Overview
This lecture explains the behavior and importance of water in the atmosphere, including phase changes, humidity concepts, the water cycle, cloud formation, cloud types, and precipitation.
Water Phases and Heat Transfer
- Water exists in all three phases (solid, liquid, gas) near Earth's surface.
- Ice has a highly ordered lattice structure held by strong intermolecular forces.
- Melting changes solid ice to liquid water, requiring latent heat of fusion (336 J or 80 cal/g).
- Vaporization (liquid to gas) requires latent heat of vaporization (2,260 J or 600 cal/g).
- Sublimation (solid directly to gas) uses 2,594 J/g (latent heat of fusion + vaporization).
- Reverse processes (condensation, freezing, deposition) release the same amount of latent heat.
Measuring Water Vapor in the Atmosphere
- Humidity: amount of water vapor in air at a given time.
- Dewpoint temperature: temperature air becomes saturated and condensation begins.
- Specific humidity: ratio of water vapor weight to total air plus water vapor weight.
- Relative humidity: ratio of actual water vapor to maximum possible at that temperature.
- Warmer air holds more water vapor, so relative humidity decreases with rising temperature even if water content stays the same.
The Hydrologic Cycle
- Water is continuously cycled between atmosphere, land, ocean, and plants.
- Greatest water transfer: ocean evaporation and ocean precipitation.
- Precipitation: any water type falling from atmosphere to surface.
- Transpiration: water vapor released from plant leaves.
- Evapotranspiration: combined evaporation and transpiration from land and plants.
- Runoff: water not evaporated moves via streams to oceans.
- Infiltration: water soaks into ground, becoming groundwater.
- The water cycle is a closed system; Earth neither gains nor loses water.
Cloud Formation and Types
- Clouds form when air is saturated (usually by cooling to the dewpoint) and condensation nuclei are present.
- Condensation nuclei include clays, sea salt, and pollution particles.
- Three major cloud types:
- Stratus: low, flat, layered; can be called nimbostratus if producing precipitation.
- Cumulus: puffy, thick, often tall; cumulonimbus refers to very tall, storm-producing variants.
- Cirrus: high-altitude, thin, wispy; composed of ice crystals.
Interpreting Cloud Names
- "Strato" = layered/flat; "cumulus" = puffy/fluffy; "cirro" = high; "alto" = middle altitude.
- Examples: altostratus (mid-altitude layered), stratocumulus (low puffy), cirrocumulus (high puffy).
- Cumulonimbus are tall, multilevel, produce extreme weather like thunderstorms.
Precipitation and Associated Hazards
- Not all clouds produce precipitation; droplets/ice must grow large enough to fall through unsaturated air.
- Precipitation types: rain, freezing rain, sleet, snow, each formed by temperature variations during descent.
- Hazards: flooding (rain), icy roads/structures (freezing rain, sleet), blizzards (snow, cold).
Key Terms & Definitions
- Latent Heat of Fusion — heat required to melt a solid.
- Latent Heat of Vaporization — heat required to vaporize a liquid.
- Humidity — amount of water vapor in the air.
- Dewpoint Temperature — temperature at which air is saturated.
- Specific Humidity — ratio of water vapor weight to air+vapor weight.
- Relative Humidity — percent of maximum water vapor air can hold at a temperature.
- Condensation Nuclei — small particles for water vapor to condense onto.
- Precipitation — water in any form falling from atmosphere to surface.
Action Items / Next Steps
- Review cloud types and associated prefixes/suffixes.
- Prepare for detailed discussion of precipitation and weather hazards in future lessons.