Insolation, the Water Cycle, Climate & Geologic History
Solar energy and the water cycle, the factors that control climate and the greenhouse effect, and how geologists read the age of rock layers.
Insolation — incoming solar energy
Insolation is incoming solar radiation. How much a place receives depends on:
- Angle of the Sun — direct (high-angle) rays deliver the most energy; they're concentrated. Low-angle rays spread thin. Insolation peaks at solar noon.
- Duration — longer daylight means more total energy (summer).
- Latitude and season — the tropics get the most; the poles the least.
Earth absorbs short-wave solar energy and re-radiates it as long-wave (infrared) energy.
The water cycle
The Sun drives a constant recycling of water:
- Evaporation (and transpiration from plants) lifts water vapor into the air.
- Condensation forms clouds as rising air cools to its dew point.
- Precipitation (the arrow pointing DOWN from a cloud as rain or snow) returns water to the surface.
- Runoff flows over land to rivers and oceans; infiltration soaks into the ground.
Climate factors and the greenhouse effect
Climate is the long-term average weather of a place, controlled by latitude, elevation, nearness to large water bodies, ocean currents, and mountain ranges. Water moderates temperature, so coastal places have milder swings than inland ones.
The greenhouse effect is real and natural: gases like carbon dioxide, water vapor, and methane absorb the long-wave energy Earth radiates and send some back down, keeping the planet warm enough to live on. Adding more of these gases enhances the effect.
Reading geologic history
Geologists work out the order of past events from the rock record:
- Superposition — in undisturbed layers, the oldest is on the bottom, the youngest on top.
- Cross-cutting — a fault or intrusion is younger than the rock it cuts through.
- Unconformity — a buried, eroded surface marking a gap of missing time (often tilted layers below, flat layers above).
- Index fossils — species that lived briefly but spread widely let us match rock layers between distant places.
- Radioactive decay gives absolute ages: each radioactive isotope has a fixed half-life (carbon-14 for recent organic remains; uranium and potassium isotopes for very old rock).