Chapter 4 Water in the Atmosphere Dew Dew point Dew point depression Drizzle Evaporation Evaporation fog Fallstreaks Fog Freezing nucleation Freezing rain Frontal fogs Frontal lifting Frost Frost point Frozen dew Graupel Hail Heterogeneous nucleation hydrophobic nuclei Hygroscopic nuclei Ice crystals Ice nuclei Immersion nucleation Lifting condensation level LCL Mammatus Mixing ratio Needle Nimbostratus Nucleation Orographic lifting Precipitation Radiation fog Rain Rain shadow Relative humidity Rime Saturated adiabatic lapse rate Saturation Saturation vapor pressure Shower Sleet Snow Snowflake Solute effect Steam fog Stratocumulus Stratus Supersaturation Updraft Upslope fog Vapor pressure Virga Warm clouds Key Terms Accretion Advection fog Aggregation Altocumulus Altostratus Anvil Bergeron Wegener Process Cirrocumulus Cirrostratus Cirrus Cloud droplet Cold clouds Collision coalescence Column Condensation nuclei Conditionally unstable environment Contact nucleation Convection Convective clouds Convergence Crystal habit Cumulonimbus Cumulus Curvature effect Dendrites Deposition nuclei Summary Water in the atmosphere exists as water vapor clouds and precipitation We can measure water vapor in a variety ways Mixing ratio vapor pressure relative humidit and dew point temperature are the most common tools for measuring water vapor concentrations The vapor pressure of saturated 100 relative humidity air increases rapidly as the air is warmed Also changing the amount of water vapor in or the air temperature changes the relative humidity In clear calm precipitation free conditions the relative humidity is usually highest at sunrise and lowest during the mid afternoon Cloud is a suspension of water droplets ice crystals or both Updrafts in the cloud keep particles aloft The formation of a cloud requires water vapor saturated air and nuclei onto which vapor can condense or deposit These nuclei can be water droplets or aerosol particles Clouds over land have more and smaller cloud droplets than clouds over the ocean Fog is a cloud that is at ground level Fogs develop through two processes that lead to saturation Air can cool to the dew point and become saturated producing radiation fog advection fog or unslope fog Air can also saturate via evaporation of water into the air This produces steam fog or evaporation fog Most other clouds are formed when air cools to saturation as it is lifted The four primary mechanisms for lifting air are orographic lifting frontal lifting convection and convergence near the surface Moisture affects how a parcel s temperature changes as it rises The saturated adiabatic lapse rate of 6 degrees Celsius per kilometer is less than the dry adiabatic lapse rate because of latent heating resulting from condensation This means that saturated air parcels cool less quickly then dry air parcels as they rise As a result an environmental with a lapse rate between the dry and saturated lapse rates can be unstable but only for saturated air parcels This is called a conditionally stable environment and promotes the growth of clouds and thunderstorms Clouds can be classified as layered strato or convective cumulo and also as low middle alto or precipitating nimbo The 10 basic cloud types are cirrus cirrostratus cirrocumulus altostratus altocumulus cumulus stratus stratocumulus nimbostratus and cumulonimbus the thunderstorm cloud Clouds play a major role in the greenhouse effect They warm the surface but also reflect sunlight cooling the surface In today s climate clouds tend to cool the planet Precipitation processes differ in warm and cold clouds In warm water only clouds precipitation sized particles grow by collision and coalescence of large and small water droplets In cold clouds with ice crystals accretion aggregation and the Bergeron Wegenor process cause precipitation In the latter process ice crystals attract water vapor more strongly than do liquid water drops in a temperature range commonly found in midlatitude clouds The most common forms of precipitation are rain snow freezing rain and sleet Freezing rain and sleet form when there is a temperature inversion near the surface The location of mountains and valets affect the type and amount of clouds and precipitation In particular regions downwind of a mountain range are drier and sunnier than the upwind slopes
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