Solar Radiation, Heat Balance and TemperatureClass 11 Fundamentals of Physical Geography NCERT Solutions

Answer: (c) 23.5 N

Explanation: As shown in 'Figure 8.1: Summer Solstice' in the chapter, on 21st June, the sun's rays fall vertically on the Tropic of Cancer, which is located at 23.5° N latitude. This event is known as the summer solstice in the Northern Hemisphere.

Answer: (b) Chandigarh

Explanation: The length of the day is a factor that influences the amount of insolation received. During the summer in the Northern Hemisphere, the duration of daylight increases with increasing latitude. Among the given options, Chandigarh is located at the highest latitude, and therefore, it will experience the longest day.

Answer: (c) Long wave terrestrial radiation

Explanation: The chapter states that the Earth's surface is heated by incoming short-wave solar radiation (insolation). The heated Earth then radiates this energy back into the atmosphere in the form of long waves. This process is called terrestrial radiation. Atmospheric gases, particularly carbon dioxide and other greenhouse gases, absorb this long-wave radiation, which heats the atmosphere from below. Therefore, the atmosphere is indirectly but mainly heated by terrestrial radiation.

The correct pairs are as follows:

Answer: (a) Subtropical areas tend to have less cloud cover than equatorial areas.

Explanation: The chapter's section on 'Spatial Distribution of Insolation at the Earth's Surface' explicitly states, "Maximum insolation is received over the subtropical deserts, where the cloudiness is the least. Equator receives comparatively less insolation than the tropics." Less cloud cover allows more direct solar radiation to reach the surface, leading to higher temperatures.

The unequal distribution of heat received from the sun across the Earth's surface creates temperature differences. According to the text, this variation causes pressure differences in the atmosphere, leading to the transfer of heat from one region to another by winds. This large-scale movement of air and energy drives weather systems and determines long-term climate patterns globally.

The factors that control the temperature distribution on the Earth's surface, as listed in the chapter, are:

Although the sun's rays are most direct after the summer solstice (June 21st), temperatures in India are typically highest in May. This is because the transparency of the atmosphere, a key factor affecting insolation, is reduced after June due to the onset of the monsoon. The increased cloud cover during the monsoon reflects a significant amount of solar radiation, preventing it from reaching the surface and causing a drop in temperature compared to the clear skies of May.

The annual range of temperature is high in the Siberian plains due to 'continentality'. The region is located deep within the vast Eurasian landmass, far from the moderating influence of oceans. Land heats up and cools down much faster than water, resulting in extremely cold winters and hot summers, which leads to a very large difference between the highest and lowest monthly mean temperatures.

Latitude and the Earth's axial tilt are the two most significant factors determining the amount of solar radiation received at the surface.

Latitude: The latitude of a place determines the angle of the sun's rays. At lower latitudes near the equator, the sun's rays are almost vertical, concentrating energy over a small area and leading to high insolation. At higher latitudes near the poles, the rays are slanted. These slant rays spread the same amount of energy over a larger area, reducing the intensity. Furthermore, slant rays must travel through a greater depth of the atmosphere, which increases the amount of energy lost to absorption, scattering, and diffusion.

Axial Tilt: The Earth's axis is tilted at an angle of 66.5° to the plane of its orbit. This tilt is the primary cause of seasons. As the Earth revolves around the sun, this tilt causes different latitudes to receive the most direct rays of the sun at different times of the year. It influences the length of the day and the angle of the sun's rays, thereby causing significant variations in the amount of insolation received in a season and in a year at any given latitude, except at the equator.

The Earth-atmosphere system maintains a stable temperature by balancing the incoming solar energy with the outgoing terrestrial energy. This is known as the Earth's heat budget. The process can be explained by assuming the total incoming solar radiation (insolation) at the top of the atmosphere is 100 units.

First, about 35 units are reflected back to space before reaching the surface. This is the Earth's albedo, with 27 units reflected by clouds, 2 by ice/snow, and 6 scattered to space by the atmosphere.

The remaining 65 units are absorbed by the Earth system. The atmosphere absorbs 14 units, and the Earth's surface absorbs 51 units.

To maintain balance, these 65 absorbed units must be radiated back to space. The Earth's surface radiates back all 51 units it absorbed in the form of long-wave terrestrial radiation. Of these 51 units, 17 are radiated directly to space. The remaining 34 units are absorbed by the atmosphere.

The atmosphere, having absorbed 14 units from insolation and 34 units from terrestrial radiation, now holds a total of 48 units. It radiates these 48 units back to space.

Finally, the total outgoing radiation is the sum of what is radiated from the Earth directly (17 units) and what is radiated from the atmosphere (48 units), which equals 65 units. This perfectly balances the 65 units absorbed, ensuring the Earth neither heats up nor cools down over time.

The global distribution of temperature in January shows significant differences between the Northern and Southern Hemispheres, primarily due to the unequal distribution of land and water.

In the Northern Hemisphere, January is winter. The vast landmasses cool down rapidly, causing isotherms to bend sharply southwards over continents like North America and Eurasia. This effect is most pronounced in the Siberian plain, where temperatures can drop below -20°C. Conversely, over the oceans, which retain heat longer, the isotherms bend northwards. The warm North Atlantic Drift current, for example, keeps the Northern Atlantic Ocean warmer than adjacent land areas at the same latitude. This creates a strong temperature contrast and irregular, wavy isotherms.

In the Southern Hemisphere, January is summer. This hemisphere is dominated by oceans, which heat up slowly and have a moderating effect on temperature. As a result, the isotherms are more regular and run more or less parallel to the latitudes. The temperature variation is much more gradual compared to the Northern Hemisphere. For instance, the isotherms of 20°C, 10°C, and 0°C run almost parallel to the 35°S, 45°S, and 60°S latitudes, respectively, reflecting the uniform heating of the vast ocean surface.

This project requires collecting data from a local meteorological observatory. The following explains how to approach each task based on the concepts in the chapter.

(i) Note the altitude, latitude of the observatory and the period for which the mean is calculated. This information is crucial as latitude determines the angle of the sun's rays and potential insolation, while altitude affects temperature due to the normal lapse rate (temperature decreases with height).

(iii) Calculate the daily mean monthly temperature. As shown in the example, this is calculated using the formula: Daily Mean Monthly Temperature = (Mean of Daily Max. + Mean of Daily Min.) / 2

(iv) Draw a graph to show the daily mean maximum, the daily mean minimum and the mean temperature. This would be a line graph with months on the x-axis and temperature in °C on the y-axis. Three separate lines would be plotted to visualize the temperature variations throughout the year.

(v) Calculate the annual range of temperature. This is calculated by finding the difference between the mean temperature of the warmest month and the mean temperature of the coldest month. Annual Range = Mean Temperature of Warmest Month - Mean Temperature of Coldest Month

(vi) Find out in which months the daily range of temperature is the highest and the lowest. The daily range for each month is (Mean of Daily Max. - Mean of Daily Min.). Calculate this for all twelve months and identify the highest and lowest values.

(vii) List out the factors that determine the temperature of the place and explain the possible causes for temperature variation. The factors from the chapter are: latitude, altitude, distance from the sea, air-mass circulation, and ocean currents. For a specific location, one would explain how these factors cause temperature variations. For example, January is cold due to lower sun angle and shorter days. May is often very hot due to high sun angle and clear skies (pre-monsoon). July's temperatures might be moderated by cloud cover and rain. October marks the transition from wet to dry season, affecting temperature and humidity.