Unit 1: Introduction to Earth Atmosphere and Meteorology

1. Elementary Concept of Atmosphere and its Composition

The atmosphere is the vast envelope of gases surrounding the Earth, held in place by gravity. It's essential for life, protecting us from harmful radiation, regulating temperature, and providing the air we breathe.

Composition

The atmosphere is a mixture of gases, with dry air primarily composed of:

• Nitrogen (N₂): ~78%
• Oxygen (O₂): ~21%
• Argon (Ar): ~0.93%
• Carbon Dioxide (CO₂): ~0.04% (highly variable and a key greenhouse gas)

Other trace gases include neon, helium, methane, krypton, and hydrogen. A crucial variable component is water vapor (H₂O), which drives weather processes.

2. Introduction to Atmospheric Dynamics

Atmospheric dynamics is the study of air motion. This motion (wind) is fundamentally caused by unequal heating of the Earth's surface by the sun, which creates differences in temperature and pressure.

Basic Conservation Laws

The complex motions of the atmosphere are governed by three fundamental principles of physics:

1. Conservation of Mass: Air is neither created nor destroyed; it just moves and changes density. This is the basis of the continuity equation.
2. Conservation of Momentum (Newton's 2nd Law, F=ma): The wind we observe is a result of a balance of forces, including the pressure gradient force (air moving from high to low pressure), the Coriolis force (due to Earth's rotation), gravity, and friction.
3. Conservation of Energy (1st Law of Thermodynamics): Energy is conserved. The atmosphere gains energy from the sun and releases it back to space. This energy balance dictates the Earth's temperature and drives thermal air currents.

3. Thermal and Pressure Variation

Pressure Variation

Air pressure is the weight of the column of air above a certain point. It is highest at sea level and decreases exponentially with increasing altitude. This is because there is less air pressing down from above.

Thermal Variation

Temperature variation with altitude is not a simple decrease. The atmosphere's temperature profile is complex, with layers of cooling and warming. This profile is used to define the layers of the atmosphere.

4. Thermal Structure of the Atmosphere

The atmosphere is divided into four main layers based on how temperature changes with height.

[Image of thermal structure of the atmosphere layers]

• Troposphere (0 - ~12 km):
  • This is the lowest layer, where we live and where all weather occurs.
  • Temperature decreases with altitude. This is because the layer is heated from below by the Earth's surface, which absorbs sunlight.
• Stratosphere (~12 - 50 km):
  • Above the troposphere.
  • Temperature increases with altitude. This is known as a "temperature inversion."
  • This warming is caused by the Ozone Layer, which absorbs harmful high-energy ultraviolet (UV) radiation from the sun.
• Mesosphere (~50 - 85 km):
  • Above the stratosphere.
  • Temperature decreases again with altitude, reaching the coldest temperatures in the atmosphere (around -90°C).
  • This is where most meteors burn up.
• Ionosphere (and Thermosphere):
  • The uppermost layer, extending for hundreds of kilometers.
  • Temperature increases dramatically with altitude (to over 1000°C) as high-energy solar radiation (X-rays, extreme UV) is absorbed.
  • The air is extremely thin here.
  • This layer contains electrically charged particles (ions), which are crucial for reflecting radio waves and creating auroras.

5. Spectral Distribution of Solar Radiation

The sun emits energy across the full electromagnetic spectrum. This energy is known as solar radiation or "insolation."

[Image of solar radiation spectrum]

The spectrum can be broken down as follows:

• Ultraviolet (UV): Short wavelengths, high energy. Most is absorbed by the ozone in the stratosphere.
• Visible Light: The narrow band of wavelengths our eyes can see. The atmosphere is mostly transparent to visible light, which is why it reaches the surface.
• Infrared (IR): Longer wavelengths, felt as heat. Some is absorbed by water vapor and CO₂ in the atmosphere, but much of it reaches the surface.

The Earth, being much cooler, emits radiation back to space primarily in the longwave infrared (heat) range. This outgoing heat is what greenhouse gases trap.

6. Meteorological Process and Different Systems

This refers to the formation, evolution, and movement of organized weather patterns, which are driven by atmospheric instability and pressure differences.

• Low-Pressure System (Cyclone): A region where air pressure is lower than its surroundings. Air flows inward (converges) at the surface and then rises. This rising air cools, causing condensation, which leads to clouds and precipitation.
• High-Pressure System (Anti-cyclone): A region where air pressure is higher than its surroundings. Air sinks from above (subsides) and flows outward (diverges) at the surface. This sinking air warms and dries, leading to clear skies and fair weather.
• Fronts: Boundaries between large masses of air with different temperatures (e.g., a "cold front" is the leading edge of an advancing mass of cold air).

7. Overview of Meteorological Observations

To understand and forecast the weather, we must constantly measure the state of the atmosphere. This is done in two main ways:

• Surface Observations: Using ground-based weather stations to measure conditions at the surface, such as temperature, pressure, humidity, wind speed/direction, and precipitation.
• Upper-Air Observations: Using radiosondes (instrument packages on weather balloons) and satellites to measure conditions at various altitudes in the atmosphere.