CHAPTER 3 Focus Points
3.1 The Hydrologic Cycle and Water
Water in the atmosphere: About 85% of water vapor in the atmosphere originates from evaporation from the oceans. The remaining 15% comes from plant transpiration.
State changes of water: Water changes its state (solid, liquid, gas) when energy is added to or removed from water molecules. When water changes its state, hydrogen bonds linking water molecules are either formed or broken, depending on the state shift.
Evaporation and condensation: Evaporation is a cooling process because it absorbs heat from the environment. Condensation is a warming process because it releases latent heat to the environment.
Net condensation: Clouds form in environments of net condensation.
3.2 Atmospheric Humidity
Relative humidity: Changes in relative humidity happen mainly because of air temperature changes. As air cools, relative humidity rises. Saturated air has a relative humidity of 100%.
Humidity indicators: High vapor pressure, specific humidity, and dew point all indicate a high atmospheric water vapor content. Relative humidity does not reliably indicate the water vapor content of the atmosphere.
3.3 Lifting Air: Atmospheric Stability
Atmospheric stability: Lifting and cooling of air is the most common way an air parcel becomes saturated.
Lifting condensation level: Above the lifting condensation level, condensation releases latent heat and slows the rate of adiabatic cooling to the moist adiabatic rate.
Adiabatic heating and mountains: The leeward side of a mountain range is often warmer and drier than the windward side due to the release of latent heat through condensation.
Four ways to lift air: The four ways in which air is lifted are convective uplift, orographic uplift, frontal uplift, and convergent uplift.
3.4 Cloud Types
Cloud composition: Clouds are composed of suspended microscopic liquid cloud droplets and ice crystals.
Cloud forms: All clouds take one of three forms: cirriform, stratiform, or cumuliform.
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Clouds that bring precipitation: Nimbostratus and cumulonimbus clouds bring precipitation. Cumulonimbus clouds can produce severe weather.
Fog: Fog forms where surface air has been cooled to the dew point. Most fog is either radiation fog or advection fog.
3.5 Precipitation: What Goes Up …
Condensation nuclei: Cloud droplets form around condensation nuclei (such as dust, pollen, or particulate pollution) suspended in the atmosphere.
Rain and snow: Rain is formed through collision and coalescence of cloud droplets in warm clouds. Snow is formed through the ice-
Types of precipitation: The temperature profile of the atmosphere determines which form of precipitation occurs. Falling snow may become rain, sleet, or freezing rain as it falls from a cloud to the ground. Hail forms in cumulonimbus clouds.
3.6 Geographic Perspectives: Clouds and Climate Change
Clouds and temperature: Clouds affect atmospheric temperature through cloud forcings, and they are affected by atmospheric temperature in turn.
Cloud feedbacks: Clouds modify Earth’s climate through numerous positive and negative feedbacks.