Water + Atmospheric Moisture

• Water vapor is a powerful greenhouse gas
• Water can exist in 3 phases (solid, liquid, and gas) at the same temperatures
• The absorption and release of latent heat energy during phase changes are important for climate systems
• Water (H2O)


  A chemical compound of hydrogen and oxygen
• Hydrogen bond


  Positive and negative charges attract, causing water molecules to stick together
• Three States of Water

  • Solid (ice)
  • Liquid (water)
  • Gas (water vapor)
• Latent heat energy


  Energy released or absorbed as water changes from one state to another
• Sensible heat


  Heat that can be felt and measured when the molecules pass to lower energy states
• Humidity


  The amount of water vapor present in the air (dependent on the air temperature)
• Measures of humidity


  • Absolute humidity
  • Relative humidity
  • Specific humidity
  • Dewpoint
• Absolute humidity


  A measure of the actual amount of water vapor (moisture) in the air, regardless of the air's temperature
• Specific humidity


  The ratio of the mass of water vapor to the mass of the air
• Relative humidity


  The actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature
• Relative humidity depends on vapor content and temperature, so temperature variations will change relative humidity even when the vapor pressure of the atmosphere is constant
• Dew-point


  The temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity of 100%
• Atmospheric stability


  The tendency of the atmosphere to resist upward motion and instead stay in place (or sink)
• Stable atmosphere


  • Lifting is resisted, even if lifted, air will not continue to rise but will sink. No clouds, clear conditions.
• Unstable atmosphere


  • Lifting is promoted, air rises, cools, condensates and forms clouds, and potentially, rainfall.
• If you push something in a stable situation, it will return to its original location
• Atmospheric Stability


  Tendency of the atmosphere to resist upward motion and instead stay in place (or sink)
• Parcel


  A body of air with homogeneous temperature & humidity characteristics
• Stable parcel resists upward movement, and/or sinks back to its starting point
• Unstable parcel rises & continues to rise freely, on its own
• Adiabatic process


  Warming or cooling rate inside an air parcel, occurring from internal processes only, without a loss or gain of energy from the surrounding environment
• Dry Adiabatic Lapse Rate

  • 10°C per 1000 m (5.5°F per 1000 ft)
  • For rising air parcel not yet cooled to saturation
• Moist Adiabatic Lapse Rate

  • Ranges between 4 and 9°C per 1000 m (2.2–4.9°F per 1000 ft)
  • Depends on temperature, pressure, & moisture content
  • Usually use 5°C per 1000 m
• Moist Adiabatic Lapse Rate is less than Dry Adiabatic Lapse Rate because condensation releases latent heat energy
• Determining atmospheric stability


  1. Compare environmental lapse rate with dry/moist adiabatic lapse rate

  2. If parcel temperature < environmental temperature, parcel is stable

  3. If parcel temperature > environmental temperature, parcel is unstable
• Unstable atmosphere processes


  • Daytime solar heating of surface air
  • Advection of cold air over warm surface
• Stable atmosphere processes


  • Nighttime surface cooling
  • Advection of warm air over cold surface
  • Subsidence inversions
• Hydrologic Cycle 


  global flow of water to and from oceans, land, and atmosphere.
• Ocean covers 70% of Earth.
• Sublimation = ice to water vapor
• Deposition = Water vapor to ice.
• Latent Heat


  Amount of heat energy released/absorbed when water changes from one state to another.
• Sensible Heat


  The heat that can be felt/measured when molecules pass to lower energy states.
• Adiabatic Cooling


  rising air parcel --> decreasing pressure --> expands --> less molecular collisions --> cooling.
• Adiabatic Warming


  Opposite to cooling. (Descending air parcel).
• Environmental Lapse Rate


  Typically 6.5C per 1000m
• Cloud consists of water droplets, ice particles, or a mixture of both