Atmosphere and Weather Notes ch 2.1

Atmosphere and Weather

Introduction

  • This chapter covers:
    • Energy gain, loss, and transfer in the Earth-atmosphere system at a local scale.
    • Variations in the global energy budget and its link to seasonal variations.
    • Atmospheric moisture processes causing precipitation.
    • Human impact on weather and climate at global and city scales.

The Atmosphere

  • The atmosphere is a mixture of gases held to Earth by gravity.
  • Density and pressure increase towards the Earth's surface.
  • Divided into zones based on temperature variations; the lower two zones (troposphere and stratosphere) are most relevant.

Vertical Structure of the Atmosphere

  • Troposphere: The zone of weather.
    • Temperature decreases with increased altitude.
    • Contains almost all water vapor and suspended aerosols.
    • Tropopause: Temperature inversion prevents air rising into the stratosphere; varies in height from about 8 km at the poles to 18 km at the Equator.
  • Stratosphere:
    • Contains important concentrations of ozone.
    • Stratopause marks the boundary with the mesosphere.
  • Mesosphere:
    • Above the stratosphere, temperature decreases with altitude until the mesopause.
  • Thermosphere:
    • Temperature increases significantly with altitude, reaching very high temperatures.

Composition

  • Troposphere and stratosphere consist of 78% nitrogen and 20% oxygen.
  • Trace amounts of other gases like methane and low-level ozone exist in the troposphere.

Local Diurnal Energy Budgets

Factors Affecting the Daytime Energy Budget

  • The sun is the primary energy source.
  • The atmosphere derives most heat from the Earth, not directly from the sun's rays.
  • Daytime energy budget model represents an average situation.

Components of the Daytime Energy Budget

  • Incoming Short-wave Solar Radiation: The main input of energy.
  • Outputs:
    • Reflected Solar Radiation:
      • Amount of solar radiation reflected back to space from the white upper surfaces of thick clouds can be about 80%.
    • Outgoing Long-wave Terrestrial Radiation.
    • Energy Absorbed into the Earth's Surface.
    • Sensible Heat Transfer.
    • Latent Heat Transfer.
  • Surplus Energy: The remaining energy available at the surface, varies by location and time.

The Night-Time Energy Budget

  • Lacks short-wave radiation from the sun and reflected solar radiation, leading to an energy deficit.
  • Four-factor model: Long-wave radiation, sensible heat transfer, latent heat transfer, and conduction of heat to the surface.
  • As insolation stops, the ground loses heat and cools the air next to it.

Components of the Night-Time Energy Budget

  • Long-wave Earth Radiation:
    • Escapes to space through radiation 'window' under clear skies.
    • Cloud cover affects the amount of escaping long-wave radiation; clear skies lead to very cold nights.
  • Sensible Heat Transfer:
    • Convectional uplift may continue in the tropics and sub-tropics.
    • Advection: Horizontal transfers of air from warmer to colder areas.
  • Latent Heat Transfer:
    • Dew forms by condensation on cold surfaces, releasing heat.
  • Conduction of Heat to the Surface:
    • Heat absorbed during the day returns to the surface, offsetting heat loss.

Temperature Changes During a Cloudless Day

  • On cloudless nights, the Earth's surface rapidly loses heat by long-wave radiation.
  • Intense cooling occurs if the air is also calm.

Latent Heat Transfer Explained

  • Heat is absorbed from the air for changes like evaporation, leaving less energy for heating at the surface.
  • Latent heat is stored in water vapor and carried upwards via convection currents.
  • When water vapor condenses into water droplets or ice crystals, stored heat is released, warming the air - latent heat of condensation, increasing convection.
  • Significant solar radiation is used to convert snow and ice back to water in high latitudes during spring and early summer.
  • Albedo: Where snow cover is permanent, the net radiation balance may be zero or slightly negative, even with maximum insolation.

Influence of Clouds on the Daytime Energy Budget

  • High thin clouds (e.g., cirrus) allow incoming solar radiation to pass through but absorb some long-wave radiation, warming the Earth's surface.
  • Deep convective clouds (e.g., cumulonimbus) neither heat nor cool overall.
  • Overcast skies with low, thick clouds (e.g., stratus, stratocumulus) can reflect about 80% of solar radiation, cooling the Earth's surface.
  • Clouds generally have higher albedos than the surface below, so more short-wave radiation is reflected back to space, resulting in a net cooling effect.

Albedo

  • The percentage of solar radiation reflected back to space by a surface.
  • Lighter-colored surfaces have higher albedo (reflect more), while darker surfaces have lower albedo (absorb more).
  • Examples:
    • Fresh snow: 92%
    • Thick stratus cloud: 65%
    • Sandy surfaces: 40%
    • Green grass: 15%
    • Asphalt: 7%

Factors Affecting Albedo

  • Angle of the sun: Albedo of oceans varies; low albedo (4%) when the sun is high near midday, but high reflection (80%) when the evening sun is low.
  • Type of surface:
    • Snow: Dirty snow melts faster than fresh snow because it absorbs more solar radiation.
    • Crops: Albedo can vary from 15-25% depending on type and stage of growth.
    • Urban areas: Parts with dark surfaces (e.g., asphalt) have lower albedo, while parts with light surfaces (e.g., concrete) have higher albedo.

Energy Absorption into the Surface and Sub-Surface

  • Dark surfaces absorb more radiation than surfaces with high albedo.
  • Some absorbed energy is transferred into the soil and rocks by conduction.
  • Light-colored rock (e.g., limestone) is a poor conductor, so heating is confined to the surface, leading to high surface temperatures.
  • Darker rock (e.g., granite) with a low albedo absorbs heat well.
  • Soil conductivity varies with moisture content: dry sand is a poor conductor, concentrating heat at the surface; water in soil increases heat flow, cooling the surface.

Long-Wave Earth Radiation

  • Short-wave radiation from the sun is absorbed by the Earth and re-radiated as long-wave (infrared) radiation.
  • Long-wave radiation is more easily absorbed by greenhouse gases (water vapor, carbon dioxide) than short-wave radiation.
  • Clouds absorb long-wave radiation efficiently and re-radiate it back to Earth, keeping heat in via the greenhouse effect.
  • Heat loss is greatest in dry air, but only small amounts escape directly to space through 'radiation windows'.

Sensible Heat Transfer

  • Heat energy transferred by direct conduction or convection.
    • Air is a poor conductor, so only a thin layer next to the surface is warmed by conduction.
    • Warming causes air molecules to expand, become lighter, and rise through cooler, denser air (convection).
    • Warm winds near the surface can be deflected upwards by obstacles, reaching up to 600 m above the surface with strong wind turbulence.

Latent Heat Transfer

  • Occurs when water on the Earth's surface evaporates or ice melts to water vapor.
  • The heat needed for these changes is absorbed from the air, leaving less energy for surface heating.
  • This latent heat is stored in water vapor and carried upwards via convection currents.
  • During condensation, stored heat is released into the air, warming it and increasing convection speed and extent.

Solar Radiation

  • The sun emits short-wave (ultraviolet) radiation.
  • Radiation involves the transfer of heat via electromagnetic waves.
  • Half of the Earth facing the sun is being heated, while the other side remains unheated.

Passage Through the Atmosphere

  • Not all solar energy reaches the Earth's surface.
    • About 5% is scattered directly back to space by dust and smoke particles.
    • 23% is absorbed by atmospheric gases (mainly ozone and oxygen at high levels, and small amounts by carbon dioxide and water vapor near the Earth's surface).
    • 24% is reflected back to space (18% by clouds, 6% by the Earth's surface - snow, ice, water).
    • The remaining 48% reaches the Earth's surface directly.

Factors Affecting Solar Radiation Intensity

  • The angle of the sun's rays, being greatest at 90° and reducing at smaller angles.
  • Dust and smoke particles also scatter solar radiation.
  • Short-wave blue light rays are more easily scattered.

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