Introduction
The Sun is the ultimate source of energy for all atmospheric processes on Earth. Understanding how solar energy reaches the Earth, how the atmosphere and surface respond to it, and how temperature varies across time and space is fundamental to physical geography. This chapter explores the mechanisms of insolation, the heat budget, and the factors that govern temperature distribution.
Insolation
Insolation (incoming solar radiation) is the solar energy received per unit area of the Earth's surface per unit time. The Sun emits energy as short-wave radiation (primarily visible light and ultraviolet). Not all of this energy reaches the ground — the atmosphere absorbs, reflects, and scatters a significant portion.
Solar constant: The average intensity of solar radiation received just outside the Earth's atmosphere is approximately 1.36 kW/m2 (kilowatts per square metre). This value varies slightly due to Earth's elliptical orbit.
- Factors affecting insolation received at the surface:
- Angle of incidence: When solar rays strike at a low angle (oblique), energy spreads over a larger area, reducing intensity. At the equator, rays are nearly perpendicular, maximising insolation.
- Duration of sunshine: Longer days mean more hours of insolation; polar regions experience very long summer days but near-zero winter days.
- Transparency of the atmosphere: Clouds, dust, and aerosols reflect and absorb radiation before it reaches the surface.
- Distance from the Sun: Earth is closest (perihelion, ~147 million km) in early January and farthest (aphelion, ~152 million km) in early July.
Heating of the Atmosphere
The atmosphere is largely transparent to short-wave solar radiation but absorbs long-wave (terrestrial) radiation efficiently. This asymmetry is critical.
- 1.Terrestrial radiation: The Earth's surface absorbs insolation and re-radiates energy as long-wave (infrared) radiation.
- 2.Greenhouse effect: Gases like water vapour (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) trap outgoing long-wave radiation, warming the lower atmosphere. Without this natural greenhouse effect, Earth's average temperature would be about -18 degrees C instead of the present +15 degrees C.
- 3.Conduction: Direct heat transfer from the warm surface to the air layer in contact with it (important close to the ground).
- 4.Convection: Vertical movement of heated air parcels carrying energy upward — the dominant mechanism of tropospheric heating.
- 5.Advection: Horizontal transfer of heat by wind — vital in redistributing heat from equator to poles.
Heat Budget (Energy Balance)
The Earth–atmosphere system maintains a long-term heat balance: the energy received equals the energy lost.
- Of 100 units of incoming solar radiation:
- 35 units are reflected back to space (albedo): ~27 units by clouds, ~2 by atmosphere, ~6 by Earth's surface.
- 65 units are absorbed: ~14 by the atmosphere and ~51 by the Earth's surface.
The Earth re-radiates these 51 units as long-wave radiation; the atmosphere absorbs 48 of them and emits 48 to space along with its own 17 units = 65 units leaving the system.
Albedo: The fraction of incoming solar radiation reflected by a surface. Snow and ice have high albedo (~0.8); forests and oceans have low albedo (~0.1–0.2).
Temperature Distribution
- Factors controlling temperature:
- Latitude: Temperature generally decreases from equator to poles because of angle of incidence and day-length variation.
- Altitude: Temperature decreases with altitude at the Normal Lapse Rate of 6.5 degrees C per 1000 m in the troposphere.
- Distance from sea (continentality): Continental interiors experience greater annual temperature ranges (hot summers, cold winters) than coastal areas, which are moderated by the sea.
- Ocean currents: Warm currents (e.g., Gulf Stream) raise coastal temperatures; cold currents (e.g., Labrador Current) lower them.
- Prevailing winds: Onshore winds from warm oceans raise temperatures; offshore winds or winds from cold regions lower them.
Isotherms: Lines on a map joining places with equal mean temperature. They generally run parallel to latitudes but bend over continents and oceans due to differential heating.
Inversion of temperature: Normally temperature decreases with altitude, but sometimes a layer of warmer air overlies cooler air — this is called a temperature inversion. It suppresses convection, causes fog formation, and traps pollutants.
Common mistakes
- Students often confuse insolation (incoming solar) with terrestrial radiation (outgoing long-wave). Remember: the Sun sends short-wave; Earth sends long-wave.
- The heat budget does NOT mean the Earth gets hotter — it means the system is in balance over time.
- Lapse rate applies within the troposphere; temperature actually increases in the stratosphere due to ozone absorption.
- Perihelion (January) does NOT mean the Northern Hemisphere is hotter — the angle of incidence, not distance, is the dominant control.
Summary
Solar radiation drives all atmospheric processes. The Earth maintains a heat balance by absorbing insolation and re-radiating an equal amount of energy. Temperature is controlled by latitude, altitude, distance from the sea, ocean currents, and winds. Temperature inversions and isotherms are important tools for understanding spatial temperature patterns.