Temperature and Heat Budget

Temperature and Heat Budget

Air temperature of a particular place denotes the degree of hotness or coldness of air at a given place. It is measured in Celsius. Let us understand how the earth is heated. The surface of the earth is heated by the sun’s rays in the form of short wave radiation. The heat received by the earth is called ‘Solar Radiation’ or ‘Insolation’. Heating of atmosphere is an indirect process. The processes are:

a. Terrestrial radiation

The solar radiation reflected by the earth’s surface is called ‘Terrestrial radiation’. Terrestrial radiation supplies more heat energy to the atmosphere due to its long wave length.

b. Conduction

The heat energy from the earth’s surface is transferred to the lower atmosphere which is directly in contact with the surface by the process of conduction.

c. Convection and advection

The movement of air molecules in vertical and horizontal direction is called as ‘convection and advection’ respectively. This movement carries heat energy to the various parts of the earth and at different altitudes.

Heat budget

The heat energy reflected, absorbed and radiated back into the space equals the energy received by the earth. Incoming radiation and the outgoing radiation pass through the atmosphere. The earth maintains its optimum temperature.

When 100% solar radiation reaches the earth’s atmosphere, 35% is reflected back to space by clouds, water bodies and ice covered areas. This heat does not heat either the earth or atmosphere.

Of the remaining 65% of heat, 14% are absorbed by the atmosphere and 51% are absorbed by the earth’s surface (34% of direct solar radiation and 17% from scattered radiation). 51% received by the earth are radiated back to the space directly as terrestrial radiation (Figure 6.6).

In total, 17% are radiated to space directly and 48% are absorbed by the atmosphere ( 14% from insolation and 34% from terrestrial radiation) are radiated back to space gradually. Therefore, 65% heat received from the sun is balanced by the 65% radiated by the earth. 

This balance between the incoming and the outgoing heat energy is called the global heat energy balance. Distribution of Temperature Distribution of temperature varies both horizontally and vertically. Let us study it under

a.        Horizontal Distribution of Temperature

b.        Vertical Distribution of Temperature

A) Horizontal Distribution of Temperature

Distribution of temperature across the latitudes over the surface of the earth is called horizontal distribution of temperature. On maps, the horizontal distribution of temperature is commonly shown by isotherms. Isotherms are line connecting points that have an equal temperature at mean sea level.

Factors Affecting the Horizontal Distribution of Temperature

The horizontal distribution of temperature on the earth’s surface varies from place to place. Following are the factors affecting the horizontal distribution of temperature of the earth:

a.        Latitude: The angle formed by the solar radiation to the ground is called ‘angle of incidence’. The solar radiation passes vertically along the equator. The angle of incidence decreases from equator towards the poles. The area heated by the solar radiation increases towards the poles and therefore, temperature decreases from the equator to the poles.

b.        Distribution of land and water: Land is heated and cooled at a faster rate due the conduction process whereas water is heated and cooled at slower rate due to convection process. Water takes 2.5 times of heat energy to heat a unit area compared to land. Thus, the land will have higher temperature than the water in summer and vice versa during the winter. So more land mass in northern hemisphere (15.28C) leads to higher average temperature than the southern hemisphere (13.38C).

c.         Ocean currents: Warm ocean currents carry warm water from the tropical region towards the poles and increase the temperature while cold ocean currents carry cold water from Polar Regions and reduce the temperature along the coasts.

d.        Prevailing winds: Warm winds like trade wind and westerly, that carry higher heat energy, increase the temperature while cold polar easterlies carry lower heat energy from polar region reduces the temperature.

e.         Cloudiness: The cloudy sky obstructs the solar radiation from the sun to earth and reduces the temperature. But the clear sky during the day allows more solar radiation to reach the earth’s surface and increases the temperature. Meanwhile clear sky at night allows more terrestrial radiation to escape. For example, the tropical hot deserts experience higher temperature at day and lower temperature at night.

f.          Nature of the surface: The reflection from surface varies based on the nature of land cover. The more reflection from the snow surface leads to low temperature accumulation. But the dense forest, which reflects less heat energy and absorbs more heat energy, leads to higher temperature.

g.        Mountain barriers: If a wind or air mass blows towards the mountain, it influences the distribution of temperature on either side of the mountain.

For example, polar easterlies and blizzards are obstructed by Himalayas in Asia and Alps in Europe respectively. This leads to lower temperature in the northern slopes and higher temperature in the southern slopes of the respective mountains.

Factors Affecting the Vertical Distribution of Temperature

We all know that the temperature decreases with increasing altitude from the surface of the earth. The vertical decrease in temperature of troposphere is called as ‘Normal Lapse Rate’ or ‘vertical temperature (Figure 6.7) gradient’ at which the temperature reduces at the rate of 6.5 8C per 1000 meter of ascent. This is influenced by the following factors:

a. Amount of terrestrial radiation reaching the altitude and

b. Density of air to absorb the heat energy at higher altitude.

As both the above said factors decrease with altitude, the temperature also decreases (Figure 6.5).

Student Activity

If the temperature of Chennai (7 m) is 348C, calculate the temperature of Kodaikanal (2133m) using normal lapse rate.

Inversion of Temperature

The condition at which the temperature increases with altitude is called as ‘inversion of temperature’. In this condition, warm air lies over cold air.

The conditions for inversion of temperature are:

a. Long winter nights: The bottom layer of the atmosphere in contact with the ground is cooled and the upper layer remains relatively warm.

b. Cloudless sky: The higher amount of terrestrial radiation reaches the higher altitude which leads to lower temperature at low level due to clear sky.

c. Dry air near the surface: the dry air absorbs less terrestrial radiation and allows them to escape into space. 

d. Snow covered ground: During night, due to terrestrial radiation and higher albedo, most of the heat is lost to the atmosphere and the surface is cooled. 

e. Formation of fronts: the movement of warm air over the cold air during the formation of the various fronts leads to inversion condition. 

f. Mountain wind: The subsidence of cold mountain wind at the early morning leads to the displacement of warm air from the valley to higher altitude. This type of inversion is called as ‘valley inversion’.

Measurements of Temperature

Heat Zones of the World

The earth has been divided into three heat zones according the amount of insolation received. These are the Torrid Zone, the Temperate zone and the Frigid Zone.

Torrid Zone ( 23 ½ °N to 23 ½ °S)

The zone lying between the Tropic of cancer and Tropic of Capricorn is called ‘Torrid zone’ (Figure 6.8). The sun’s rays are vertical throughout the year and it receives maximum insolation. Thus, this is the hottest zone.

Temperate Zone (23 ½ °N to 66 ½ °N and 23 ½ °S to 66 ½ °S)

The temperate zone lies between the Tropic of Cancer and Arctic Circle in the northern hemisphere and the Tropic of Capricorn and Antarctic circle in the southern hemisphere. This region never experiences over head sun light but experiences longer days and shorter nights during summer and vice versa during winter. This region experiences moderate temperature and is therefore called as ‘Temperate zone’.

Polar Zone (Frigid Zone – 66 ½ °N to 90°N and 66 ½ °S to 90°S )

The region between North pole and Arctic Circle in the northern hemisphere and South pole and Antarctic Circle in the southern hemisphere is called ‘Polar Zone’. This region always receives more oblique rays of the sun and so the temperature is very low. It is the coldest zone. This region experiences 24 hours of day and night during peak summer and winter respectively.

From the above discussion, it is clear that the energy for the earth is from the sun.

Green House Effect: As seen in the heat budget, the longer wavelengths are absorbed by greenhouse gases in the atmosphere, increases the temperature of atmosphere. These greenhouse gases act like a green house and retains some of the heat energy would otherwise be lost to space. The retaining of heat energy by the atmosphere is called the ‘greenhouse effect’.

Global Warming

Global warming is observed in a century scale. The temperature increase over theyearshas been due to the green house gas concentration such as carbon dioxide (CO₂), water vapour, methane and ozone. Greenhouse gases are those gases that contribute to the greenhouse effect. The largest contributing source of greenhouse gas is the burning of fossil fuels leading to the emission of carbon dioxide from industries, automobiles and domestic.

Urban Heat Island (UHI)

An urban heat island is an urban area or metropolitan area that is significantly warmer than its surrounding rural area due to high concentration of high rise concrete buildings, metal roads, sparse vegetation cover and less exposure of soil. These factors cause urban regions to become warmer than their rural surroundings, forming an “island” of higher temperatures (Figure. 6.9).

Ways to reduce the impact of urban heat island:

a.        Increase shade around your home: Planting trees and other vegetation, provides shade and cooling effect through evapotranspiration and it lowers the surface and air temperature.

b.        Install green and cool roofs.

c.         Use energy-efficient appliances and equipments.

d.        Shift all industries away from the urban area.

e.         Reduce emission from automobiles.