Solar Radiation, Heat Balance and Temperature

SOLAR RADIATION, HEAT BALANCE AND TEMPERATURE

SOLAR RADIATION AND INSOLATION

**Definition**: **Insolation** refers to **incoming solar radiation** — the total solar energy received at the top of the earth's atmosphere measured as 1.94 calories per square cm per minute on average.

The sun is the primary energy source for the earth-atmosphere system. The earth intercepts only a tiny fraction of the sun's total energy output due to its spherical shape and the oblique angle at which solar rays strike the atmosphere.

Variations in Earth's Distance from Sun

**Aphelion**: Earth's farthest position from the sun (152 million km) occurring on **4th July**

**Perihelion**: Earth's nearest position to the sun (147 million km) occurring on **3rd January**

On 3rd January, the earth receives slightly more insolation than on 4th July, but this variation is masked by other factors like land-sea distribution and atmospheric circulation patterns

VARIABILITY OF INSOLATION AT EARTH'S SURFACE

The amount and intensity of insolation vary during a day, season, and year. **Five major factors cause these variations**:

1. **Rotation of earth on its axis**: Determines which parts receive direct sunlight

2. **Angle of inclination of sun's rays**: Varies with latitude — determines whether rays are perpendicular or slant

3. **Length of the day**: Duration of daylight varies seasonally and by latitude

4. **Transparency of the atmosphere**: Atmospheric conditions affect radiation penetration

5. **Configuration of land (aspect)**: Slope orientation affects radiation reception (less influential than others)

Critical Factor: Earth's Axial Tilt

The earth's axis makes an angle of **66½°** (or 23½° tilt) with the plane of its orbit around the sun. This axial inclination is the primary reason for:

Variation in insolation at different latitudes

Seasonal changes in temperature

Unequal distribution of solar energy

Angle of Inclination of Rays

The **angle of incidence** (angle between incoming rays and earth's surface) depends on latitude:

**Higher latitudes**: Rays are slant/oblique, making smaller angles with earth's surface

**Lower latitudes**: Rays are more perpendicular to earth's surface

**Why slant rays receive less insolation per unit area**:

Same energy is spread over a larger surface area

Slant rays must pass through greater atmospheric depth, causing increased **absorption**, **scattering**, and **diffusion**

More atmospheric interference reduces energy reaching the surface

**Example**: The same energy spread over area A at the equator is concentrated over area B at higher latitudes, thus B receives less energy per unit area.

PASSAGE OF SOLAR RADIATION THROUGH THE ATMOSPHERE

**The atmosphere is largely transparent to short-wave solar radiation** but interacts with it in specific ways:

Absorption

**Water vapour**, **ozone**, and other gases absorb much of the **near-infrared radiation** within the troposphere

Direct absorption reduces the amount reaching the earth's surface

Scattering

Very small suspended particles (dust, smoke, water droplets) scatter visible light spectrum

Scattering occurs both toward space (away from earth) and toward the earth's surface

This scattering adds colour to the sky:

**Red/orange colour at sunrise and sunset**: Long wavelengths are scattered less; short wavelengths are scattered away, allowing red wavelengths to reach the observer

**Blue colour of the sky**: Short blue wavelengths are scattered in all directions; blue light dominates what we see from below

SPATIAL DISTRIBUTION OF INSOLATION AT EARTH'S SURFACE

Insolation varies significantly across the globe:

**Tropics**: 320 Watt/m² (highest insolation)

**Poles**: 70 Watt/m² (lowest insolation)

**Subtropical deserts**: Maximum insolation due to least cloud cover (e.g., Sahara, Arabian deserts)

**Equator**: Comparatively less insolation than tropics due to greater cloud cover

**Continents vs. Oceans**: At the same latitude, insolation is more over continents than oceans

**Seasonal variation**: Middle and higher latitudes receive less radiation in winter than in summer

**Example**: The Atacama Desert in South America receives more insolation than the Congo Rainforest despite being at similar latitudes, due to cloud cover differences.

HEATING AND COOLING OF THE ATMOSPHERE

The atmosphere cannot be directly heated by short-wave solar radiation (it is largely transparent to it). Instead, the atmosphere is **indirectly heated** through three main processes:

1. CONDUCTION

**Definition**: Direct transfer of heat between two bodies in contact, from warmer to cooler body until thermal equilibrium is reached.

**Mechanism**: Heat flows through molecular collision; molecules in contact with warm earth surface gain kinetic energy and transfer it upward to cooler layers

**Importance**: Crucial for heating lower atmospheric layers (troposphere)

**Limitation**: Very slow process; only affects a thin layer near the surface

**Time-dependent**: Continues until both bodies reach the same temperature or contact breaks

**Example**: During the day, soil heats up from insolation; it then conducts this heat to the air immediately above it, warming the lower atmosphere.

2. CONVECTION

**Definition**: Vertical transfer of heat through the movement of air currents. Heated air rises, cooler air descends.

**Mechanism**:

Earth's surface is heated by insolation

Air in contact with warm earth surface becomes warm, expands, becomes less dense, and rises vertically

Upper layers in contact with rising warm air also get heated

Cooler air sinks to replace the rising air, creating convection cells

**Spatial limitation**: Confined only to the **troposphere**; does not extend to stratosphere

**Effectiveness**: More significant than conduction for atmospheric heating

**Example**: Thermal convection in equatorial regions causes rising air and subsequent cloud formation; trade winds result from subsiding cooler air at subtropics

3. ADVECTION

**Definition**: Horizontal transfer of heat through movement of air masses (wind).

**Mechanism**: Wind carries warm or cold air masses horizontally from one region to another

**Importance**: In **middle latitudes**, most day-to-night (diurnal) variations in daily weather are caused by advection

**Relative significance**: More important than vertical movement (convection) in many regions

**Example in India**: The "**loo**" — hot, dry local winds of northern India during summer season — is an outcome of advection process, bringing extremely hot air from continental interiors

TERRESTRIAL RADIATION

Definition and Process

After being heated by insolation, the **earth's surface becomes a radiating body**. It radiates energy back to the atmosphere in the form of **long-wave radiation** (or **long-wave electromagnetic radiation**).

**Wavelength**: Terrestrial radiation consists of longer wavelengths than incoming solar radiation

**Direction**: Radiates upward from earth's surface toward the atmosphere

**Atmospheric absorption**: Long-wave radiation is absorbed by atmospheric gases, particularly:

**Carbon dioxide (CO₂)**

**Other greenhouse gases** (methane, nitrous oxide, water vapour)

Indirect Heating of Atmosphere

The atmosphere is **transparent to incoming short-wave solar radiation** but **opaque to outgoing long-wave terrestrial radiation**

Atmospheric gases absorb terrestrial radiation and re-emit it in all directions, including back toward the earth's surface

This creates a **warming effect** in the lower atmosphere

This mechanism is the basis of the **Greenhouse Effect**

HEAT BUDGET (HEAT BALANCE) OF PLANET EARTH

The Heat Budget Concept

The earth neither accumulates nor loses heat over long periods — it maintains a constant average temperature. This is only possible because:

**Amount of heat received from sun = Amount of heat radiated back to space**

The Heat Budget Equation (Based on 100 Units of Insolation)

Consider 100 units of insolation reaching the top of the atmosphere:

**Reflection and Albedo (35 units returned to space)**:

27 units reflected from cloud tops

2 units reflected from snow/ice-covered areas

6 units scattered back by atmosphere

**Total albedo**: 35 units

**Absorption and Transmission (65 units reach earth's surface)**:

14 units absorbed by atmosphere

51 units absorbed by earth's surface

**Total reaching surface**: 65 units

**Terrestrial Radiation from Earth (51 units)**:

Earth's surface radiates 51 units as long-wave radiation

17 units escape directly to space

34 units absorbed by atmosphere (greenhouse effect)

6 units direct absorption

9 units through convection and turbulence

19 units through latent heat of condensation

**Total Atmospheric Absorption (48 units)**:

14 units from incoming solar radiation

34 units from terrestrial radiation

Total: 48 units

**Final Balance**:

Radiation returning to space: 17 units (direct terrestrial) + 48 units (from atmosphere) = **65 units**

This balances the 65 units that reached the surface

**Conclusion**: Total radiation returning = Total radiation received (65 = 65), maintaining equilibrium and constant earth temperature.

VARIATION IN NET HEAT BUDGET AT EARTH'S SURFACE

**Surplus and Deficit Zones** exist based on latitudinal position:

**Surplus radiation balance**: Between **40°N and 40°S** (tropical and subtropical regions)

These regions receive more insolation than they radiate back

Excess heat accumulates in lower latitudes

**Deficit radiation balance**: Near **poles (beyond 40° latitude)**

These regions radiate more heat than they receive in insolation

Extreme deficit due to low angle of sun's rays and long winter darkness

Redistribution of Heat

The surplus heat from tropics is redistributed **poleward** through:

1. **Atmospheric circulation**: Trade winds, westerlies, and general circulation patterns

2. **Ocean currents**: Warm currents (e.g., Gulf Stream) transport heat poleward

3. **Jet streams**: High-altitude wind systems redistribute thermal energy

**Consequence**: This redistribution prevents:

Tropics from getting progressively hotter

Polar regions from becoming permanently frozen

Extreme temperature variations between regions

TEMPERATURE

Definition

**Temperature** measures the **degree of hotness or coldness** of a place in degrees (°C or °F).

**Distinction from Heat**:

**Heat**: Represents the **total molecular movement** (kinetic energy) of all particles in a substance

**Temperature**: Measurement of **average kinetic energy** of particles; a property of the substance

FACTORS CONTROLLING TEMPERATURE DISTRIBUTION

1. LATITUDE

**Primary factor**: Temperature depends on insolation received, which varies with latitude

**Pattern**: Temperature generally decreases from equator toward poles

**Relationship**:

Lower latitudes (closer to equator) receive more direct, perpendicular rays → higher temperatures

Higher latitudes receive slant rays → lower insolation and temperatures

**Example**: Equator receives approximately 250 Watt/m² while poles receive only 70 Watt/m²

2. ALTITUDE (Elevation)

**Mechanism**: Atmosphere is indirectly heated by terrestrial radiation from below, not directly by sun

**Temperature decrease**: Places at higher elevation have lower temperatures

**Normal lapse rate**: Temperature decreases with height at a rate of **6.5°C per 1,000 m**

**Example**: Delhi at sea level has mean temperature of 25°C; at 2,000 m elevation in Himalayas, temperature drops to about 12.9°C (approximately 6.5 × 2 = 13°C decrease)

**Implication**: Mountainous regions are cooler than surrounding plains at the same latitude

3. DISTANCE FROM THE SEA (Continentality)

**Land heating characteristics**:

Heats up quickly during day

Cools down quickly at night

Greater daily and annual temperature range

**Sea/Ocean heating characteristics**:

Heats up slowly (high specific heat capacity)

Loses heat slowly

Much smaller daily and annual temperature range

**Coastal locations**:

Come under **moderating influence** of land and sea breezes

Temperatures remain relatively stable

Less extreme variations

**Continental locations** (far from sea):

Extreme temperatures (very hot summers, very cold winters)

Large annual temperature range

**Example**: Coastal city like Mumbai has annual range of ~8°C; inland city like Delhi has annual range of ~18°C (continental effect)

4. AIR-MASS AND OCEAN CURRENTS

**Air masses**:

Warm air masses bring higher temperatures

Cold air masses bring lower temperatures

Example: Arctic air masses bring extreme cold to North America in winter

**Warm ocean currents** (e.g., Gulf Stream, North Atlantic Drift):

Raise coastal temperatures

Make coastal areas warmer than inland areas at same latitude

Example: Norway's Atlantic coast remains ice-free due to warm North Atlantic Drift

**Cold ocean currents** (e.g., Humboldt Current, Benguela Current):

Lower coastal temperatures significantly

Create dry, cool coastal regions despite equatorial location

Example: Peru's coast remains cool despite being near equator due to cold Humboldt Current

5. LOCAL ASPECTS (Minor Factor)

**Slope orientation** and aspect (direction slope faces)

**Natural vegetation** and surface type

**Urban vs. rural areas**: Cities show higher temperatures (urban heat island effect)

GLOBAL DISTRIBUTION OF TEMPERATURE

Temperature Distribution Shown by Isotherms

**Isotherm Definition**: Lines joining places or points having equal temperature.

Temperature distribution is best understood by comparing **January and July** maps.

JANUARY TEMPERATURE DISTRIBUTION

**Northern Hemisphere Characteristics**:

**Greater deviation from latitude**: Isotherms do NOT run parallel to latitude lines

**Reason**: Larger land surface area in Northern Hemisphere

**Pattern over oceans**:

Isotherms bend toward **north** over oceans

Due to **warm ocean currents**: Gulf Stream and North Atlantic Drift keep North Atlantic warmer

Example: 0°C isotherm runs much farther north in Atlantic than in continental Asia

**Pattern over continents**:

Isotherms bend sharply toward **south** over land

Due to rapid cooling of continental interiors

**Most pronounced in Siberian plain**:

At 60°E longitude, mean January temperature is **–20°C at both 80°N and 50°N latitudes**

Shows extreme continentality effect in Asia

**Temperature ranges in January**:

**Equatorial oceans**: Over 27°C

**Tropics**: Over 24°C

**Middle latitudes**: 2°C to 0°C

**Eurasian continental interior**: –18°C to –48°C (extreme cold)

**Southern Hemisphere Characteristics**:

**More parallel isotherms**: Run nearly parallel to latitude lines

**Reason**: Larger ocean area; smaller land mass

**Gradual temperature change**: More uniform temperature distribution

**Specific patterns**:

20°C isotherm runs parallel to 35°S latitude

10°C isotherm runs parallel to 45°S latitude

0°C isotherm runs parallel to 60°S latitude

Pattern shows land has less effect on isotherms here

JULY TEMPERATURE DISTRIBUTION

**Isotherms generally run parallel to latitude**: Effect of latitude becomes dominant

**Temperature ranges in July**:

**Equatorial oceans**: Over 27°C (warmest)

**Subtropical continental regions** (along 30°N in Asia): Over 30°C (hottest areas)

**Along 40°N latitude**: 10°C isotherm

**Along 40°S latitude**: 10°C isotherm

**Why less deviation than January**:

Entire earth experiences more uniform heating in summer

Both land and oceans warm up significantly

Reduced contrast between land and sea

ANNUAL TEMPERATURE RANGE

**Definition**: Difference between mean temperature of warmest month and coldest month.

**Temperature Range Distribution**:

**Highest range**: Over **60°C in north-eastern Siberian region** (Asian continental interior)

Due to **continentality**: extreme summer heat (over 30°C) and extreme winter cold (–30°C or lower)

**Least range**: Approximately **3°C between 20°S and 15°N**

Equatorial regions maintain relatively uniform temperature year-round

Equatorial oceans remain warm throughout the year

**Pattern**: Range increases poleward from equator; most extreme in continental interiors

**Example**: New Delhi (28.6°N) has annual range ~18.5°C; Kolkata (22.6°N, closer to sea) has range ~12°C

INVERSION OF TEMPERATURE

Definition and Normal Condition

**Normal lapse rate**: Temperature **decreases with increase in elevation** at rate of 6.5°C per 1,000 m.

**Temperature inversion**: Situation where normal lapse rate is **reversed** — temperature **increases with height** in a particular layer of atmosphere.

Characteristics of Temperature Inversion

**Duration**: Usually short-lived, lasting few hours to overnight

**Frequency**: Common occurrence despite being abnormal

**Ideal conditions**:

Long winter night with **clear skies**

**Still air** (no wind to mix layers)

**Calm atmospheric conditions**

Formation Mechanism

Heat radiated by earth during night escapes to space

By early morning, **earth's surface becomes cooler than air above**

Cool air cannot rise; instead sinks and gets trapped

Results in cooler layer at surface, warmer layer above

Types of Temperature Inversion

#### 1. Surface Inversion (Most Common)

Occurs at earth's surface

Created by rapid cooling of ground at night

Found in valleys and low-lying areas

**Effects**:

Promotes **atmospheric stability**: Prevents vertical air movement

**Smoke and dust accumulation**: Particles trapped below inversion layer spread horizontally, creating smog

**Dense morning fogs**: Common especially in winter season

**Duration**: Usually breaks within few hours after sunrise when sun warms earth

#### 2. Inversion in Hills and Mountains (Air Drainage)

**Mechanism of air drainage**:

Cold air produced during night flows downslope under gravity

Being **heavy and dense**, cold air acts like water

Flows downslope and collects in **pockets and valley bottoms**

Warm air remains above due to density differences

**Advantage**: Protects plants from frost damage by trapping warmer air above

**Example**: Frost-prone areas in hills experience less damage than expected due to air drainage

Polar Temperature Inversion

**Normal condition**: Temperature inversion is permanent throughout the year in polar regions

**Reason**: Continuous extreme cold and minimal solar radiation input

**Pattern**: Cold surface layer persists; temperature increases with height

PLANCK'S LAW

**Statement**: "Hotter a body, the more energy it will radiate and shorter the wavelength of that radiation."

**Application**:

Sun (very hot) radiates short-wave radiation

Earth (cooler) radiates long-wave radiation

**Significance**: Explains why atmosphere is transparent to solar radiation but opaque to terrestrial radiation

SPECIFIC HEAT

**Definition**: Energy needed to raise temperature of **one gram** of substance by **one degree Celsius**

**Variation by substance**:

**Water**: High specific heat (takes more energy to warm)

**Land/soil**: Low specific heat (warms quickly)

**Implication**: Water bodies heat and cool slowly compared to land

EXAM-IMPORTANT POINTS

1. **Insolation variation**: Aphelion (July) vs. Perihelion (January) — effect is masked by other factors

2. **Heat budget balance**: 65 units received = 65 units returned; earth temperature constant

3. **Atmospheric heating**: Indirect — through terrestrial radiation, not direct solar radiation

4. **Three heating processes**: Conduction (slow, limited), Convection (vertical, troposphere only), Advection (horizontal, most important)

5. **Latitude effect**: Primary control on temperature; isotherms more parallel in July, deviate in January (Northern Hemisphere)

6. **Continental effect**: Land shows 60°C+ range; oceans show 3°C range

7. **Subtropical maximum**: Higher insolation than equator due to low cloud cover

8. **Temperature inversion**: Reverses normal lapse rate; creates fog, traps pollution, protects frost

9. **Albedo**: 35% of insolation reflected back; rest absorbed

10. **Poleward heat redistribution**: Prevents polar freezing and tropical overheating