Cloud Formation

Review LCL & Dew Point The Sun’s radiation heats Earth’s surface, the surrounding air is heated due to conduction and rises because of convection. Unstable air rises and contains water vapor Through adiabatic cooling, the temperature and pressure decrease with higher altitudes. This causes the air parcel to expand & cool as it rises. Remember, as temperature drops, the air cannot hold the same amount of water vapor causing it to condense At a certain altitude in the troposphere the rising air condenses. This is called the ‘lifting condensation level’. This occurs because the water vapor has reached its Dew Point. The Dew Point is when the temperature of water vapor condenses into water droplets and forms a cloud This is why a cloud appears to be flat at the base

Why Clouds Form Relative Humidity has gone over 100% (supersaturated) Cloud Condensation Nuclei (CCN) Solid particles suspended in the atmosphere where water vapor condenses around to form a cloud Particles come from various sources Car exhausts Volcanic activity Forest fires Sea salt (ocean spray) Soil Dust Sand Fossil fuel combustion. They attract water droplets above the LCL Nearly all cloud droplets or ice particles form around a CCN

Warm Clouds Collision and Coalescence Mechanism A warm cloud form when temperature is above 0⁰C Cloud droplets are carried through the air in air currents within the cloud itself These droplets bump into each other – This is called a collision As droplets collide, they stick together – This is called Coalescence When this occurs, the cloud droplet get bigger When the cloud gets heavy enough, rain will occur

Cold Clouds Bergeron Process- Supercooling Water in the liquid state that is below 0⁰C Supercooled water will freeze if it touches a solid object (such as Condensation Nuclei) Supersaturation When relative humidity is over 100% If liquid water is 100%, then ice in supersaturated (over 100%) Ice Crystals cannot coexist with water droplets Excess water vapor in the air become ice crystals, which lowers the relative humidity near water droplets This causes surrounding water droplets to evaporate into water vapor creating and in turn creating more ice crystals This is how snowflakes are made in clouds

Types of Clouds Clouds are classified by their form and height Clouds form at different Altitudes High Clouds (~6000m) Middle Clouds (~4000m) Low Clouds (~2000m) Forms Cirrus- “curl of hair”; High clouds, white, thin Cumulus - “a pie”; rounded individual cloud masses ; flat base, dome structured Stratus- “a layer”; look like sheets or layers that cover the sky; no distinct breaks in-between clouds

Development of a Thunderstorm Thunderstorms form when warm, humid air rises in an unstable environment. 3 stages Cumulus Stage Mature Stage Dissipating Stage Updraft- Rising air Downdraft- sinking air

Stage 1- Cumulus Stage Warm, moist air rises, and the water vapor within the air condenses to form a cumulus cloud Cloud grows above the freezing point in the atmosphere creating both warm and cold cloud

Stage 2- Mature Stage Condensation continues as the cloud rises and becomes a dark cumulonimbus cloud (thunderstorm cloud) Amount of water held in cloud become too much for the updraft (upward movement of air) to support Heavy, torrential rain and hail storms begin to fall from the cloud

Stage 3- Dissipating Stage Strong downdrafts (downward movement of air) stop air currents from rising Cooling effect from falling precipitation and colder air high above cause the storm to die down Life span of a cumulonimbus cloud within a thunderstorm is only about an hour As storm moves, new warm moist air generate new clouds which causes storms to last longer than an hour