Test 2 Review I Atmospheric Humidity a air parcels and adiabatic processes adiabatic temperature changes condensation occurs when air is cooled to dew point temp may produce dew fog or clouds cloud formation often takes place during the warmest part of the day therefore something else must be acting to cool air sufficiently when you apply energy to compress the air the motion of the gas molecules increase and thus the temperature increases adiabatic temperature changes heat was neither added nor subtracted when air is compressed it warms and when air is allowed to expand it cools adiabatic cooling and condensation parcel imaginary volume of air considered to be a few hundred cubic meters in volume and acts independently of the surrounding air as a parcel moves upwards it passes through successively lower pressures of air ascending air expand and cools adiabatically unsaturated air cools at a constant rate of 10 C 1000m descending air conversely heats 10 C 1000m applies only to unsaturated air dry adiabatic rate if parcels rises high enough it will cool to its dew point and trigger condensation lifting condensation level altitude at which a parcel reaches saturation and here latent heat that was absorbed by the water vapor when it evaporated is cloud formation begins released as sensible heat release of latent heat slows the rate of cooling wet adiabatic rate varies from 5 C to 9 C because dependent on moisture present b measures of atmospheric humidity humidity general term used to describe the amount of water vapor in the air 1 Absolute Humidity mass of water vapor in a given volume of air mass of water vapor g volume of air cubic meter changes in pressure and temperature caused changes in it volume when volume changes absolute humidity changes even if no vapor added 2 Mixing Ratio mass of water vapor in a unit of air compared to remaining mass of air mass of water vapor g mass of dry air kg not affected by changes in pressure or temp bc measured by mass 3 Vapor Pressure increase in pressure that is the result of the motion of the water vapor molecules the part of the total atmospheric pressure attributable to its wv content initially many more molecules will leave the water surface evaporate than return condense as more molecules evaporate from the water surface the increasing vapor in the air above forces more condensation eventually a balanced is reached in which the evaporation equals the condensation molecules returning molecules leaving increase in saturation vapor pressure pressure exerted by the motion of the water vapor 4 Relative Humidity ratio of airs actual water vapor content compared with the amount of water vapor required for saturation at that temperature and pressure how near the air is to saturation rather than the actual quantity of wv in the air How RH changes RH changes when wv is added or removed from the atmosphere bc amount of moisture required for saturation is a function of air temp RH varies with temperature when wv is added to a parcel of air through evaporation the RH of the air increases until saturation occurs 100 RH if exceeds 100 the excess water vapor condenses to form liquid water think of shower and condensation on the mirror air temperature affects RH when amount of WV remains contant a in temp RH air cooled below temp that saturation occurs wv condenses to form water droplets clouds when wv content of air remains constant a decrease in air temp causes an increase in RH and conversely an temp causes a decrease in RH in nature there are three major was that air temps change to cause corresponding changes in RH 1 Daily changes in temperature night and day 2 air changes that result as air moves horizontally wind 3 temp changes caused as air moves vertically in the atmos c Dew Point Temperature temp to which air needs to be cooled to reach saturation unlike RH which is a measure of how near the air is to being saturated dew point temp is a measure of the actual moisture content of a parcel of air saturation vapor pressure is temp dependent and that for every 10 C increase in temperature the amount of water vapor needed for saturation doubles bc dew point is the temp at which saturation occurs we can conclude that high dew points temps equate to moist air and low indicate dry air for every 10 C increae in dew point temp the air contains about twice as much wv if dew point exceeds 65 F 18 C most people consider air to feel humid Dew point depression Temperature Dew point temp when T and TD are far apart there is large DPD and RH is low close together small DPD Relative Humidity is high Temp tells you how much wv can hold DPT tells how much water is actually in air II Condensation Dew Frost Fog and clouds atmospheric moisture is important because it is needed for formation of clouds b condensation and condensation nuclei when a parcel of air ascends it cools due to expansion from lower pressure lifting condensation level cooled to dew point temp and condensation occurs two conditions must be met air must be saturated and must be surface for air during formation of dew objects at or near the ground serve as a surface for water to condense on vapor condensation however when condensation occurs in the atmosphere tiny particles known as condensation nuclei serve as surface for condensation without these nuclei RH of well above 100 would be needed for cloud forming hygroscopic nuclei water seeking most effective sites for condensation when exposed to humid air they absorb moisture water droplet form on salt above ocean hydrophobic nuclei water repelling not efficient condensation nuclei but droplets will form on them when RH reaches 100 cloud with its base at or very near the ground results from cooling when the air becomes saturated through the addition of wv Fogs formed by cooling when the temperature of a layer of air in contact with the ground falls below its dew point condensation produces fog radiation fog results from radiation cooling of the ground and air nighttime phenom requiring clear skies and high RH under clear skies the ground and air immediately above cools rapidly and because of high RH a small amount of will lower the temp to the dew point bc air containing the fog is relatively cold and dense it flows downslope in hilly terrain therefore thickest is dissipates within one to three hours after sunrise and lifts sun warms ground so closest to ground warms and dissipates first advection fog warm moist air blows over a cold surface