Introduction to Meteorology: Scientific Inquiry and Atmospheric Structure

The Nature of Scientific Inquiry

• Science is defined as a systematic process for learning about the world and testing our understanding of it. Its primary goal is to apply scientific principles to solve problems.

• The scientific method is a specific technique used for testing ideas, involving several key components:

  • Hypothesis: An explanation of observed facts that requires testing.

  • Theory: A hypothesis that has successfully passed many tests and is widely accepted. A theory is not a guess; it is a large construct with extremely strong confidence behind it. Examples include Darwin’s theory of evolution, atomic theory, cell theory, the big bang theory, plate tectonics, and general relativity.

  • Law: A large construct that is generally accepted as scientific fact.

• Scientific methods are not rigid "recipes" but involve creativity, insight, and an iterative process:

  • Observing facts and making measurements.

  • Raising questions that lead to hypotheses.

  • Collecting data to test these hypotheses.

  • Accepting or rejecting hypotheses based on evidence.

  • Sharing results with the scientific community for further, independent testing.

Case Study: Western U.S. Wildfires and Scientific Observation

• Direct empirical observations show that wildfires are increasing in frequency and the wildfire season is lengthening in the Western United States.

• Numerical Data on Wildfires:

  • Average number of large wildfires (bigger than $1,000$ acres) per year:

    • 1980–1989: ~$140$ wildfires.

    • 1990–1999: <$160$ wildfires.

    • 2000–2012: ~$250$ wildfires.

  • Average length of wildfire season:

    • Early 1970s: $5$ months.

    • Today: $7+$ months.

• Possible Working Hypotheses for Increasing Fires:

  1. Human-caused fires are increasing.

  2. Lightning strikes are increasing.

  3. Forests are drier due to mega-droughts.

  4. Long-term forest management has increased the amount of fuel in forests.

  5. Long-term climate change.

• Evidence Linking Climate Change to Wildfire Risk:

  • Average annual temperatures in the Western U.S. have increased by $1.9^{\circ}F$ since 1970.

  • Winter snowpack melts up to $4$ weeks earlier than in previous decades.

  • Earlier snowmelt leads to longer periods where forests are dry and primed for ignition.

Weather versus Climate

• Weather: The state of the atmosphere at any given time and place. It is highly variable and changes from day to day. A weather map shows current conditions like temperature ($10s$ to $110s^{\circ}F$), rain, thunderstorms, snow, ice, and cloud cover (sunny, partly cloudy, mostly cloudy).

• Climate: The average weather and generalized weather variation for a given place. It is a long-term description and is very predictable over time, though it varies by location based on latitude, altitude, and proximity to water (upwind ocean vs. land).

• Primary Distinction: "Climate is what you expect, but weather is what you get."

• Climate data cannot be used to predict specific daily weather events due to high variability. For example, New York City shows a range of approximately $45^{\circ}F$ between record highs and record lows on any given date.

• Elements of Weather and Climate: Both are measured using the same six basic variables:

  1. Temperature.

  2. Humidity.

  3. Type and amount of cloudiness.

  4. Type and amount of precipitation.

  5. Air pressure.

  6. Speed and direction of wind.

Earth as a System: The Four Spheres

• A system is defined as a group of interacting or interdependent parts that form a complex whole. The Earth system consists of four interconnected spheres:

Geosphere

• The solid Earth extending from the surface to the center ($6371\,km$ or $3960\,miles$).

• Chemical Layers:

  • Crust: Thin outer layer of light silicate materials.

  • Mantle: Silicate materials rich in iron and magnesium.

  • Core: Primarily iron with nickel and sulfur.

• Physical Properties:

  • Lithosphere: Rigid outer layer (~$100\,km$ or $60\,mi$).

  • Asthenosphere: Plastic/ductile layer (~$700\,km$ or $430\,mi$).

  • Mesosphere: Solid/rigid lower mantle layer (~$2885\,km$ or $1800\,mi$).

  • Outer Core: Liquid iron-rich layer (~$5155\,km$ or $3200\,mi$).

  • Inner Core: Rigid/solid center.

Hydrosphere

• The liquid water component of Earth.

• Oceans cover $71\%$ of the Earth’s surface and contain $97\%$ of the Earth’s water volume.

• Freshwater accounts for only about $3\%$ of all water:

  • Glaciers: $1.76\%$.

  • Groundwater: $0.77\%$.

  • Lakes, rivers, and streams: $0.03\%$.

• The atmosphere contains less than $1\%$ of Earth’s water.

• Hydrologic Cycle: Involves water vapor emitted by volcanoes, evaporation from oceans, transpiration from plants, condensation (cloud formation), precipitation (rain or snow), infiltration into the ground, and surface flow back to oceans.

Biosphere

• Includes all life on Earth: on land, in oceans, in soil, and on air currents in the lower atmosphere.

• Modern coral reefs host about $25\%$ of all marine species.

• Interactions: Plants contribute water vapor through transpiration and remove carbon dioxide. Vegetation coverage also affects how much sunlight is reflected (albedo).

Atmosphere

• A thin gas envelope protecting the Earth. While space is considered to begin around $100\,km$ ($60\,miles$), $99\%$ of the atmosphere is concentrated within the first $30\,km$ ($19\,miles$) of the surface.

Composition of the Atmosphere

Major Gaseous Components

• Nitrogen ($N_2$): $78\%$.

• Oxygen ($O_2$): $21\%$.

• Together, these two account for $99\%$ of the atmosphere's volume.

Variable Components and Trace Gases

• Carbon Dioxide ($CO_2$):

  • Makes up $0.0400\%$ ($400\,ppm$) of the atmosphere.

  • Absorbs outgoing energy, contributing to atmospheric warming.

  • The "Keeling Curve" tracks the steady increase of $CO_2$ since the Industrial Revolution, primarily due to burning fossil fuels. It shows annual cycles where concentrations drop during northern hemisphere summers (leaf growth) and rise in winters (leaf loss).

• Water Vapor:

  • The source of all clouds and precipitation.

  • Varies by location but represents less than $4\%$ of the total volume.

• Aerosols:

  • Microscopic particles such as dust, soot, sea salts, ash, and sulfur dioxide ($SO_2$).

  • They absorb radiation (warming) and scatter sunlight (cooling).

  • They serve as nuclei for cloud formation and cause red/orange sunrises and sunsets.

• Ozone ($O_3$):

  • Concentrated in the stratosphere ($10-50\,km$).

  • Absorbs harmful ultraviolet (UV) radiation.

  • Synthetic Chlorofluorocarbons (CFCs), used in refrigerants and aerosols, break down the ozone. This led to the thinning of the ozone layer (the "ozone hole") over the Southern Hemisphere, measured in Dobson units. The Montreal Protocol (1987) now regulates these substances.

• Greenhouse Gases: Include $CO_2$, Methane ($CH_4$), CFCs, Nitrous Oxide ($N_2O$), Fluorinated Gases, and Water Vapor ($H_2O$).

Vertical Structure of the Atmosphere

Pressure Changes

• Air pressure is the weight of the atmosphere above you. As elevation increases, air pressure decreases at a decreasing rate.

• Average air pressure at sea level is $1013\,mb$.

Temperature Changes and Thermal Layers

• Environmental Lapse Rate: In the lowest layer, temperature decreases with height at an average rate of $6.5^{\circ}C$ per $1\,km$ or $3.5^{\circ}F$ per $1000\,ft$.

• The Four Layers:

  1. Troposphere ($0-12\,km$): The lowest layer where practically all weather occurs. Ends at the Tropopause.

  2. Stratosphere ($12-50\,km$): Contains the highest concentration of ozone. Temperatures increase in this layer due to UV absorption. Ends at the Stratopause.

  3. Mesosphere ($50-80\,km$): Temperature decreases again, reaching the coldest atmospheric temperatures. Ends at the Mesopause. This is where meteors typically burn up.

  4. Thermosphere ($80\,km+$): The uppermost layer with no defined upper limit. Contains the Ionosphere.

The Ionosphere and Auroras

• The Ionosphere is an electrically charged layer located between $80\,km$ and $400\,km$ above the surface, coinciding with the lower thermosphere.

• It contains atoms of oxygen and nitrogen that are "energized" into ions (positively or negatively charged particles) by solar radiation.

• These ions emit light, creating auroras when Earth's magnetic field traps solar particles (solar flares):

  • Aurora Borealis: Northern Hemisphere (Northern Lights).

  • Aurora Australis: Southern Hemisphere (Southern Lights).