5a_AtmosphericWater

Water, Weather, and Climate Systems

• Atmospheric Water

  • Water and Atmospheric Moisture

  • Water on Earth

  • Unique Properties of Water

  • Humidity

  • Atmospheric Stability

  • Clouds and Fog

Three Phases of Water

• Water Vapor (Gas Phase)

  • Molecules move independently

  • Compressible gas

• Liquid Water

  • Reaches greatest density at 4°C (39°F)

  • Density decreases below this temperature

• Ice (Solid Phase)

  • Less dense than liquid water (ice floats)

• Phase Changes

  • Heat energy must be added or released:

    • Sublimation: Direct change from water vapor to ice or ice to water vapor

Water's Heat Energy Characteristics

• Heat Energy Transfer

  • Significant for understanding atmospheric conditions

Humidity

• Measurements

  • Specific Humidity: in g/kg

  • Relative Humidity (RH): percentage (%)

  • Dew Point: °C or °F

  • Wet Bulb/Dry Bulb Temperatures: °C or °F

  • Vapor Pressure: in mb

• Specific Humidity

  • Definition: Mass of water vapor per mass of air at a given temperature

    • Formula: Grams of water vapor/Kg of air

• Maximum Specific Humidity

  • Increases with temperature: warmer air holds more vapor

• Relative Humidity

  • Definition: RH = (Mass of Water in Air / Mass of Water Air Can Hold) x 100%

  • Trends: Decreases with higher temperatures; increases with lower temperatures

Dew Point

• Dew Point Temperature

  • At which water vapor begins to condense

• Measurement Tools

  • Wet Bulb Temperature ( Twb): indicated by a moistened thermometer bulb exposed to airflow

  • Dry-Bulb Temperature ( Tdb): measured by a normal thermometer in open air

Vapor Pressure

• Definition

  • Pressure exerted by water vapor in the atmosphere

• Saturation

  • As more vapor enters, pressure increases until saturation is reached

  • Higher temperature leads to greater saturation vapor pressure

Atmospheric Stability

• Density Differences

  • Warmer air: lower density

  • Cold air: higher density

• Buoyant Forces

  • Upward buoyancy force vs. downward gravitational force

  • Less dense air parcels rise, denser air descends

Adiabatic Processes

• Definition

  • Rates of warming and cooling for expanding or compressing air

• Rates

  • Dry Adiabatic Rate (DAR): Rate at which dry air cools (10°C/1000 m)

  • Moist Adiabatic Rate (MAR): Rate for moist air (6°C/1000 m)

  • MAR < DAR due to latent heat of condensation reducing cooling

Atmospheric Conditions and Stability

• Normal Lapse Rate

  • Average temperature drop with increasing altitude

• Environmental Lapse Rate

  • Actual rate at a specific place and time

  • Can vary from normal rate

• Stable Conditions

  • Environmental lapse rate < DAR and MAR

  • Air parcels remain cooler and settle back

• Unstable Conditions

  • Rising air parcel warmer than surrounding air, continues rising

Clouds

• Formation

  • Aggregation of moisture droplets and ice crystals

  • Occur when rising air cools to dew point, causing condensation

• Types of Clouds

  • Stratiform: Flat and layered

  • Cumuliform: Puffy and globular

  • Cirroform: Wispy

Cloud Altitudes**

• Low Clouds: Surface up to 2000 m

• Middle Clouds: 2000 – 6000 m

• High Clouds: Above 6000 m

• Vertically Developed Clouds: Cumulonimbus formations often indicate storms

Types of Clouds by Composition

• Cirrus Clouds: Thin, wispy, ice crystal composition

• Stratus Clouds: Low to middle altitude, dull appearance, potential for precipitation

• Cumulus Clouds: Low-level, fluffy, fair weather indications

Fog Forms**

• Advection Fog: Forms with air migration that causes saturation

• Evaporation Fog: Water evaporating into cold air

• Upslope Fog: Moist air forced upward, cools and condenses

• Valley Fog: Cool air settles in low areas

• Radiation Fog: Surface cooling chills air above ground to dew point