Lifting Mechanisms and Cloud Types

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.