Solar Radiation, Heat Balance and Temperature — NCERT Solutions

Answer:

Given: The Summer Solstice occurs on 21st June, yet the highest temperatures in India are recorded in May.

Reason:

1. Onset of Monsoon: After the summer solstice (21st June), the Southwest Monsoon arrives over most parts of India. The monsoon brings heavy cloud cover and rainfall, which:

• Reduces incoming solar radiation reaching the surface (clouds reflect and absorb insolation).
• Increases latent heat loss through evaporation, cooling the surface.

2. In May, the skies are largely clear and cloudless, allowing maximum insolation to heat the land surface. The sun is also nearly overhead over the Indian subcontinent.

3. Therefore, despite June 21 being the day of maximum solar altitude, the cooling effect of monsoon clouds and rain after the solstice keeps temperatures lower than in May.

Conclusion: The maximum day temperature in India occurs in May due to clear skies and intense insolation, before the monsoon clouds arrive to reduce surface heating.

Answer:

Given: The Siberian plains are located deep in the interior of the Eurasian continent at high latitudes.

Reasons for high annual range of temperature:

1. Continentality: Siberia is far from the moderating influence of the sea. Land heats up and cools down much faster than water (low specific heat of land). This leads to very hot summers and extremely cold winters.

2. High Latitude: Being at high latitudes, Siberia experiences great variation in day length between summer and winter — very long days in summer (intense heating) and very short days in winter (minimal insolation and intense cooling).

3. No Oceanic Moderation: The absence of warm ocean currents and the blocking of maritime air masses by mountain ranges means there is no moderating influence on temperature extremes.

4. As a result, the annual range of temperature in north-eastern Siberia can exceed $60°\text{C}$, making it one of the highest in the world.

Conclusion: The combination of continentality and high latitude is responsible for the extreme annual range of temperature in the Siberian plains.

Answer:

1. Effect of Latitude:

• The Earth is spherical. At the equator, solar rays strike the surface at a right angle (90°), concentrating energy over a small area — hence maximum insolation.
• At higher latitudes, solar rays strike at an oblique angle. The same amount of solar energy is spread over a larger surface area, reducing the intensity of insolation per unit area.
• Additionally, at higher latitudes, solar radiation passes through a greater thickness of the atmosphere, leading to more absorption, reflection, and scattering, further reducing the energy reaching the surface.
• Mathematically, insolation received is proportional to $\sin\theta$, where $\theta$ is the angle of incidence.

2. Effect of Axial Tilt ($23.5°$):

• The Earth's axis is tilted at $23.5°$ to the plane of its orbit (ecliptic). This tilt causes seasons.
• Summer Solstice (21 June): The Northern Hemisphere is tilted towards the sun. The sun is directly overhead at the Tropic of Cancer ($23.5°$ N). The Northern Hemisphere receives more radiation with longer days.
• Winter Solstice (22 December): The Northern Hemisphere is tilted away from the sun. The sun is overhead at the Tropic of Capricorn ($23.5°$ S). The Northern Hemisphere receives less radiation with shorter days.
• Equinoxes (21 March & 23 September): The sun is directly overhead at the equator; both hemispheres receive equal radiation.
• The tilt also determines the length of day and night, which directly affects the duration of insolation received at any location.

Conclusion: Latitude determines the angle of solar radiation (intensity), while the axial tilt determines the seasonal variation in the angle and duration of insolation. Together, they are the primary controls of the spatial and temporal distribution of solar radiation on Earth's surface.

Answer:

The Earth receives solar energy and must lose an equivalent amount to maintain a heat balance (also called the radiation budget), ensuring that the Earth's average temperature remains relatively constant over time.

Incoming Solar Radiation (Insolation):

• Consider the total incoming solar radiation as 100 units.
• Of this, about 35 units are reflected back to space (by clouds ~27 units, by the earth's surface ~2 units, and by scattering ~6 units). This is the Earth's albedo.
• About 14 units are absorbed by the atmosphere (by ozone, dust, water vapour, etc.).
• The remaining 51 units are absorbed by the Earth's surface.

Heat Transfer Processes (Surface to Atmosphere):
The 51 units absorbed by the surface are transferred back to the atmosphere through:

1. Terrestrial Radiation (Long wave): The Earth radiates energy as long wave infrared radiation. The atmosphere absorbs most of this ($34$ units) and re-radiates it — some back to Earth (counter radiation / greenhouse effect) and some to space.
2. Convection and Conduction: Sensible heat transfer — the heated surface warms the air above it directly ($7$ units approximately).
3. Latent Heat: Evaporation of water from the surface absorbs heat; when water vapour condenses in the atmosphere, this latent heat ($9$ units approximately) is released, warming the atmosphere.

Outgoing Radiation:

• The atmosphere eventually radiates energy to space.
• Total outgoing radiation = $35$ (reflected) + $65$ (emitted by atmosphere and surface) = 100 units, balancing the incoming radiation.

Conclusion: Through the processes of reflection, absorption, terrestrial radiation, convection, and latent heat transfer, the earth-atmosphere system maintains a heat balance, ensuring that the Earth neither continuously warms nor cools over long periods.

Answer:

Global Distribution of Temperature in January:

Northern Hemisphere (Winter in January):

1. Low Temperatures over Land: January is winter in the Northern Hemisphere. Continental interiors, especially in Siberia and Central Asia, experience extremely low temperatures (below $-40°\text{C}$ in parts of Siberia) due to continentality and long nights.
2. Isotherms Bend Equatorward over Land: Over the cold continental interiors, isotherms bend towards the equator (southward), indicating that land is colder than the surrounding oceans at the same latitude.
3. Isotherms Bend Poleward over Oceans: Over the relatively warmer oceans (e.g., North Atlantic warmed by the Gulf Stream), isotherms bend towards the poles, showing that oceans are warmer than land at the same latitude in winter.
4. Crowding of Isotherms: Isotherms are more closely spaced in the Northern Hemisphere in January, indicating a steep temperature gradient between the cold interior and the warmer coasts.

Southern Hemisphere (Summer in January):

1. Higher Temperatures: January is summer in the Southern Hemisphere. Temperatures are generally higher, especially over the subtropical continental regions of Australia, southern Africa, and South America.
2. Isotherms are More Regular: The Southern Hemisphere has a larger proportion of ocean compared to land. Oceans moderate temperatures, so isotherms run more parallel to the latitudes (east-west), showing less variation.
3. Less Extreme Temperatures: Due to the dominance of oceans, the Southern Hemisphere does not experience the extreme cold or heat seen in the Northern Hemisphere's continental interiors.
4. Temperature Decreases Southward: From the subtropics towards Antarctica, temperatures decrease steadily and regularly.

Key Comparison:

Conclusion: In January, the Northern Hemisphere is colder with irregular isotherms due to large landmasses, while the Southern Hemisphere is warmer with regular isotherms due to the dominance of oceans.

Project Work — Solved Example Using New Delhi (Safdarjung) Observatory

(i) Observatory Details:

(ii) Definitions of Temperature Terms:

• Mean of Daily Maximum Temperature: The average of the highest temperatures recorded each day over a month.
• Mean of Daily Minimum Temperature: The average of the lowest temperatures recorded each day over a month.
• Highest Recorded Temperature: The absolute highest temperature ever recorded at the station in that month.
• Lowest Recorded Temperature: The absolute lowest temperature ever recorded at the station in that month.
• Daily Mean Monthly Temperature: The average of the mean daily maximum and mean daily minimum temperatures for a month.
• Annual Range of Temperature: The difference between the mean temperature of the warmest month and the mean temperature of the coldest month.
• Daily Range of Temperature: The difference between the maximum and minimum temperature on a given day.

(iii) Calculation of Daily Mean Monthly Temperature:

Using the formula:
$$\text{Daily Mean Monthly Temperature} = \frac{\text{Mean Daily Max} + \text{Mean Daily Min}}{2}$$

$$\text{January} = \frac{21.1 + 7.3}{2} = \frac{28.4}{2} = 14.2°\text{C}$$

$$\text{May} = \frac{39.6 + 25.9}{2} = \frac{65.5}{2} = 32.75°\text{C}$$

(iv) Graph (Description — to be drawn on graph paper):

Plot the following on a line graph with months on the X-axis and temperature ($°\text{C}$) on the Y-axis:

• Line 1: Daily Mean Maximum Temperature (highest curve)
• Line 2: Mean Temperature (middle curve)
• Line 3: Daily Mean Minimum Temperature (lowest curve)

The graph will show a peak around May–June for maximum temperatures and a trough in December–January for minimum temperatures, reflecting the seasonal pattern of a continental subtropical location.

(v) Calculation of Annual Range of Temperature:

$$\text{Annual Range} = \text{Mean Temperature of Warmest Month} - \text{Mean Temperature of Coldest Month}$$

Using the example data:
$$\text{Annual Range} = 32.75°\text{C} - 14.2°\text{C} = 18.55°\text{C}$$

(vi) Months with Highest and Lowest Daily Range of Temperature:

• Daily Range = Mean Daily Maximum $-$ Mean Daily Minimum
• For January: $21.1 - 7.3 = 13.8°\text{C}$
• For May: $39.6 - 25.9 = 13.7°\text{C}$

Generally, for New Delhi:

• Highest daily range: Occurs in October–November (post-monsoon, clear skies, dry air allow rapid heating by day and rapid cooling at night).
• Lowest daily range: Occurs in July–August (monsoon season — cloud cover reduces daytime heating and nighttime cooling, keeping temperatures more uniform).

(vii) Factors Determining Temperature and Causes of Variation:

Factors determining temperature of New Delhi:

1. Latitude ($28.5°$ N) — subtropical location, receives intense insolation in summer.
2. Altitude (216 m) — slightly elevated, minor moderating effect.
3. Distance from sea — Delhi is far from the sea; continental climate with extreme temperatures.
4. Prevailing winds — hot, dry winds (loo) in summer; cold northwesterly winds in winter.
5. Cloud cover — monsoon clouds in July–August moderate temperatures.

Temperature Variation by Month: