The Atmosphere

  • Properties

  • Clouds and Precipitation

  • Motion
        - Circulation

  • Air Masses

  • Last Chapter for Exam 2

Properties

Composition of the Atmosphere

  • The atmosphere is primarily composed of nitrogen (N₂) at 78.08% and oxygen (O₂) at 20.95%.

  • It contains trace amounts of other gases, including:
        - Carbon Dioxide (CO₂) - 0.04%
        - Methane (CH₄)
        - Ozone (O₃)
        - Nitrous Oxide (N₂O)
        - Sulfur Dioxide (SO₂)
        - Carbon Monoxide (CO)
        - Neon (Ne)
        - Helium (He)
        - Argon (Ar)
        - Hydrogen (H₂)
        - The balance of greenhouse gases is crucial:
            - Too little greenhouse gases would result in a world below freezing.
            - Too much would lead to excessive warming.

Layers of the Atmosphere

  • Troposphere: Temperature warms due to infrared radiation from Earth’s surface; most weather occurs here.

  • Stratosphere: Temperature warms as ozone absorbs ultraviolet light.

  • Mesosphere: Temperature cools due to the lack of ozone.

  • Thermosphere: Temperature warms because it absorbs high-energy radiation.

Weather vs. Climate

  • Weather: Describes short-term atmospheric conditions, including phenomena like thunderstorms, tornadoes, hurricanes, and floods; it represents the state of the atmosphere on any given day.

  • Climate: Refers to long-term atmospheric patterns, including average conditions over years or decades and trends in temperature and precipitation.

Describing Weather

To evaluate weather conditions, the following elements are considered:

  • Temperature

  • Air pressure

  • Relative humidity

  • Wind

  • Precipitation

Temperature and Heat

  • Temperature: A measure of the kinetic energy of atoms, indicating how hot or cold a substance is.

  • Heat: Represents the total amount of energy contained within molecules. Energy can be absorbed or released during phase changes.

Pressure

  • Air Pressure: The force exerted by air molecules over an area, averaged at 1,013.25 millibars (or 14.7 lb/in²).

  • Examples of air pressure levels:
        - Normal pressure: 1,013.25 mb (14.7 lb/in²)
        - Hurricane low pressure: 979 mb (14.2 lb/in²)

  • Factors affecting pressure:
        - Warm air leads to lower density and lower pressure as molecules move apart.
        - Cool air results in higher density and higher pressure as molecules come closer together.

Critical Thinking 1

  • How do warm and cold air move in the atmosphere?
        - Warm air rises as it is less dense; cold air sinks as it is more dense.

Density

  • Air pressure and density decrease with increasing altitude:
        - 50% of atmospheric molecules are located below 5.6 km (or 18,372 ft).
        - 99% are found below 30.5 km (or 100,065 ft).
        - 100 km marks the boundary of space (or 328,084 ft).

Humidity

  • Humidity: The amount of water vapor present in the air.

  • Relative Humidity: A measure of how close the air is to being saturated with moisture.

Humidity Characteristics

  • Warm air can hold more water vapor than cooler air:
        - Cold air generally has high relative humidity (RH).
        - Warm air usually has low relative humidity (RH).

Wind

  • Wind represents the horizontal movement of air, which can also move vertically in updrafts and downdrafts.

  • The direction of wind is indicated based on its origin.

Reading Wind Maps

  • Wind maps use isobars (lines of equal pressure) to denote wind speeds;
        - Closely spaced lines indicate stronger winds.
            - Example: A wind map may show different wind speeds, e.g., from the W at 10 knots, from the SW at 5 knots, etc.

Radar

  • Radar technology utilizes microwaves that reflect off precipitation such as rain and hail:
        - High reflectivity indicates heavy rain.
        - Low reflectivity indicates light drizzle.

Satellites

  • Satellites detect cloud cover and moisture in the atmosphere.

Clouds and Precipitation

  • Clouds are visible water droplets or ice crystals that form when air becomes supersaturated with water vapor, causing water to condense on microscopic particles known as aerosols.

Cloud Formation

  1. Warm air rises over cooler, denser air (Front)

  2. Air is forced upward over a mountain range

  3. Ground warming causes air to rise

Warm and Cold Clouds

  • Warm Clouds: Form at temperatures above freezing.
        - Water droplets larger than 0.5 mm will fall, growing larger by colliding with other droplets until reaching 4 mm.

  • Cold Clouds: Form at temperatures below freezing and consist of ice crystals that grow through:
        1. Snowflakes: Water freezes to ice nuclei gradually.
        2. Rime: Instant freezing of supercooled droplets to ice nuclei.

Hail Formation

  • Microscale aerosols act as nucleation sites for crystallization.
        - Large snowflakes aggregate to form larger crystals.
        - Hail is a lump of ice formed from extreme riming on graupel particles, which resemble snowballs.

Latent Heat

  • Refers to the energy absorbed or released as water transitions between states (solid, liquid, gas):
        - Evaporation draws heat from the environment leading to cooling effects.
        - Condensation releases heat into the environment causing warming effects.

Motion

Wind Mechanisms

  • Pressure-Gradient Force: The greater the difference in pressure, the faster the air moves.
        - High pressure areas have closely spaced isobars indicating strong winds.
        - Low pressure areas have widely spaced isobars indicating weak winds.

  • Friction: Slows air movement due to molecular collision and interactions with landscapes, mountains, and buildings.

Coriolis Force

  • An apparent force arising from Earth’s rotation, which varies with latitude:
        - Equator: Rotation speed is 1,670 km/hr (1,038 mph).
        - Poles: Rotation speed is 0.

  • The Coriolis effect deflects moving air masses:
        - Northern Hemisphere: Curves to the right.
        - Southern Hemisphere: Curves to the left.

Rising and Sinking Air

  • Rising warm air creates low-pressure areas, while sinking cool air induces high-pressure zones.

Integrated Atmospheric Motion

  • The interplay of pressure gradient force, friction, and Coriolis force determines wind direction:
        - Wind spirals inward counterclockwise towards low-pressure systems.
        - Wind spirals outward clockwise from high-pressure systems.

Geostrophic Wind

  • Winds that travel parallel to isobars where friction is negligible, leading to balance among pressure-gradient and Coriolis forces.

High-Altitude Winds

  • The jet stream represents a type of geostrophic wind occurring at approximately 10 km (6 mi) altitude, reflecting variations in pressure and wind speed.

Circulation

Circulation Cells

  • Three main circulation cells help transfer heat from the equator to the poles:
        - Hadley Cell: Located in the tropics.
        - Ferrel Cell: Present in mid-latitudes.
        - Polar Cell: Found near the poles.

Characteristics of Hadley Cells

  • Warm air rises near the equator and spreads outward before sinking at approximately 25-30° latitude, resulting in high-pressure areas that control rainfall in low latitude regions.

Trade Winds

  • Surface wind patterns develop from the Hadley cell flows toward the equator:
        - Northern Hemisphere: Winds blow southwest.
        - Southern Hemisphere: Winds blow northwest.

Intertropical Convergence Zone (ITCZ)

  • A belt near the equator where trade winds from both hemispheres converge, shifting north during northern summer and south during southern summer.

Walker Circulation

  • Represents the east-west flow of air and water in the equatorial Pacific Ocean, influenced by temperature variances across the ocean surface:
        - High-pressure and cold water dominate the eastern Pacific.
        - Low-pressure and warm water characterize the western Pacific.

El Niño and La Niña

El Niño

  1. Occurs when the trade winds weaken or reverse, causing warm water to pool in the eastern Pacific, impacting global weather patterns, particularly during winter.

  2. Results in significant sea-surface temperature anomalies, influencing atmospheric conditions and precipitation patterns.

La Niña

  • Represents a strengthening of the Walker circulation, leading to contrary effects compared to El Niño, enhancing cold ocean surface conditions and altering weather patterns as well.

Monsoons

  • Regional climate patterns that arise from the seasonal winds flowing over both sea and land masses:
        - Summer Monsoon: Characterized by heavy rainfall in regions like the Indian plains.
        - Winter Monsoon: Represents a dry season affecting the same regions.

Air Masses

North American Air Masses

  • Mid and high-latitude weather phenomena are largely governed by air masses:
        - Defined as bodies of air exhibiting uniform temperature and humidity characteristics.

Fronts

  • Boundaries separating different air masses, where the majority of stormy weather occurs:
        - Cold Front: A cold air mass advances under a warm air mass, forcing the latter to rise rapidly, often resulting in thunderstorms.
        - Warm Front: A warm air mass pushes into a retreating cold air mass, causing gradual rises and leading to clouds and light rain.
        - Occluded Front: Occurs when an advancing cold front overtakes a warm front, lifting all warm air.
        - Stationary Front: A static boundary where cold air flows parallel to the front and warm air rises above it, resulting in clouds and light rain.

Jet Stream

  • Formed by large temperature differences that create strong pressure gradients, resulting in strong winds often referred to as the polar jet stream.

Ridges and Troughs

  • The jet stream moves along high (ridges) and low (troughs) pressure systems, influencing weather patterns significantly.

Houston Cold Analysis

  • The effect of a jet stream trough on areas like Houston can bring cold polar air, leading to drastic temperature changes and precipitation patterns in affected areas.

Mid-Latitude Cyclones

  • Coriolis force contributes to the counterclockwise flow around low-pressure systems, which are central to the formation of mid-latitude cyclones responsible for severe weather in temperate climates.