GEOG 1114 1st Edition Lecture 12 Outline of Last Lecture Continuation of Pressure and Atmospheric motion Atmospheric Moisture Transfer of Water to the Atmosphere Measuring moisture in the Atmosphere Outline of Current Lecture Dew Point and sensible heat Condensation Adiabatic Processes Precipitation Atmospheric lifting Current Lecture Dew Point The temperature at which saturation occurs 100 humidity and condensation begins The temperature at which saturation occurs 100 humidity and condensation begins Varies with the moisture content of air High Dew point indicate high moisture content in the air Sensible Temperature Temperature sensed by a person s body Involves actual air temp relative humidity and wind These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Warm humid days seem warmer than actual thermometer reading when sensible temperature is high Cool humid days coldness felt more Sensible temp is low Condensation Process by which water vapor in the atmosphere changes phase to become a liquid or a solid This happens when moist air is cooled to saturation dew point by the adiabatic process Two situations are needed 1 Water vapor content of at least one layer of the atmosphere must closely approach saturation 2 Condensation nuclei must be present The nuclei act as collections centers for water molecules and the growth of water droplets attach themselves to dust molecules in the atmosphere and the water adheres to it forming clouds Adiabatic Processes The process by which large air masses can be cooled to the dew point The major mechanism for the development of clouds and the production of rain As air rise it expands and cools adiabatically Unsaturated air cools at the dry adiabatic lapse rate If the air mass rises up enough it cools to the dew point condensation begins and clouds form Altitude at which this occurs is known as the lifting condensation level LCL As Condensation begins latent heat is released If air continues to rise cooling continues a lower rate due to release of latent heat at the saturated adiabatic lapse rate Adiabatic warming occurs when air descends As air warms its capacity to hold more moisture increases which causes saturated air to become unsaturated Thus descending air warms at the dry adiabatic lapse rate The Results Dew and frost Dew Usually originates from terrestrial radiation earth s reflected energy Moisture condensation on surfaces that have been cooled to saturate Frost Occurs when the saturation point is below 0 degrees C 32 F Fog Formed when Air at earth s surface cools below its dew point When enough water vapor is added to their air to saturate Clouds Visible expression of condensation they contain collections of water droplets and or ice crystals Three General Types Cirrus Thin and wispy Stratus Continuous Cumulus Billowy with vertical component White or gray clouds White equal reflection within the air mass Gray unequal reflection The term nimbo or nimbus refers to clouds that are producing precipitation Nimbostratus low level cloud blankets that result in steady rains Cumulonimbus high level clouds that can elevations of 50 000 feet and frequently fan out at the top in the shape of an anvil Result Thunderstorms and downpours Buoyancy of air Air moves up and down to adjust to the temperature gradient Conditional instability the release of latent heat during condensation warms rising air to make it unstable Visual determination of instability Precipitation Originates from clouds Condensation alone insufficient to form raindrops Other processes important Needs Evaporation from the ocean and evaporation from the plants The Process Rainfall is a rare event because it requires three different things to for rain PPTN precipitation Water Droplet Formation Uplif Droplet Formation Condensation which produces tiny drops is not enough to produce precipitation Two mechanisms are thought to be responsible for droplet formation Ice Crystal Formation Raindrops form as ice crystals found in clouds with temp Below freezing grow by attracting water vapor to them They grow until they are heavy enough to fall Known as the Bergeron Process Collision Coalescence Raindrops form as small droplets collide and begin to merge and form larger droplets If large enough they begin to fall and as they do they continue to collide and merge causing larger drops to form that will eventually fall in the form of rain Merging only occurs if a positively charged droplet collides with a negatively charged one Forms of Precipitation Rain condensation PPTN in ascending air that has temp above freezing Snow Solid PPTN in the form of ice crystals pellets or flakes Sleet In US small raindrops that freeze during descent and reach ground as iceelsewhere mixture of rain and snow Glaze rain that turns to ice when it hits the surface Black ice Hail Forms in cumulonimbus clouds As a result of strong updrafts In continual cooling air water droplets form and process is repeated several times Hail pellets develop and drop when reach downdraft Four Types of Atmospheric Lifing 1 Convective Unequal heating of surface areas by conduction causes a parcel of air near the ground to be warmed more that the surrounding air Density of the heated air drops as air expands and it begins to rise convectively Uplift is more likely to happen if unstable air is involved Pressure of the unstable airdrops as it rises and it cools adiabatically to the dew point Condensation and clouds develop If temperature humidity and stability conditions are just right the clouds will grow into a cumulonimbus thunderhead and a downpour of showery rains or hail and lightning and thunder will result 2 Orographic Air is forced to rise over a mountain barrier and it is cooled to the dew point Windward side side in which air is forced to rise Typically West Leeward side Side in which air descends Little PPTN Rain shadow 3 Frontal Lifing When a cool and warm air mass meet a boundary or front is established between them The warm air mass is forced to rise above the adjacent cold front If the warm air is cooled to saturation precipitation can occur along the front 4 Convergent When air converges there is a general uplift If lots of crowding occurs get more lift and instability and precipitation can occur Less common than others happens in ITCZ often associated with hurricane formation