सौर विकिरण, ऊष्मा संतुलन एवं तापमान (Solar Radiation, Heat Balance and Temperature)

• Insolation: The incoming solar radiation intercepted and received by the Earth's surface in the form of shortwave electromagnetic radiation.
• Solar Constant: The quantity of solar energy received per square centimeter per minute at the outer boundary of the atmosphere on a surface held perpendicular to solar rays: $1.94 \text{ calories / cm}^2 / \text{minute}$ ($1,368 \text{ W/m}^2$).
• Variation by Angle of Incidence:
1. Vertical Rays (Equator): Strike at $90^\circ$, concentrating radiant heat over a small surface area, and pass through a thin atmospheric column with minimal scattering, maximizing surface heating.
2. Slanted Rays (High Latitudes): Strike at oblique angles, spreading the same amount of solar energy over a vastly larger surface area, and travel through a thick atmospheric path where dust, clouds, and gases scatter and absorb significant energy, minimizing surface heating.

1. (a) Terrestrial Radiation: The Earth, heated by incoming solar shortwaves, radiates heat back into space as Longwave Infrared Radiation. Greenhouse gases ($CO_2, H_2O$) absorb this longwave terrestrial heat, heating the atmosphere from below.
   2. (b) Conduction: Direct molecular heat transfer between the warm Earth's ground and the immediate contacting layer of air.
   3. (c) Convection: The vertical upward movement of heated air. As air warms at the ground, it expands, becomes less dense, and rises vertically in convection currents, transferring heat to upper levels.
   4. (d) Advection: The horizontal transfer of heat by moving winds and air masses. Advection is responsible for most middle-latitude weather variations (e.g., the hot, dry "Loo" winds in northern India).

Assume total incoming solar shortwave radiation = 100 Units:
1. Planetary Albedo (Lost directly to space):
• Reflected by clouds = 27 units
• Scattered by atmosphere = 6 units
• Reflected by Earth's snow/ice = 2 units

$$\text{Total Albedo} = 27 + 6 + 2 = \mathbf{35 \text{ Units}}$$

(Reflected directly without heating).
2. Absorbed Energy (Net Input):
• Absorbed directly by atmosphere ($O_3$, water vapor, dust) = 14 units
• Absorbed by Earth's surface (land and oceans) = 51 units

$$\text{Total Absorbed} = 14 + 51 = \mathbf{65 \text{ Units}}$$

3. Terrestrial Heat Radiated Back to Space:
• Radiated directly from surface to space = 17 units
• Radiated by atmosphere to space = 48 units (14 original solar + 34 transferred from surface via conduction, convection, and latent heat)

$$\text{Total Radiated} = 17 + 48 = \mathbf{65 \text{ Units}}$$

Conclusion: $\text{Net Absorbed (65)} = \text{Net Emitted (65)}$. Net heat balance is ZERO, maintaining Earth's mean global temperature at $15^\circ\text{C}$!

• Albedo: The fraction or percentage of total incoming solar radiation that is reflected directly back into space by a surface without being absorbed or converted into heat.
Earth's overall planetary albedo is approximately 35% (0.35).
• Highest Albedo Surface: Fresh, clean snow and polar ice caps, which reflect 80% to 90% of all incoming sunlight back into space, explaining why polar regions remain freezing even during 24-hour summer daylight!

Land heats and cools much faster than oceans due to four physical properties:
1. Specific Heat Capacity: Water has a very high specific heat capacity, requiring roughly three times more thermal energy to raise its temperature by $1^\circ\text{C}$ compared to dry soil or rock.
2. Transparency: Solar rays penetrate several meters deep into transparent ocean water, distributing heat over a massive volume. Opaque land absorbs heat only in the top thin millimeter layer.
3. Convective Mixing: Water is fluid; ocean currents and wave action mix warm surface water with cold deep water. Land is solid and immovable.
4. Evaporation: Continuous evaporation over oceans consumes immense latent heat, exerting a strong cooling effect on water.

In January (Northern Hemisphere winter), landmasses cool down drastically due to terrestrial radiation, while adjoining oceans remain relatively warm due to water's high thermal inertia and warm ocean currents:
• Over Continents: Because land is much colder than the ocean at the same latitude, isotherms bend sharply equatorward (southward) over North America and Eurasia.
• Over Oceans: Because ocean water is warmed by currents (like the North Atlantic Drift), isotherms bend sharply poleward (northward) over the North Atlantic and North Pacific.

A Temperature Inversion is a meteorological condition where temperature increases with increasing altitude, directly reversing the normal lapse rate (a layer of warm air sits atop a layer of cold air).
Three Ideal Conditions:
1. Long Winter Nights: Provides a prolonged duration of darkness during which outgoing terrestrial longwave radiation exceeds incoming solar radiation.
2. Clear, Cloudless Sky: Absence of clouds allows terrestrial infrared heat to escape directly into outer space without being trapped.
3. Calm, Still Air: Absence of wind prevents the turbulent mixing of cold ground air with warmer air aloft.

• Valley Inversion (Air Drainage): On cold, calm winter nights, the air in contact with high mountain slopes loses heat rapidly through terrestrial radiation, becoming extremely cold, dense, and heavy. Under gravity, this freezing air slides down the mountain slopes (Katabatic Winds) and pools in the bottom of the valley, displacing lighter warm air upward.
• Why Orchards are on Slopes: The valley floor becomes a freezing "frost pocket" where temperatures plunge below freezing, which would kill delicate fruit blossoms and freeze tree sap. The mountain slopes remain warm and frost-free due to the displaced warm inversion layer aloft. Therefore, farmers plant apple orchards on slopes.