Solar Radiation, Heat Balance and Temperature

Insolation and the Heat Budget

Insolation: What Controls It

• Insolation, incoming solar radiation, averages 1.94 calories per sq. cm per minute at the top of the atmosphere.
• Insolation varies with the earth’s rotation, the angle of the sun’s rays, day length, atmospheric transparency, and land configuration.
• The earth’s axis tilts 66.5 degrees from its orbital plane, strongly shaping how much insolation different latitudes receive. This incoming energy first passes through the layered atmosphere before any of it reaches the ground.
• On 4 July the earth sits farthest from the sun, at aphelion, 152 million km away. On 3 January it is nearest, at perihelion, 147 million km away, so it receives slightly more insolation then. Land-sea distribution and atmospheric circulation mask this small variation, so it barely affects daily weather.

Conduction, Convection and Advection

• Conduction moves heat from a warmer body to a cooler one in direct contact, warming the atmosphere’s lowest layers.
• Convection carries heated air upward as rising currents, but this transfer is confined only to the troposphere.
• Advection, horizontal air movement, drives most day-to-day weather change in the middle latitudes. India’s hot summer “loo” wind, part of the broader Indian monsoon climate system, is a real example.

The Heat Budget: 100 Units In, 65 Out

• Of 100 units of insolation, 35 are reflected straight back to space before reaching the surface. This reflected share is the earth’s albedo.
• The remaining 65 units are absorbed: 14 within the atmosphere, and 51 by the earth’s surface itself.
• The earth radiates its 51 units back, 17 straight to space and 34 absorbed by the atmosphere. Combined with the atmosphere’s own 14 units, a total of 48 units return to space from the atmosphere.
• 17 units from earth plus 48 from the atmosphere equals the original 65 absorbed, which is why the earth neither steadily warms nor cools. This same balance underlies the climate zones mapped in Koeppen’s climate classification.

Temperature Patterns and the Exam Angle

Five Factors Controlling Temperature

• Temperature depends on insolation received by latitude, and decreases with height at the normal lapse rate of 6.5°C per 1,000 m.
• Because the sea heats and cools more slowly than land, coastal places see less temperature variation than inland places.
• Warm air masses and warm ocean currents raise coastal temperatures; cold ones lower them.

Isotherms, Inversion and Net Radiation Balance

• Isotherms are lines joining places of equal temperature, generally running parallel to latitude.
• Isotherms bend far more in January than July, especially in the Northern Hemisphere, where a much larger landmass sharpens land-ocean temperature contrasts. Warm currents like the Gulf Stream bend isotherms northward over the ocean, while temperatures drop faster over nearby land.
• Temperature inversion reverses the normal lapse rate, usually briefly, most often on clear winter nights with still air. Over the poles, inversion is normal all year round, not just an occasional event.
• Cold, dense air flows downhill at night into valley bottoms under gravity, a process called air drainage that protects hillside plants from frost.
• Regions between 40°N and 40°S run a radiation surplus, while polar regions run a deficit. Winds redistribute this surplus heat poleward, which is why the tropics don’t keep heating up and the poles don’t freeze permanently solid.