Lecture Notes Flashcards

Lifting Mechanisms

  • Review of Previous Concepts:
    • Convection: Warm air passively rising.
    • Orographic Lifting: Air forced up and over topography (e.g., mountains).

Orographic Lifting

  • Washington State as a Laboratory:
    • Ideal location to observe orographic lifting.
    • Air cools as it rises on the windward side, warms as it descends.
    • Rain shadow effect on the leeward side (dry side) of mountains.
  • Rainfall Totals Example:
    • Significant contrast in rainfall between the windward and leeward sides of the Olympic Mountains.
    • Sequim, WA, receives only 16 inches of rain per year due to the rain shadow effect.
  • Global Application of Orographic Lifting:
    • Orographic lifting is driven by wind direction and intersecting topography.
    • Example: Oahu, Hawaii
      • Prevailing winds: Northeast trade winds due to its location at 15 degrees north.
      • Highest precipitation on the northeast side of the island where winds are lifted.
      • Dry conditions on the leeward (west) side.
      • Can be used to predict wet and dry conditions and vegetation communities.
  • Factors Affecting Cloud Formation:
    • Moisture in the air (specific humidity) and temperature.
    • Air may not always reach the freezing point, even when cooled. Clouds might pass over mountains without rain if the air isn't saturated.

Frontal Lifting

  • Definition:
    • Occurs when two different air masses meet with different temperatures.
    • Warm air is forced aloft as cold air pushes underneath or when warm air encounters stationary cold air.
  • Temperature Contrast:
    • Driven by temperature differences between air masses.
    • Involves cold fronts and warm fronts.
  • Mid-Latitude Cyclones:
    • Storm systems formed by different types of fronts. More to be discussed in future lectures.

Convergent Lifting

  • Local and Global Relevance:
    • Relevant on a local scale (e.g., the Olympics) and globally (e.g., ITCZ).
    • ITCZ (Intertropical Convergence Zone): Amplified by convergence and convection.
  • ITCZ Mechanism:
    • Northeast and Southeast trade winds converge, causing warm air to rise.
    • Combination of convection and convergent lifting.
  • Puget Sound Convergence Zone:
    • Specific to the Northwest; an area of convergent lifting east of the Olympics.
    • Winds moving onshore split around the Olympics due to low elevation gaps.
    • Air converges on the leeward side, causing lifting and precipitation.
    • Typically located near Everett, north of Seattle.
  • Wind Patterns and Convergence Zones:
    • Orientation of the convergence zone depends on wind direction (westerly or southwesterly).
    • Rain shadow effect and convergence both occur simultaneously.
      • Air is sinking in the rain shadow, causing dry conditions.
      • Convergence behind it causes precipitation.

Cloud Types

  • Basic Concepts:
    • Classifying clouds is complex with varying ways to describe them.
    • Focus on common types and their implications for weather.

Cloud Classification Criteria:

  • Vertical Development:
    • Vertically developed clouds: Tall, reaching up to 20,000 feet.
    • Minimally developed clouds: Thin, flat, sheet-like.
  • Cloud Altitude
  • Latin-Based Naming:
    • Cloud names derived from Latin words.
  • Key Terms:
    • Stratus: Flat, sheet-like clouds.
    • Cumulus: Puffy clouds with vertical development.
    • Cirrus: Thin, wispy clouds.
    • Nimbus/Nimbo: Rain-producing clouds.

Specific Types of Clouds

  • Stratus and Nimbostratus Clouds:
    • Flat, sheet-like clouds, often rain-producing (nimbostratus).
    • Common in the Pacific Northwest due to wet, stable air masses.
    • Associated with light, widespread, gentle showers.
  • Cumulus Clouds:
    • Show vertical development.
    • Indicator of added moisture in the atmosphere, cooled to the dew point.
    • Classic warm, sunny, summer afternoon sky, indicating convection.
    • Pyrocumulus clouds: Formed by rising warm air from fires.
  • Cumulonimbus Clouds:
    • Very vertically developed, rain-producing clouds.
    • Associated with thunderstorms, heavy precipitation, hail.
    • Involve warm air rising and cool air falling, creating large droplets and potential for hail.
  • Cirrus Clouds:
    • Thin, high clouds made of ice crystals.
    • Often indicate a change in weather, preceding lower, thicker clouds.
    • Contrails from airplanes are a type of cirrus cloud, formed from exhaust.
  • Lenticular Clouds:
    • Mountain wave clouds formed as air flows over mountains.
    • Air is displaced upward, cools to the dew point, forming ice crystal clouds.
    • Appear to hover due to constant feeding of fresh air.
    • Indicator of upper-level moisture and fast upper-level winds.

Frontal Lifting - Air Masses and Fronts

Air Masses

  • Temperature and Moisture Characteristics:
    • Air masses take on characteristics of the surface they flow over.
  • Source Region:
    • Informs the characteristics of an air mass.
  • Examples:
    • Continental Polar (cP): Dry and cold.
    • Maritime Polar (mP): Cool and humid.
  • Mid-Latitudes:
    • Battleground between warm and cold air masses.
  • Pacific Northwest (PNW):
    • Primarily influenced by maritime polar air.
  • Midwestern US:
    • Intersection of multiple air masses.

Specific Humidity and Air Mass Characteristics

  • Measurement:
    • Grams of water vapor per kilogram of air.
  • Seasonal Patterns:
    • Winter: Tropical air masses smaller, continental air masses larger.
    • Summer: Continental air masses retreat.
  • Continental Arctic Air:
    • Very dry due to cold temperatures and small container size.
  • Continental Polar Air:
    • Also quite dry.
    • Can cause low indoor relative humidity when warmed.
  • Continental Tropical Air:
    • Hot and dry, emerges in the summer.
    • Can hold more water vapor than continental or maritime polar air due to higher temperature, but is still relatively dry.
  • Maritime Tropical Air:
    • Super warm, wet, and humid.

Fronts and Air Mass Boundaries

  • Definition:
    • Boundary between two different air masses.
    • Air on either side has different temperature and moisture characteristics.
  • Identification:
    • Fronts are identified on weather maps using surface weather station data.
  • Naming Convention:
    • Fronts are named for the temperature of the air behind them.
  • Polar Front:
    • A cold front where cold air sags south.
  • Jet Stream:
    • Formed on top of the Polar Front.
    • Divides cold air to the north and warm air to the south.
    • Bends and irregularities (Rossby waves) can lead to mixing of air masses.
  • Mid-Latitude Cyclones:
    • Combinations of cold air sinking south, warm air coming up, and being pulled together around a low-pressure center.

Frontal Lifting Mechanisms - Stationary, Cold, Warm Fronts

Stationary Fronts

  • Definition:
    • Boundary where warm and cold air masses are adjacent but not moving, doesn't want to mix.
    • The disturbance along the boundary is a low.
  • Symbol:
    • Alternating warm and cold front symbols.

Cold Fronts

  • Definition:
    • Warm air in place at the surface is invaded by colder air.
    • Where air masses meet, frontal lifting occurs.
  • Mechanism:
    • Cold air bulldozes warm air rapidly upward.
  • Associated Weather:
    • Heavy precipitation, thunderstorms, vertically developed clouds. (cumulonimbus)
  • Progression:
    • Drop in temperature as cold air floods in.
    • Series of cloud and sky occurrences.
  • Analogy:
    • Pushing snow with a shovel.