Clouds, Fog, and Atmospheric Stability

Clouds and Fog

• There are three pure forms of clouds: cirrus, stratus, and cumulus
• Nimbus clouds, which are low altitude, produce rain
• Most clouds are combinations of the pure forms
• Cloud height classifications include:
  • High clouds prefixed with "Cirr," formed from ice crystals and contrails at high altitudes
  • Middle clouds prefixed with "alto," forming large, undefined sheets
  • Low clouds, which do not have a prefix

Atmospheric Stability

• An air parcel warmer than its surroundings results in an unstable atmosphere
• An air parcel cooler than its surroundings results in a stable atmosphere
  • Cool air parcels are heavier and denser
• A rising parcel cooler than its surroundings when unsaturated, but warmer when saturated, leads to a conditionally unstable atmosphere
• The adiabatic process involves no heat exchange as an air parcel cools and expands or warms and compresses (Moist AR = 6°C/km; Dry AR = 10°C/km)
• The lifting condensation level, or LCL, is the altitude at which an air parcel reaches condensation
• An atmosphere is considered stable when:
  • An air parcel is colder than the surrounding environment
  • The ELR(Environmental Lapse Rate) is less than the MAR( Moist Adiabatic Rate)/DAR (Dry Adiabatic Rate), expressed as DAR > MAR > ELR

Cloud Development

• There are four ways for cloud development
  • Convection through thermal heating
  • Organic lifting over mountains or hills
  • Surface air convergence, where more air enters an area than leaves
  • Frontal uplift along weather fronts

Windward vs. Leeward

• The windward side of a mountain, closest to the ocean, receives the most rain, moisture, and greenery
• The leeward side is on the opposite side, behind the "rain shadow"
  • Leeward areas are dry, like the Great Basin in the Sierra Nevada

Precipitation

• Precipitation processes include:
  • Collision and coalescence
  • Ice crystal process
• Collision and Coalescence
  • This occurs in warmer places like the tropics and subtropics
  • It happens in warmer clouds, where the top is warmer than 5°F or -15°C
  • Large droplets fall and collect smaller ones, reaching the surface without evaporating
• Ice Crystal Process
  • Supercooled water is water in a liquid state below 0°C or 32°F
  • Ice crystals form and grow until too large to be held by the cloud
  • Hail forms when a large cumulonimbus cloud's updraft lifts ice crystals, freezes them, and then they fall until the updraft fails and they fall to earth

Air Pressure

• An isoline connects points of equal values; an isoline map contains multiple isolines
• Different types of isoline maps include:
  • Contour map for elevation and height
  • Isobar map for air pressure (always from high to low pressure areas)
  • Isotherm map for temperature
  • Isohyets map for rainfall
• Air pressure varies with:
  • Altitude, where higher elevations have less air pressure
  • Temperature, where colder air is denser and hot air rises
  • Humidity, where less humidity means less air pressure
  • Convergence and divergence, where converging air increases pressure and diverging air decreases it