible compared to that received from sun.
- Earth receives Sun’s radiation (heat) in the form of short waves (visible light + wavelengths below visible light – most of it is ultraviolet radiation) which are of electromagnetic nature. The earth absorbs short wave radiation during daytime and reflects back the heat received into space as long-wave radiation (mostly infrared radiation) during night.
Ways of Transfer of Heat Energy
- The heat energy from the solar radiation is received by the earth through three mechanisms—
- Radiation == Heat transfer from one body to another without actual contact or movement. It is possible in relatively emptier space, for instance, from the sun to the earth through space.
- Conduction == Heat transfer through matter by molecular activity. Heat transfer in iron and other metals is by conduction. Generally, denser materials like water are good conductors and a lighter medium like air is a bad conductor of heat.
- Convection == Transfer of heat energy by actual transfer of matter or substance from one place to another. (heat transfer by convection cycles in atmosphere as well as oceans)
Factors Affecting Temperature Distribution
The Angle of Incidence or the Inclination of the Sun’s Rays
Duration of Sunshine
- Heat received depends on day or night; clear sky or overcast, summer or winter etc..
Transparency of Atmosphere
- Aerosols (smoke, sooth), dust, water vopour, clouds etc. effect transparency.
- If the wavelength (X) of the radiation is more than the radius of the obstructing particle (such as a gas), then scattering of radiation takes place.
- If the wavelength is less than the obstructing particle (such as a dust particle), then total reflection takes place.
- Absorption of solar radiation takes place 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.
- Albedo of land is much greater than albedo of oceans and water bodies. Especially snow covered areas reflect up to 70%-90% of insolation.
- Average penetration of sunlight is more in water – up to 20 metres, than in land – where it is up to 1 metre only. Therefore, 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. (more while studying salinity and temperature distribution of oceans)
- The specific heat of water is 2.5 times higher than landmass, therefore water takes longer to get heated up and to cool down.
- Winds transfer heat from one latitude to another. They also help in exchange of heat between land and water bodies.
- The oceanic winds have the capacity to take the moderating influence of the sea to coastal areas – reflected in cool summers and mild winters. This effect is pronounced only on the windward side (the side facing the ocean).
- The leeward side or the interiors do not get the moderating effect of the sea, and therefore experience extremes of temperature.
Aspects of Slope
- The direction of the slope and its angle control the amount of solar radiation received locally. Slopes more exposed to the sun receive more solar radiation than those away from the sun’s direct rays.
- Slopes that receive direct Sun’s rays are dry due to loss of moisture through excess evaporation. These slopes remain barren if irrigational facilities are absent. But slopes with good irrigational facilities are good for agriculture due to abundant sunlight available. They are occupied by dense human settlements.
- Slopes that are devoid of direct sunlight are usually well forested.
- Ocean currents influence the temperature of adjacent land areas considerably. (more while studying ocean currents).
- With increase in height, pressure falls, the effect of greenhouse gases decreases and hence temperature decreases (applicable only to troposphere).
- The normal lapse rate is roughly 1⁰ C for every 165 metres of ascent.
Earth’s Distance form 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
- On 3rd January, the earth is the nearest to the sun (147 million km). This position is called
- Therefore, the annual insolation received by the earth on 3rd January is slightly more than the amount received on 4th July.
- However, the effect of 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 the solar output does not have great effect on daily weather changes on the surface of the earth.
Latitudinal Heat Balance
- The amount of insolation received varies from latitude to latitude.
- Regions within the equator and 40° N and S latitudes receive abundant sunlight and hence more heat will be gained than lost. Hence they are energy surplus regions.
- Regions beyond 40° N and S latitudes lose more heat than that gained from sunlight. Hence they are energy deficit regions (This is because of slant sunlight and high albedo of polar regions).
- Going by this logic, the tropics should have been getting progressively hotter and the poles getting progressively cooler. And the planet would have been inhospitable except for few regions near mid-latitudes. But, in reality, this does not happen.
- The atmosphere (planetary winds) and the oceans (ocean currents) transfer excess heat from the tropics (energy surplus region) towards the poles (energy deficit regions) making up for heat loss at higher latitudes.
- And most of the heat transfer takes place across the mid-latitudes (30° to 50°)[more while studding jet streams and cyclones], and hence much of the stormy weather is associated with this region.
- Thus, the transfer of surplus energy from the lower latitudes to the deficit energy zone of the higher latitudes, maintains an overall balance over the earth’s surface.
- The earth receives a certain amount of Insolation (short waves) and gives back heat into space by terrestrial radiation (longwave radiation). Through this give and take, or the heat budget, the earth maintains a constant temperature.
The 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.
- Effects of altitude is not considered while drawing an isotherm. All the temperatures are reduced to sea levels.
General characteristics of isotherms.
- Generally follow the parallels: Isotherms have 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 effects temperature distribution)
- Narrow spacing between isotherms indicate rapid change in temperature (high thermal gradient).
- Wide spacing between isotherms indicate 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 sub polar regions. in continents due to the effect of continentiality.
- Diurnal and annual range of temperatures are highest in the interiors of continents due to the effect of continentiality (in continental interiors these will no moderating effect of oceans).
- 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 (sun is almost overhead the entire year) and high temperature gradients over middle and higher latitudes (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 predominance of land over water in the north, the northern hemisphere is warmer. The thermal equator (ITCZ) lies generally to the north of 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.
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 equal temperature.
- In general the effect of 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 land mass and the 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 the Westerlies are able to carry high temperature into the landmasses.
- The temperature gradient is close to the eastern margins of continents. The isotherms exhibit a more regular behavior in the southern hemisphere.
- The isotherms deviate to the north over the ocean and to the south over the continent. This can be seen on 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 are able to carry high temperatures poleward.
- An equator ward bend of the isotherms over the northern continents shows that the landmasses are overcooled and that polar cold winds are able to penetrate southwards, even in the interiors. It is much pronounced in the Siberian plain.
- Lowest temperatures are recorded over northern Siberia and Greenland.
- The effect of the ocean is well pronounced in the southern hemisphere. Here the isotherms are more or less parallel to the latitudes and the variation in temperature 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 geographical equator (because the Intertropical Convergence Zone or ITCZ has shifted southwards with the apparent southward movement of the sun).
Seasonal Temperature Distribution – July
- During July, it is summer in the northern hemisphere and winter in the southern hemisphere. The isothermal behavior is the opposite of what it is in January.
- In July the isotherms generally run parallel to the latitudes. The equatorial oceans record warmer temperature, more than 27°C. Over the land more than 30°C is noticed in the subtropical continental region of Asia, along the 30° N latitude.
- The highest range of temperature is more than 60° C over the north-eastern part of Eurasian continent. This is due to continentiality. 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 are able to go far into the northern interiors.
- The isotherms over the northern oceans show an equator ward shift indicating that the oceans are cooler and are able to 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 southeastern USA. The temperature gradient is irregular and follows a zig-zag path over the northern hemisphere.
- The gradient becomes regular over the southern hemisphere but shows a slight bend towards the equator at the edges of continents. Thermal equator now lies to the north of the geographical equator.
Condensation of water vapour, thunderstorms, cyclonic and anticyclonic conditions etc. depend on Adiabatic Lapse Rate. Adiabatic Lapse Rate determines the Rate Of Condensation and the rate of condensation determines the amount of Latent Heat of Condensation Released.
- Lapse rate is rate of change in temperature observed while moving upward through the Earth’s atmosphere (troposphere to be specific).
- The lapse rate is considered positive when the temperature decreases with elevation,zero when the temperature is constant with elevation, and negative when the temperature increases with elevation(temperature inversion).
- The lapse rate of non-rising air – commonly referred to as the normal, or Environmental, Lapse Rate (ELR) – is highly variable, being affected by radiation, convection, and condensation; it averages about 5 °C per kilometer in the lower atmosphere (troposphere).
Why does temperature fall with elevation
- When we move up a hill, we notice a fall in temperature. This fall in temperature with elevation is primarily due to two reasons.
- With increase in elevation, the atmospheric pressure falls. Fall in pressure implies that the temperature also falls [Pressure is directly proportional to Temperature and vice versa]
- With increase in elevation, the concentration of greenhouse gasesdecrease (Water vapor and carbon dioxide fall sharply with elevation). Hence the heat absorption capacity of atmosphere will also decrease.
This sort of fall in temperature with elevation is called Temperature Lapse and the rate at which it happens is called Temperature Lapse Rate or simply Lapse rate.
Adiabatic Lapse rate
- Lapse rate is the rate of fall in temperature of atmosphere with elevation.
- Adiabatic Lapse Rate is the rate of fall in temperature of a rising or a falling air parcel adiabatically.
- Adiabatic or adiabatically: Heat doesn’t enter or leave the system. All temperature changes are internal.
- Adiabatic Lapse rate is governed by Gas law.
- According to gas law Pressure ‘P’ is directly proportional to Temperature ‘T’ when Volume ‘V’ is a constant.
Adiabatic Process: A Parcel of Rising or Falling Air
- An air bubble rises in water whereas stone sinks. This is obvious. The stone is denser (heavier than water) and it sinks whereas the air bubble is less denser (lighter than water) and it rises.
- Similarly, a parcel of air rises when it is less denser than the surrounding environment and it falls when its density becomes greater than the surrounding environment.
A Parcel of Rising Air
- When an air parcel is subjected to differential heating compared to the surrounding air, it becomes lighter (less denser) or heavier (more denser) depending on whether the air parcel is heated or cooled.
- When an air parcel receives more heat than the surrounding air, its temperature increases leading to an increase in volume (Increase in Volume == Fall in Density). The air parcel becomes lighter than the surrounding air and it starts to rise. This process is non-adiabatic (there is heat exchange between the air parcel and the external environment).
- But when the air parcel starts to rise, the ambient pressure on it starts to fall [The atmospheric pressure decreases with height, so the pressure on the air parcel decreases with height]. With the fall in ambient pressure, the temperature falls and the volume increases. This is adiabatic[there is no heat exchange between the air parcel and the external environment. All the temperature changes are internal. Temperature changes are only due to change in pressure or volume or both].
- This fall in temperature with the rising of the air parcel is called Adiabatic Temperature Lapse. And the rate at which it happens is called Adiaba