Chapter 4: Water in the Atmosphere

• Chapter 4 covers the topic of "Water in the Atmosphere".
• This includes the hydrologic cycle, atmospheric humidity and instability, cloud types, precipitation, and cloud science.

Hydrologic Cycle and Water

• Evaporation involves solar energy converting liquid water into atmospheric water vapor.
• Condensation is the processes where water vapor forms liquid cloud droplets in the atmosphere.
• Precipitation occurs when cloud droplets combine becoming heavy enough for gravity to precipitate rain or snow toward earth.
• Runoff: If precipitation falls as snow, it returns to the ocean as stream runoff after melting.
• Lithosphere: Surface runoff reaches the ocean in days to months, ground infiltration takes hundreds to thousands of years.
• Hydrosphere: Lakes delay water movement. Some water evaporates back into the atmosphere instead of flowing to the ocean.
• Biosphere: Plants temporarily divert liquid water, returning water vapor through transpiration.
• The Hydrologic Cycle involves water movement within the atmosphere, biosphere, lithosphere, and hydrosphere.
• The hydrologic cycle is solar powered.
• 85% of atmospheric water vapor originates from ocean evaporation.
• 15% of atmospheric water vapor comes from plant transpiration.

Phase Changes of Water

• Evaporation is the change of water from liquid to gas.
• Condensation is the change of water from gas to liquid.
• Sublimation is the direct change of ice to water vapor.
• Deposition is the direct formation of ice from water vapor.
• Precipitation is solid or liquid water falling from the atmosphere to the ground, due to condensation/deposition.

Properties of Water

• Hydrogen Bond: Attraction between one water molecule's positive end and another's negative end.
• When water changes state (solid, liquid, gas), hydrogen bonds either form or break.
• Energy must be added or removed for water to change state.
• Evaporation is a cooling process because it absorbs heat from the environment.
• Condensation is a warming process because it releases heat to the environment.
• Clouds form from net condensation in the atmosphere.
• Net condensation occurs when air reaches saturation, at which point the water vapor content is equal to the water vapor holding capacity.
• Latent Heat: Energy absorbed or released during a substance's state change, such as evaporation or condensation.
• The solid state of water is less dense than the liquid state.
• Water has a higher specific heat than most materials.
• Water is known as the universial solvent.
• Cohesion attracts water to itself as a result of electrical polarity.

Atmospheric Humidity

• Humidity is the water vapor content of the air, sourced from evapotranspiration (evaporation + transpiration).
• Vapor Pressure: The part of air pressure exerted exclusively by water vapor molecules.
• Saturation Vapor Pressure: The vapor pressure at which saturation occurs.
• Millibar (mb): A measure of atmospheric pressure.
• Sea level pressure = 1013 mb
• Thermal expansion causes matter to expand when heated and contract when cooled.
• Specific Humidity: Water vapor content of the atmosphere expressed in weight, unit: grams of water per kg of air (g/kg)
• Relative Humidity (RH): Ratio of water vapor content to water vapor capacity, expressed as a percent (%).
• Saturated air = relative humidity of 100%.

Heat-Index Temperature

• This is the temperature perceived by people based on atmospheric humidity and air temperature.
• High atmospheric humidity slows evaporation, which can cause bodies to overheat.
• Heat waves are a #1 meteorological killer in the United States.