Chapter 12 - Moisture, Clouds, and Precipitation

Water Vapor and Atmospheric Processes

• Water vapor is an odorless, colorless gas that can change states of matter at Earth's temperatures and pressures.
• It is the only substance that exists as a solid, liquid, and gas in the atmosphere.
• Composed of 2 hydrogen atoms bonded to an oxygen atom.

States of Matter of Water

• Solid (ice):
  • Water molecules are held together by molecular attractions, forming a tight, orderly arrangement.
  • Molecules vibrate about fixed sites and are not free to move about.
  • Melting occurs when molecular movements increase, breaking bonds between water molecules.
• Liquid (water):
  • Water molecules are tightly packed but move fast enough to slide past one another.
  • As molecular movement increases, remaining molecular attractions are broken, allowing molecules to escape and become water vapor.
• Gas (water vapor):
  • Water molecules are widely spaced and exhibit energetic, random motion.
  • Water vapor is compressible and distinguished from liquid water in this way.

Energy and State Changes

• Changing states of matter requires heat (energy) to be absorbed or released.
• Measured in calories.
• Latent heat:
  • Latent heat of melting: 80 calories to change 1 gram of ice at 0°C to liquid.
  • Latent heat of fusion (freezing): 80 calories to change 1 gram of liquid water at 0°C to solid.
  • Latent heat of vaporization: approximately 600 calories to change 1 gram of water at 100°C to gas.
  • Latent heat of condensation: approximately 600 calories to change 1 gram of water vapor at 100°C to liquid.

Additional State Changes

• Sublimation: conversion of a solid directly into a gas without passing through the liquid state.
• Deposition: conversion of a gas directly into a solid without passing through the liquid state.

Water Vapor in the Atmosphere

• Water vapor comprises a small amount of gases within the atmosphere (1/10 of 1% to 4%).
• Most important gas in understanding atmospheric processes.
• Humidity is the general term for the amount of water vapor in air.

Measures of Water Vapor Content

• Mixing ratio: mass of water vapor in a unit of air compared to the remaining mass of dry air.
• Relative humidity: ratio of air's actual water vapor content compared with the amount required for saturation at that temperature.
• Dew-point temperature: temperature to which a parcel of air would need to be cooled to reach saturation.

Condensation and Cloud Formation

• Condensation occurs when air is cooled to its dew point temperature.
• Requires a surface on which water vapor can condense (e.g., particulate matter, condensation nuclei).
• Clouds can consist of water droplets, ice crystals, or both.

Cloud Formation Mechanisms

• Adiabatic temperature change: change in temperature that occurs without heat being added or removed.
• Orographic lifting: air is forced to rise over a mountainous region, causing adiabatic cooling and cloud formation.
• Frontal wedging: warmer, less dense air is forced over cooler, denser air, causing it to rise and cool.
• Convergence: pileup of horizontal airflow results in upward movement.
• Localized convective lifting: unequal surface heating causes localized pockets of air to rise due to buoyancy.

Air Stability

• Stable air: when a parcel of air is cooler than the surrounding air, it will sink.
• Unstable air: when a parcel of air is warmer than the surrounding air, it will rise.
• Environmental lapse rate: rate of temperature change with altitude.
• Adiabatic temperature change: changes in temperature caused by expansion or compression.

Types of Air Stability

• Absolute stability: develops when the environmental lapse rate is less than the wet adiabatic rate.
• Absolute instability: develops when the environmental lapse rate is greater than the dry adiabatic rate.
• Conditional instability: develops when moist air has an environmental lapse rate between the dry and wet adiabatic rates.### Clouds
• Clouds are visible aggregates of minute droplets of water or tiny ice crystals
• Classified by form and height:
  • Cirrus: high, white, and thin; can occur as patches, veil-like sheets, or wispy fibers
  • Cumulus: globular individual cloud masses; normally have a flat base and the appearance of rising domes or towers
  • Stratus: sheets or layers that cover much or all of the sky; typically have no distinct individual cloud units
• Three levels of cloud height:
  • High clouds: have bases above 20,000 feet
  • Middle clouds: occupy heights from 6,500 to 20,000 feet
  • Low clouds: form below 6,500 feet

Cloud Classification

• High clouds:
  • Cirrus
  • Cirrostratus
  • Cirrocumulus
• Middle clouds:
  • Altocumulus
  • Altostratus
• Low clouds:
  • Stratus
  • Stratocumulus
  • Nimbostratus
• Clouds of vertical development:
  • Cumulus clouds can grow dramatically in height under correct conditions, forming cumulonimbus

Fog

• Fog is a cloud with its base at or very near the ground
• Typically reduces visibility
• Officially reported when visibility is reduced to 1 km (0.6 miles) or less
• Types of fog:
  • Fogs Caused by Cooling:
    • Advection fog: occurs when warm, moist air moves over a cool surface
    • Radiation fog: forms on cool, clear nights when the Earth's surface cools rapidly by radiation
    • Upslope fog: occurs when relatively humid air moves up a sloping terrain
  • Evaporation Fogs:
    • Steam fog: occurs when cool air moves over warm water, producing saturation
    • Frontal (or precipitation) fog: occurs when frontal wedging occurs, producing precipitation that evaporates to produce fog

Precipitation

• Cloud droplets are very small (20 micrometers), requiring a large size to reach the Earth's surface
• Two processes result in large volume droplets:
  • Bergeron process:
    • Relies on supercooling and supersaturation
    • Dominant in mid-latitudes where cold clouds exist
  • Collision-coalescence process:
    • Occurs in warm clouds where large droplets form
    • Requires giant condensation nuclei or hygroscopic particles
• Types of precipitation:
  • Rain: drops of water with a diameter of at least 0.5 millimeters
  • Drizzle: fine uniform drops of water with a diameter of less than 0.5 millimeters
  • Snow: precipitation in the form of ice crystals or aggregates of crystals
  • Sleet: small particles of ice that are clear to translucent
  • Glaze: raindrops that fall and freeze, resulting in a thick coating of ice
  • Hail: hard, rounded, pellets or irregular lumps of ice
  • Rime: deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects

Measuring Precipitation

• Rain is measured using a rain gauge, which collects and conducts rain into a cylindrical measuring tube
• Snow has two measurements: depth and water equivalent
  • General ratio: 10 parts snow equals 1 part water
  • Ratio varies based on physical properties of snow
• Weather radar is used to estimate rainfall and rate of rainfall
  • Radio waves are transmitted and reflected by larger raindrops, ice crystals, or hailstones
  • Reflected signal (echo) is displayed

Learn about the physical properties of water vapor, its states of matter, and how temperature affects molecular motion. Understand the importance of water vapor in atmospheric processes.