🌦️Atmospheric Science Unit 5 – Atmospheric Moisture and Clouds

Key Concepts and Definitions

• Humidity measures the amount of water vapor present in the atmosphere
• Saturation occurs when the air cannot hold any more water vapor at a given temperature
• Dew point temperature when air becomes saturated and condensation begins
• Relative humidity ratio of actual water vapor content to the maximum possible at a given temperature
• Condensation process of water vapor changing to liquid water, often forming clouds or fog
• Evaporation conversion of liquid water into water vapor, driven by heat energy
  • Occurs from water surfaces, soil moisture, and plant transpiration
• Sublimation direct transition of water from solid (ice) to gas (water vapor) without passing through the liquid phase
• Latent heat energy released or absorbed during phase changes of water

The Water Cycle and Atmospheric Moisture

• Water cycle continuous movement of water through the Earth system, driven by solar energy
  • Includes evaporation, transpiration, condensation, precipitation, and runoff
• Evaporation adds moisture to the atmosphere from oceans, lakes, rivers, and soil
  • Rate depends on temperature, wind speed, and humidity
• Transpiration release of water vapor from plants through stomata in leaves
• Atmospheric circulation transports water vapor from source regions to other areas
• Condensation forms clouds and precipitation, releasing latent heat
• Precipitation (rain, snow, hail) removes moisture from the atmosphere
• Runoff and groundwater flow return water to oceans and lakes, completing the cycle
• Atmospheric moisture content varies with temperature, pressure, and air mass characteristics

Types of Humidity and Measurement

• Absolute humidity mass of water vapor per unit volume of air (g/m³)
• Specific humidity ratio of water vapor mass to total air mass (g/kg)
• Mixing ratio mass of water vapor per unit mass of dry air (g/kg)
• Relative humidity percentage of actual water vapor pressure compared to saturation vapor pressure at a given temperature
  • Varies with temperature changes even if absolute moisture content remains constant
• Dew point temperature at which air becomes saturated and condensation begins
  • Directly related to absolute moisture content
• Wet-bulb temperature lowest temperature achieved by evaporative cooling, depends on humidity
• Psychrometer instrument that measures wet-bulb and dry-bulb temperatures to determine humidity
• Hygrometer device that directly measures humidity using changes in electrical resistance or capacitance of a moisture-sensitive material

Cloud Formation Processes

• Cooling of air to its dew point temperature leads to condensation and cloud formation
• Adiabatic cooling occurs when air rises and expands due to lower pressure
  • Common in convection, orographic lift, and frontal wedging
• Conductive cooling from contact with cold surfaces (e.g., ice, snow, cold water)
• Radiative cooling loss of heat to space during clear nights
• Mixing of air masses with different temperatures and humidity levels
• Condensation nuclei tiny particles (dust, salt, smoke) that provide surfaces for water vapor to condense upon
  • Hygroscopic nuclei (salt) attract water vapor and enhance condensation
• Updrafts lift air parcels, causing cooling and increasing relative humidity
• Cloud droplets form when relative humidity exceeds 100% and condensation occurs on nuclei
• Ice crystals form at temperatures below freezing through deposition or freezing of cloud droplets

Cloud Classification and Characteristics

• Clouds classified by altitude, shape, and composition
• High clouds (above 6 km) include cirrus, cirrostratus, and cirrocumulus
  • Composed of ice crystals due to cold temperatures
• Middle clouds (2-6 km) include altostratus and altocumulus
  • May consist of water droplets, ice crystals, or a mixture
• Low clouds (below 2 km) include stratus, stratocumulus, and nimbostratus
  • Primarily composed of water droplets
• Vertical development clouds (cumulus, cumulonimbus) span multiple altitude ranges
  • Associated with strong updrafts, convection, and precipitation
• Cloud cover reported in oktas (eighths) or percentages
• Cloud opacity varies from transparent (cirrus) to opaque (nimbostratus)
• Cloud thickness and density affect solar radiation and Earth's energy balance

Precipitation Types and Mechanisms

• Precipitation forms when cloud droplets or ice crystals grow large enough to fall due to gravity
• Coalescence merging of cloud droplets through collision, forming larger drops
  • Efficient warm rain process in tropical and maritime clouds
• Bergeron process ice crystal growth at the expense of water droplets due to lower saturation vapor pressure over ice
  • Dominant precipitation mechanism in mixed-phase clouds
• Riming accumulation of supercooled water droplets on ice crystals, forming graupel or hail
• Rain liquid precipitation with drop diameters greater than 0.5 mm
• Drizzle small, uniform liquid drops with diameters less than 0.5 mm
• Snow solid precipitation composed of ice crystals or aggregates
  • Forms in cold clouds or when falling through subfreezing air
• Sleet ice pellets that form when falling snow partially melts and refreezes
• Hail large, layered ice particles that form in strong updrafts of thunderstorms
• Precipitation intensity measured in millimeters per hour or inches per hour

Atmospheric Stability and Moisture

• Stability refers to the atmosphere's resistance to vertical motion
• Stable conditions suppress vertical mixing and cloud development
  • Characterized by strong inversions and poor air quality
• Unstable conditions promote rising motion, convection, and cloud growth
  • Associated with cumulus clouds, thunderstorms, and precipitation
• Conditional instability potential for instability if air becomes saturated
• Moisture content affects stability by modifying air density and lapse rates
• Moist adiabatic lapse rate (MALR) cooling rate of saturated rising air, typically ~5°C/km
• Dry adiabatic lapse rate (DALR) cooling rate of unsaturated rising air, ~9.8°C/km
• Environmental lapse rate (ELR) actual temperature change with height in the atmosphere
• Comparing ELR to MALR and DALR helps determine atmospheric stability
• Latent heat release during condensation warms the air and increases instability

Weather Patterns and Forecasting

• Atmospheric moisture plays a crucial role in weather systems and patterns
• High humidity and instability favor thunderstorm and heavy precipitation development
• Low humidity and stable conditions promote clear skies and fair weather
• Moisture convergence zones (ITCZ, monsoon troughs) are regions of enhanced cloudiness and rainfall
• Orographic precipitation occurs when moist air is forced up mountain slopes
• Frontal systems (cold, warm, occluded) often bring cloudiness and precipitation
• Tropical cyclones (hurricanes, typhoons) form over warm, moist oceans and transport heat and moisture poleward
• Numerical weather prediction models simulate moisture transport and phase changes
• Moisture data from radiosondes, satellites, and surface stations improve forecast accuracy
• Ensemble forecasting accounts for uncertainty in moisture and stability conditions
• Long-term moisture patterns influence drought and flood risk assessment