Heat & Temperature

• It all starts with the Sun, where the fusion of hydrogen creates an immense amount of energy, heating the surface to around 6000°K; the Sun then radiates energy outwards in the form of ultraviolet and visible light, with a bit in the near-infrared part of the spectrum.
• By the time this energy gets out to the Earth, its intensity has dropped to a value of about 1370 W/m² —as we just saw this is often called the solar constant (even though it is not truly constant — it changes on several timescales)
• Earth intercepts only one in two billion partsof solar radiation. This intercepted  radiation  is called
• Insolation is the 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 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 during night.
• The insolation is not constant over the surface of the Earth — it is concentrated near the equator because of the curvature of the Earth.
• But, the situation is complicated by the fact that the Earth’s spin axis is tilted by 23.4° relative to a line perpendicular to the Earth’s orbital plane, so that as Earth orbits around the Sun, the insolation is concentrated in the northern hemisphere (the northern hemisphere summer) and then the southern hemisphere (winter in the northern hemisphere). This tilt of the spin axis, also called the obliquity, is the main reason we have seasons.

• 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.
• Earth’s energy balance is governed by the first law of thermodynamics, also known as the law of conservation of energy.
• This law states that energy can be transferred from one system to another in many forms, but it cannot be created or destroyed.
• Therefore, any energy “lost” during one process will equal the same amount of energy “gained” during another.
• The energy flows and the storage in and between each of the Earth’s subsystems involve many components.

• Each of these parts represents either input of radiation to the planet (solar heating), its output from the planet (infrared cooling), storage or release of heat within the climate system (evaporation, condensation, melting and freezing), or transport of heat from one part of the climate system to another (wind and ocean currents).
• Together, these processes serve as the driving forces of the climate system.
• The total energy available to drive all climate processes comes mainly from the distribution of solar radiation arriving and leaving the Earth.
• To maintain a constant global average temperature, all of the sun’s radiation that enters Earth’s atmosphere must eventually be sent back to space. This is achieved through Earth’s energy balance.

• Of the total solar energy entering Earth’s atmosphere, about 50% is absorbed by the Earth’s surface (the land and oceans), 30% is directly reflected back to space by clouds, the Earth’s surface and different gases and particles in the atmosphere and 20% is absorbed by the atmosphere and clouds.
• The 70% of the sun’s energy that is absorbed by the surface, clouds, and atmosphere causes warming.
• Any object or gas that has a temperature emits radiation outward which is eventually reradiated back into space as long-wave radiation 24 hours a day.
• Most of the energy emitted from the earth’s surface does not go directly out to space. This emitted energy is reabsorbed by clouds and by the gases in the atmosphere.
• Some is redistributed by convection, while even more energy is released into the atmosphere through condensation.
• The majority of the energy is absorbed by the greenhouse gases, methane, nitrous oxide, ozone, carbon dioxide and water vapor. These gases constantly emit the sun’s energy back into the atmosphere and keep the Earth at a habitable temperature. Eventually, most of the energy makes its way back out to space and Earth’s energy balance is sustained.
• As solar energy strikes the Earth, some of it is absorbed, some of it is scattered, and some of it is transmitted directly to lower levels of the atmosphere.
• Absorption, scattering and transmission do not occur equally within the atmosphere.
• A variety of molecules, particles or surface features absorb, transmit or scatter energy with very different energy levels, depending upon the wavelengths of the transmitted energy.
• Molecules tend to scatter radiation. The degree of scattering depends on the distance, the wavelength and the characteristics of the particles (size, shape, density).
• The molecules that compose the gases of the atmosphere are all very small relative to the wavelengths of most sunlight. For this reason, shorter wavelength radiation is scattered more effectively than longer wavelengths through a process known as Rayleigh scattering.
• The sky is blue during the day because the scattering of sunlight by the many tiny molecules of the Earth’s atmosphere favors shorter wavelengths, such as blue light.
• For the much larger particles, such as soil, dust or sulfuric acid, which make up atmospheric aerosols, the scattering efficiency is much more uniformly distributed across the visible wavelengths.
• Some of the scattering from clouds and dust results in back scattering, where a fraction of the incoming solar energy is scattered back, or reflected to space.
• Albedo is another name for reflectivity determines how much sunlight will be absorbed and warm the surface compared to another surface that reflects most of the light and does not change temperature.
• Something that appears white reflects most of the light that hits it and has a high albedo, while something that looks dark absorbs most of the light that hits it, indicating a low albedo.
• Gases and particles in the atmosphere allow about half of the solar rays to pass through to the Earth’s surface. However, not all of it passes directly through to the surface uninterrupted.
• Much of it – virtually all of it on a cloudy day – arrives as diffuse radiation, having been scattered by atmospheric particles and molecules.
• About one third of the total of the Sun’s radiant energy that reaches the Earth eventually hits the surface without being scattered and about 25% reaches the surface as diffuse radiation.
• At the surface, about 85% of the total amount is absorbed.
• Over dark a surface such as the oceans, more than 90% is absorbed.
• In the seas or in very wet, vegetated areas this absorbed heat is used to evaporate water.
• Over bright surfaces, such as deserts and snowfields, 40-80 percent is reflected.
• Over deserts for example, as little as 1 percent of the absorbed energy is used to evaporate water: the rest simply warms the surface.

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.

Horizontal Distribution of Temperature

The distribution of temperature across latitude over the Earth’s surface is known as the horizontal distribution of temperatures. The horizontal distribution of temperature on Earth is shown by Isotherms. Isotherms are the line joining points that have an equal temperature. When the isotherm map is analyzed, it can be observed that the horizontal distribution of temperature is uneven.

Some of the factors affecting the temperature distribution are:

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 scatteringof radiation takes place.
• If the wavelength is less than the obstructing particle (such as a dust particle), then total reflectiontakes 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.

Differential heating of land and water

• 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.

Prevailing Winds

• 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

• Ocean currents influence the temperature of adjacent land areas considerably. (more while studying ocean currents).

Altitude

• 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 from the 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.

Vertical Distribution of Temperature

• The temperature in the troposphere decreases with increase in altitudes but the rate of decrease in temperature changes according to seasons.
• The decrease of temperatures is known as vertical temperature gradient or normal lapse rate which is 1000 times more than the horizontal lapse rate.
• The decrease of temperature upward in the atmosphere proves the fact that the atmosphere gets heat from the Earth surface through the process of conduction, radiation, and convection.
• Hence, it is obvious that as the distance from the Earth’s surface (the source of direct heat energy to the atmosphere) increases (i.e as the altitude increases), the air temperature decreases.

Global Distribution of Temperature

• The global distribution of temperature can be effectively understood by considering the temperature distribution for the month of January and July.
• The distribution of temperature is usually shown on the map using the isotherms.
• The isotherms are line joining places of equal temperature.
• Generally, the effects of latitude is well shown on the map as isotherms are generally parallel to the latitudes.
• The deviation from this trend is more generally observed in January rather than in July, especially in the northern hemisphere.
• The land surface is much larger in the northern hemisphere than the southern hemisphere. Hence, the effects of land masses and ocean currents are well observed.

Temperature Distribution in January

• In January, there is winter in the Northern hemisphere and summers in the southern hemisphere.
• The western margins of continents in January are much higher than the Eastern counterparts as the westerlies can carry high temperatures into the landmasses.
• The temperature gradient is much closer to the Eastern margins of continents. The isotherms observe more steady behavior in the southern hemisphere.

Temperature Distribution in July

• During July, it is winter in the Southern hemisphere and summers in the Northern hemisphere. The isotherm behavior is the opposite of what it was in January.
• The isotherms are generally parallel to the latitudes in July.
• The equatorial oceans record warmer temperatures more than 27 degrees celsius.
• More than 30 degrees celsius is noticed over the land in the subtropical continent region of Asia, along the 30 ° N latitude.

• Temperature inversion, also called thermal inversion, a reversal of the normal behaviour of temperature in the troposphere (the region of the atmosphere nearest Earth’s surface), in which a layer of cool air at the surface is overlain by a layer of warmer air. (Under normal conditions air temperature usually decreases with height.)
• Inversions play an important role in determining cloud forms, precipitation, and visibility.
• An inversion acts as a cap on the upward movement of air from the layers below.
• As a result, convection produced by the heating of air from below is limited to levels below the inversion.
• Diffusion of dust, smoke, and other air pollutants is likewise limited.
• In regions where a pronounced low-level inversion is present, convective clouds cannot grow high enough to produce showers and, at the same time, visibility may be greatly reduced below the inversion, even in the absence of clouds, by the accumulation of dust and smoke particles.
• Because air near the base of an inversion tends to be cool, fog is frequently present there.
• Inversions also affect diurnal variations in air temperature.
• The principal heating of air during the day is produced by its contact with a land surface that has been heated by the Sun’s radiation.
• Heat from the ground is communicated to the air by conduction and convection.
• Since an inversion will usually control the upper level to which heat is carried by convection, only a shallow layer of air will be heated if the inversion is low and large, and the rise in temperature will be great.

Ideal Conditions for Temperature Inversion

• Long nights, so that the outgoing radiation is greater than the incoming radiation.
• Clear skies, which allow unobstructed escape of radiation.
• Calm and stable air, so that there is no vertical mixing at lower levels.

Types of Temperature Inversion:

There are five kinds of inversions: ground, turbulence, subsidence, air drainage and frontal.

Ground Inversion:

• A ground inversion develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere; this occurs most often on clear nights, when the ground cools off rapidly by radiation.
• If the temperature of surface air drops below its dew point, fog may result.
• Topography greatly affects the magnitude of ground inversions.
• If the land is rolling or hilly, the cold air formed on the higher land surfaces tends to drain into the hollows, producing a larger and thicker inversion above low ground and little or none above higher elevations.

Turbulence Inversion

• A turbulence inversion often forms when quiescent air overlies turbulent air.
• Within the turbulent layer, vertical mixing carries heat downward and cools the upper part of the layer.
• The unmixed air above is not cooled and eventually is warmer than the air below; an inversion then exists.

Subsidence Inversion:

• A subsidence inversion develops when a widespread layer of air descends.
• The layer is compressed and heated by the resulting increase in atmospheric pressure, and, as a result, the lapse rate of temperature is reduced.
• If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes, producing a temperature inversion.
• Subsidence inversions are common over the northern continents in winter and over the subtropical oceans; these regions generally have subsiding air because they are located under large high-pressure centres.

Air drainage type of Inversion

• Sometimes, the temperature in the lower layers of air increases instead of decreasing with elevation. This happens commonly along a sloping surface.
• Here, the surface radiates heat back to space rapidly and cools down at a faster rate than the upper layers. As a result the lower cold layers get condensed and become heavy.
• The sloping surface underneath makes them move towards the bottom where the cold layer settles down as a zone of low temperature while the upper layers are relatively warmer.
• This condition, opposite to normal vertical distribution of temperature, is known as Temperature Inversion.
• In other words, the vertical temperature gets inverted during temperature inversion.
• This kind of temperature inversion is very strong in the middle and higher latitudes. It can be strong in regions with high mountains or deep valleys also.

Frontal Inversion

• A frontal inversion occurs 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 below.
• This kind of inversion has a considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high, and clouds may be present immediately above it.

Economic Importance of Temperature Inversion:

• Sometimes, the temperature of the air at the valley bottom reaches below freezing point, whereas the air at higher altitude remains comparatively warm. As a result, the trees along the lower slopes are bitten by frost, whereas those at higher levels are free from it.
• Due to inversion of temperature, air pollutants such as dust particles and smoke do not disperse in the valley bottoms. Because of these factors, houses and farms in intermontane valleys are usually situated along the upper slopes, avoiding the cold and foggy valley bottoms. For instance, coffee growers of Brazil and apple growers and hoteliers of mountain states of Himalayas in India avoid lower slopes.
• Fog lowers visibility affecting vegetation and human settlements.
• Less rainfall due to stable conditions.