Climatology

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• Solar Radiation:
• • solar radiation reaching the Earth’s surface varies widely geographically spatially and through time.
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• • Insolation varies from about 320 Watt/m2 at the tropics to about 70 Watt/m2 at the poles.
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• Factors for variation are:
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• • The Inclination of the Sun’s Rays: large angle concentrated sunlight.
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• • Geoid Shape of the Earth: length of the time the sun shines over equator.
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• • Transparency of Atmosphere: Cloud cover affects, Maximum insolation: Desert
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• • Aspects of Slope: direction of slope and its angle control amount of solar radiation.
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• • Altitude: Lower layer warmer being close to earth, temp decreases as we move up.
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• Ultraviolet (UV) radiation that reaches the Earth’s surface lies in wavelengths between 290 and 400 nm.
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• Effect of UV radiation on life on earth
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• • The shorter the wavelength, the greater is the potential for harm. longer wavelengths called UV‐A, plays a role in formation of Vitamin D
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• UV radiation at shorter wavelengths i.e. UV‐B has effects as:
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• • It causes damage to the DNA, may induce skin cancer
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• • may also suppress the body’s immune response to Herpes simplex virus.
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• • Over‐exposure to UV‐B may cause cataracts, snow blindness, and other ailments,
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• • Affects both in humans and animals.
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• • UV‐B impairs photosynthesis & increases plants’ susceptibility to disease.
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• • Overexposure to UV‐B reduces size, productivity, and quality in many of the crop plant.
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• • By affecting organisms that move nutrients and energy through the biosphere, it can alter biogeochemical cycles. For example, reducing populations of phytoplankton would significantly
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• impact the world’s carbon cycle.
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• Heat Zones:
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• • The latitudinal heat zones of Earth are majorly governed by variability of the insolation at Earth's surface and geoidal shape of the Earth. However, the boundaries of the zones are finally delineated by taking into account the role of planetary winds and ocean currents which maintain heat balance on earth.
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• • This gives rise to the three heat zones on the earth surface:
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• Ø Torrid or tropical zone: This zone lies between the Tropic of Cancer and the Tropic of Capricorn. The apparent movement of the sun during a year is between these two latitudes. The sun's rays here are almost vertical and it receives maximum insolation.
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• Ø Temperate zones: This zone lies between the torrid zone and the Frigid zone. It shares features of both the surrounding zones. Sun is never directly overhead, thus it possess moderate temperature.
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• Ø Frigid zones: Here the sun rays fall at low angles because the sun does not rise much above the horizon. Thus, these zones are the coldest regions of Earth.
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• • Isotherms: The temperature distribution on Earth is represented with the help of isotherms, which are the lines joining places having equal temperatures. Generally parallel to latitude. However, they take sudden bends where land‐water contrasts are maximum. Thus, there are deviations in the isotherms in northern and southern hemispheres.
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• Pattern of isotherm in Northern hemisphere:
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• • 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.
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• • In January 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.
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• • The presence of warm ocean currents, Gulf Stream and North Atlantic drift, make the Northern Atlantic Ocean warmer and the isotherms bend towards the north. Over the land the temperature decreases sharply and the isotherms bend towards south in Europe.
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• • These deviations are more pronounced in January than in July. In July, the isothermal behavior is opposite of what it is in January. Isotherms bend poleward over the landmass as they are overheated.
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• Pattern of isotherm in Southern hemisphere
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• • In the southern hemisphere, the effect of the ocean is well pronounced. Thermal equator lies to the south of Geographical equator.
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• • Here the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual.
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• • However, the gradient shows slight bend towards the equator at the edges of the continent as shown in the figure.
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• Heat Budget:
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• Earth’s Heat Engine and Heat Budget
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• • Earth is powered by the Sun, but the Sun doesn‘t heat the Earth evenly, because Earth is nearly a sphere, therefore Sun heats equatorial regions more than Polar Regions.
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• • The atmospheric and oceanic systems that work to even out solar heating imbalances are collectively called Earth’s heat engine.
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• Incoming heat being absorbed by the Earth in the form of short wave radiations and outgoing heat escaping the Earth in
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• the form of long wave radiation are both perfectly balanced.
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• If they were not balanced, then Earth would be getting either progressively warmer or progressively cooler with each passing year. This balance between incoming and outgoing heat is known as Earth’s heat budget.
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• • The climate‘s heat engine must not only redistribute solar heat from the equator toward the poles, but also from the Earth‘s surface and lower atmosphere back to space. Otherwise, Earth would endlessly heat up. Earth‘s temperature doesn‘t infinitely rise because the surface and the atmosphere are simultaneously radiating heat to space. This net flow
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• of energy into and out of the Earth system is Earth‘s energy budget.
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• Effect of CO2 on Heat Budget of Earth
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• • increasing concentration of CO2 disturbs this balance. Just as the major atmospheric gases (oxygen and nitrogen) are transparent to incoming sunlight, they are also transparent to outgoing thermal infrared.
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• • green house gases, like water vapor, carbon dioxide etc. are opaque to many wavelengths of thermal infrared energy and absorb them.
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• • The earth surface radiates the net equivalent of 21 percent of incoming solar energy as thermal infrared. However, the amount that directly escapes to space is only about 16 percent of incoming solar energy. The remaining fraction—a net 5‐6 percent of incoming solar energy—is transferred to the atmosphere when greenhouse gas molecules absorb thermal infrared energy radiated by the surface
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• • Because greenhouse gas molecules radiate heat in all directions, some of it spreads downward and ultimately comes back into contact with the Earth‘s surface, where it is absorbed. The temperature of the surface becomes warmer than it would be if it were heated only by direct solar heating.
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• • As long as greenhouse gas concentrations continue to rise, the amount of absorbed solar energy will continue to exceed the amount of thermal infrared energy that can escape to space. The energy imbalance will continue to grow, and surface temperatures will continue to rise.
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• • Hence, continuous and determined steps should be taken to achieve INDCs, implement Green India
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• Mission and promoting renewable sources of energy.
• Sun‐spot cycle:
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• • is the solar magnetic activity cycle ‐ average time period of eleven years.
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• • changes in the Sun's activity
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• Ø including changes in the levels of solar radiation and ejection of solar material
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• Ø appearance (changes in the number and size of sunspots, flares, and other manifestations).
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• Sun‐spots are regions where the solar magnetic field is very strong.
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• • The number of sunspots observed on the "surface" of the Sun varies from year to year. This rise and fall in sunspot counts varies in a cyclical way is called "the Sunspot Cycle".
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• Solar minimum ‐
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• • period of least solar activity in the eleven year solar cycle.
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• • sunspot and solar flare activity diminishes.
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• • NASA ‐ a solar minimum is about to occur in 2019‐
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• 20.
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• • The sun does not become dull, rather the solar activity simply changes form.
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• • Coronal holes can last for a longer time: Coronal holes are vast regions in the sun’s atmosphere where the sun’s magnetic field opens up and allows streams of solar particles to escape the sun as the fast solar
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• wind.
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• Effects of solar minimum on earth:
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• • could enhance the effects of space weather events including geomagnetic storms, auroras, potentially disrupting power supply, satellites, communications and navigation systems.
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• • upper atmosphere cools down,
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• Ø which reduces the frictional drag in the low Earth orbit.
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• Ø the space debris has the tendency to hang around longer
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• The sun’s magnetic field weakens and provides less shielding from the cosmic rays. This can pose an increased threat to
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• astronauts travelling through space.
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• ITCZ
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• • the low‐pressure region near the equator, from about 5° north and 5° south, where the northeast trade winds and southeast trade winds converge.
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• • also known as the Equatorial Convergence Zone or the Monsoon Trough.
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• Annual Fluctuations and Shifting of ITCZ
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• • location varies throughout the year.
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• • While it remains near the equator, the ITCZ over land varies more in the north or south, than the ITCZ over the oceans, due to the variation in land temperatures.
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• • It can vary as much as 40° to 45° of latitude north or south of the equator based on the pattern of distribution of land and ocean.
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• • The position of sun influences the movement of the thermal equator, shifting the belts of planetary winds and pressure systems to the north and to the south annually.
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• • the directions of the planetary winds change according to the Coriolis effect imparted by Earth’s rotation.
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• Variation in location of the ITCZ affects rainfall in the equatorial regions, as it is the region of low atmospheric pressure pulling the moisture laden winds and causing them to rise. This results in wet or dry seasons and even droughts, depending on the location of ITCZ.
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• Figure 1: Approximate location of the Thermal Equator in the months of July and January ITCZ’s Significance for India Significance of ITCZ for India is its contribution to the Indian monsoon
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• • In July when ITCZ is located in the north, it creates the Monsoon Trough. This encourages the development of thermal low over the North and Northwest India.
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• • Due to this shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40ºE and 60ºE longitudes and start blowing from southwest to northeast due to the Coriolis force. It becomes the Southwest monsoon.
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• • In winter, the ITCZ moves southward, and so the reversal of winds, from northeast to south and southwest, takes place thus leading to the Northeast monsoon.
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• • The amount and intensity of rainfall follows the movement of the ITCZ, as the regions lying along its location are regions of high rainfall.
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• Figure 2: Location of the ITCZ contextualizing Monsoonal
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• winds across India
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• Therefore, the location and variations in the ITCZ influence the global circulation systems that, in turn, determine the weather patterns and precipitation across regions
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• Seasonal shifting of pressure belts
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• Pressure belts on earth's surface appear to move along with the Sun. due to inclination of earth to its axis. With the apparent shift of the Sun between the tropics, both the thermally formed pressure belt (EQLP) and the dynamically formed pressure belt (STHP ‐ North and South) move along. For example, in summers, when Sun is directly above tropics, rather than at equator, the entire belt system (EQLP and STHP) shifts northwards. Similarly the Equatorial low also shifts upwards, varying considerably over the landmass and the ocean. On the Indian landmass, it can reach up to 20‐22 degrees North because of immense heating of North Indian landmass and the consequent low pressure. On Oceans, the belts are fairly stable because the variances in temperature
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• are not much pronounced. Impact on formation of climates:
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• • Inhibition of cloud formation under the HP Belt. As
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• HPBs move over an area, it experiences lesser rainfall.
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• • STHP lies over Mediterranean region in summers, leading to aridity. When it moves south along with the apparent shift of the Sun, the region receives rainfall. During winters, Westerlies prevail and cause rain, where as in summers, the dry trade winds blow offshore.
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• • On Monsoon: Only on the northward shifting of the EQLP in the form of ITCZ (Inter‐tropical Convergence Zone) do the south‐east trade winds cross the equator and reach as monsoon winds.
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• • Besides convergence, convectional uplifting which causes rainfall in the northern plains also occurs in the shifted EQLP i.e. ITCZ.
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• • Sahara desert remains almost entirely in the region where STHP is found. The edges of Sahara experience some rainfall and therefore a transitional climatic
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• zone has developed there.
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• Socio‐economic significance:
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• • Mediterranean climate is conducive for growing citrus fruits and therefore it has developed as major supplier of fruits as well as wine worldwide.
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• • Similar climate in Natal (South Africa), Southern Australia and California has given to similar social setup there based on vineyards and fruit production.
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• • Monsoon determines the socio‐economic setup of
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• India via its agricultural economy.
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• Jet Streams:
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• • concentrated narrow bands of fast flowing and strong winds in the upper troposphere of the earth that significantly affect global weather phenomena.
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• There are three types of jet streams:
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• • Polar jet stream – between Ferrel and Polar cells
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• • Sub‐Tropical jet stream – between Hadley and Ferrel cells
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• • Temporary jet streams – e.g.‐ Somali Jet Stream,
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• Tropical Easterly Jet Stream
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• Fig.1 – Types of Jet Stream
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• Properties
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• • Geostrophic winds – due to Coriolis force, they blow in a direction perpendicular to the pressure
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• • gradient force.
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• • Circumpolar – circle around the earth with poles as their centers.
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• • Westerlies – i.e. blow in west to east direction
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• • Rossby waves because of meandering path
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• • High velocity – 150‐250 kmph
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• • Upper atmospheric wind circulation – blow just below the tropopause
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• Role of Jet Streams in Weather Phenomenon:
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• • Help in maintenance of latitudinal heat balance by mass exchange of air.
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• • Influence the weather of mid‐latitudes by influencing the path of temperate cyclones and the distribution of precipitation.
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• • Impact the movement of air masses, which may cause prolonged drought or flood conditions. Eg. polar vortex cold wave over North America in 2014
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• winters.
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• Role of Jet Streams in Rainfall in India:
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• Sub‐tropical jet stream and some temporary jet streams together influence Indian Monsoon patterns, winter rainfall and tropical cyclones.
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• • Winter rainfall: The Sub‐Tropical westerly jet stream transports the western disturbances (temperate cyclones) originating over the Mediterranean Sea and brings rain to northwestern regions of India – Punjab, Haryana, Himachal Pradesh.
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• • Southwest monsoon: The sub‐tropical jet stream and easterly jet stream play an important role in the monsoon system of India.
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• • The withdrawal of sub‐tropical jet stream from the south of Himalayas paves the way for the onset of monsoon in Indian sub‐continent.
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• • The easterly jet stream steers the tropical depressions into India, which play a significant role in the distribution of rainfall during the SW monsoon period.
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• Tropical cyclones – The Easterly jet stream steers tropical depressions and cyclones from the Pacific ocean towards Indian Ocean region causing rainfall predominantly over the eastern coastal region. Eg. cyclone Mora
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• Air masses
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• • When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area.
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• • This air with distinctive characteristics in terms of temperature and humidity is called an air mass.
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• • It is defined as a large body of air having little horizontal variation in temperature and moisture.
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• Classification of airmasses
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• The air masses are classified according to their source regions. There are five major source regions.
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• • Warm tropical and subtropical oceans;
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• • The subtropical hot deserts;
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• • The relatively cold high latitude oceans;
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• • The very cold snow covered continents in high latitudes;
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• • Permanently ice covered continents in the Arctic and Antarctica.
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• Accordingly, following types of airmasses are recognised:
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• • Maritime tropical (mT);
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• • Continental tropical (cT);
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• • Maritime polar (mP);
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• • Continental polar (cP);
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• • Continental arctic (cA).
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• Airmasses and Frontogenesis:
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• • When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis.
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• • There are four types of fronts: (a) Cold; (b) Warm; (c) Stationary; (d) Occluded.
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• • When the front remains stationary, it is called a stationary front.
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• • When the cold air moves towards the warm air mass, its contact zone is called the cold front, whereas if the warm air mass moves towards the cold air mass, the contact zone is a warm front.
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• • If an air mass is fully lifted above the land surface, it is called the occluded front.
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• The fronts occur in middle latitudes and are characterized by steep gradient in temperature and pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause
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• precipitation.
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• Temperature Inversion
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• Normal lapse rate
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• • temperature decreases with increase in altitude.
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• Inversion of temperature
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• • temperature increases with altitude.
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• Various mechanisms during winter season.
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• • long winter night with clear skies and still air
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• • The heat of the day is radiated off during night, and by early morning hours, the earth is cooler than the air above.
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• • Over polar areas, temperature inversion is normal throughout the year.
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• in hills and mountains due to air drainage.
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• • Cold air at hills & mountains, produced during night, flow under the influence of gravity & moves down the slope to pile up deeply in pockets & valley bottoms with warm air above.
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• • frontal inversion: When a cold air mass undercuts a warm air mass and lifts it aloft; the front between the two air masses then has warm air above and cold air
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• below.
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• Climatic significance ‐Inversion of temperature
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• • promotes stability in the lower layers of the atmosphere due to which smoke and dust particles get collected beneath the inversion layer and spread horizontally causing dense fog in the morning during winters.
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• • causes frost when the condensation of warm air due to its cooling by cold air below occurs at a temperature below freezing points.
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• • causes atmosphere stability which stops upward and downward movement of air, a condition unfavourable for rainfall.
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• • Hills top are warmer during freezing winter.
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• Economic significance
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• Positives
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• • Sometimes fog are also favourable for some crops such as coffee plants in Yemen hills of Arabia where fogs protect coffee plants from direct strong sun’s
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• rays.
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• Negatives
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• • lower visibility affecting traffic movements.
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• • Due to Temperature inversion, air pollutants fine air pollutants do not disperse in the valley bottom forcing houses and farms in intermountain valleys to relocate along upper slopes.
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• • Generally fogs are unfavourable for many agricultural crops such as grams, peas, mustard plants, wheat etc. but
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• • Frost caused due to inversion damages crops in foothills, whereas trees and vegetation at top of hills and mountains are not damaged. The valley floors in the hills of Brazil are avoided for coffee cultivation because of frequent frosts.