Weather and Humidity Concepts

Overview:

• Upcoming lectures will cover humidity and fog, followed by lifting mechanisms.
• Lifting mechanisms will be the main focus for the remainder of the week and relevant for lab five.
• The final exam is cumulative but will emphasize material covered after the previous exam.

Humidity Concepts:

• An air mass can be described by its capacity to hold water vapor based on its temperature.
• Example: If air is warm, it can hold more moisture, likening the concept to buckets of different sizes for water.
• Specific Humidity: The actual amount of water vapor present in the air.
• Relative Humidity: A comparison of specific humidity to the air's capacity to hold that moisture, expressed as a percentage.
• Graphical Representation:
  • Temperature and relative humidity are inversely related; warmer air has lower relative humidity if specific humidity is constant.
  • Warm, afternoon air has lower humidity; it peaks at cooler temperatures, like overnight.

Daily Temperature and Humidity Patterns:

• Midday/Afternoon: Warmest temperatures lead to lowest relative humidity (20% capacity example).
• Overnight: Cooling air allows for potential saturation, leading to fog or clouds.

Essential Definitions:

• Saturated Air: Air at capacity with a relative humidity of 100%.
• Air can reach saturation by:
• Adding more water vapor (increasing specific humidity) or
• Lowering the temperature.
• Fog is common in the morning due to colder temperatures leading to saturation.

Apparent Temperature Concepts:

• The perceived temperature can differ from actual due to humidity (example with heat index).
• Body cooling through sweating is hindered by high humidity; warm, moist air decreases evaporation efficiency.
• Latent Heat Transfer: Energy stored or released during evaporation and condensation, crucial in weather dynamics.

Key Types of Fog:

• Radiation Fog: Forms by cooling at night, when air temperature near the ground drops below the dew point.
• Advection Fog: Forms when warm air moves over a colder surface, cooling and reaching saturation (common along coastlines).
• Evaporation Fog: Results from adding water vapor to the air (e.g., steam above a hot tub).

Lifting Mechanisms:

• Convective Lifting: Air rises due to heating from the surface, commonly leads to thunderstorms in heated environments.
• Orographic Lifting: Air is forced up and over mountains, leading to cloud formation and precipitation on the windward side and creating rain shadows on the leeward side due to descending, warming, dry air.

Impacts of Orographic Lifting:

• Rain Shadow Effect: Precipitation is significantly higher on windward mountains than in the leeward regions (e.g., Bellingham vs. Spokane).
• Annual precipitation analysis shows dramatic contrasts in rainfall across small geographic areas driven by local topography.

Key Exam Concepts:

• Understand mechanisms for how fog forms and the conditions under which each type occurs.
• Recognize relationships between temperature, specific humidity, relative humidity, and how they affect weather patterns.
• Familiarize with the implications of latent heat in weather phenomena and its influence on temperature and precipitation distribution.