Weather and Climate Overview

Weather vs. Climate

• Weather describes short-term atmospheric conditions (hours, days, weeks) like storms and heatwaves.
• Climate describes long-term atmospheric patterns (decades, centuries) and averages. A "climate normal" is a 30-year average.

Climate Variability vs. Climate Change

• Climate Variability: Short-term fluctuations like seasonal droughts or floods, not necessarily part of a larger trend.
• Climate Change: Long-term, persistent changes in the climate system (e.g., increased average temperatures, sea level rise), driven by natural or human factors.

Atmospheric Layers

• Troposphere: Lowest layer, contains 75-80% of the atmosphere's gases, weather occurs here; temperature decreases with altitude.
• Stratosphere: Contains the ozone layer, absorbing harmful UV radiation; temperature increases with altitude.
• Mesosphere: Low air pressure, extreme cold; meteors burn up here.
• Thermosphere: Site of auroras and satellites; temperature fluctuates with solar activity.

Earth's Energy Budget

• Earth receives mostly energy from the sun.
• Greenhouse effect: Greenhouse gases (CO2, CH4) trap heat, keeping Earth's surface ~15°C instead of -18°C. Human activity intensifies this, causing global warming.

Atmospheric Processes and Circulation

• Low-pressure: Rising air cools, forms clouds and precipitation.
• High-pressure: Descending air warms, creating clear skies.
• Global circulation: Driven by Earth's rotation and uneven heating, creates pressure differences which move air (Hadley cells, prevailing winds). This distributes heat and shapes climate zones.

Key Weather Phenomena

• Thunderstorms: Require moisture, unstable air, and a lifting mechanism; stages: developing, mature, dissipating.
• Tropical Cyclones (Hurricanes/Typhoons): Rapidly rotating systems fueled by warm water (≥26.5°C), low wind shear, and the Coriolis effect; weaken over land.

Climate Patterns

• El Niño-Southern Oscillation (ENSO):
  • El Niño: Warmer-than-average Pacific waters, weaker trade winds, global weather impacts (e.g., droughts, heavy rains)
  • La Niña: Cooler-than-average Pacific waters, stronger trade winds, opposite effects of El Niño.
• Pacific Decadal Oscillation (PDO): Long-term climate variability (warm/positive & cool/negative phases) impacting global weather and interacting with ENSO.

Milankovitch Cycles

• Orbital changes affecting solar radiation:
  • Eccentricity: Shape of Earth's orbit.
  • Obliquity: Tilt of Earth's axis.
  • Precession: Wobble of Earth's rotation.
• Drives glacial and interglacial cycles.

Human and Natural Climate Drivers

• Natural: Volcanic eruptions, tectonic shifts, solar activity.
• Human: Fossil fuel burning, deforestation: increasing CO2 levels (greenhouse gases).

Earth's Past Climates

• Earth has undergone significant climate shifts (ice ages, warmer periods).
• Current warming rates are unprecedented in recent Earth history due to human influence.

Additional details about natural drivers

• Volcanic Eruptions: Release sulfur dioxide (SO2) which creates aerosols, reflecting sunlight & leading to short-term cooling. (Mount Pinatubo, 1991).
• Plate Tectonics: Changes ocean circulation and atmospheric patterns, impacting long-term climate. Mountain building alters precipitation patterns.
• Solar Activity: Variations in solar irradiance influence short term climatic changes (Maunder Minimum).