MET1010 Exam 2 Ch 4 5 Chapter 4 Circulation of Water in the Atmosphere evaporation the process by which a liquid changes into a gas condensation the process by which water vapor becomes a liquid precipitation any form of water particles liquid or solid that falls from the atmosphere and reaches the ground hydrologic cycle a model that illustrates the movement and exchange of water among the earth atmosphere and oceans Evaporation Condensation and Saturation saturated air atmospheric condition whereby the level of water vapor is the max possible at the existing temp and pressure in saturation the number of molecules condensating and evaporating are equal water molecules evaporate condensate at the surface more molecules evaporating than condensating net evaporation warm water evaporates faster than cold water can only have condensation if air is saturated condensation occurs primarily when the air is cooled condensation nuclei tiny particles on which water molecules can attach themselves to less than 1 micrometer larger is better hygroscopic attracts water is better stick to molecules cold not warm and help condensate warm molecules bounce off Humidity humidity a general term that refers to the air s water vapor content not an actual measure Vapor Pressure actual vapor pressure the pressure exerted by the water vapor molecules in a given volume of air high actual vapor pressure large number of vapor molecules low actual vapor pressure small number of vapor molecules approximately 1 2 10 20mb of pressure in a parcel is from water vapor saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature put another way it is the pressure that the water vapor molecules would exert if the air were saturated with water at a given temperature increases with increasing temperature Relative Humidity relative humidity RH ratio of the amount of water vapor in the air compared to the amount required for saturation RH water vapor content water vapor capacity or RH actual vapor pressure saturation vapor pressure x100 percent air with 100 RH is saturated because it is filled to capacity with water vapor does not actually measure vapor content related to human comfort air with greater than 100 RH is supersaturated supersaturated a condition whereby the atmosphere contains more water vapor than is needed to produce saturation with respect to a flat surface of pure water or ice and the RH is greater than 100 Can change RH in two ways 1 By changing the air s water vapor content increased water vapor content increased RH closer to saturation decreased water vapor content decreased RH further from saturation 2 By changing the air s temperature increased temp decreased RH further from saturation decreased temp increased RH closer to saturation When air is cool morning RH is high assuming clear and calm weather water When air is warm afternoon RH is low assuming clear and calm weather water evaporates slowly evaporates quickly Warm air can hold more vapor Cold air can hold less vapor RH changes daily even if amount of vapor is constant Relative Humidity and Dew Point dew point temperature temperature to which air must be cooled for saturation to occur RH 100 constant even if temperature changes unlike RH warm values mean lots of vapor high water vapor high dew point cool values mean drier air low water vapor low dew point always less than or equal to air temperature Polar Desert Less water vapor More water vapor Higher RH Lower RH Lower dew point Higher dew point Relative Humidity and Human Discomfort wet bulb temperature the lowest temperature that can be obtained by evaporating water into the air measured with a sling psychrometer colder than air temp evaporation required 600cal g of energy colder than air temperature as wet bulb temperature approaches air temperature less cooling is able to occur heat stroke a physical condition induced by a person s overexposure to high air temperatures especially when accompanied by high humidity when temperature and RH are high wet bulb temperature becomes higher than skin temperature causing skin temperature to rise and heat stroke to become imminent heat index HI an index that combines air temperature and relative humidity to determine an apparent temperature apparent temperature how hot or cold the temperature feels combination of temperature and RH Measuring Humidity psychrometer an instrument used to measure the water vapor content of the air It consists of two thermometers dry bulb and wet bulb After whirling the instrument the dew point and RH can be obtained with the aid of tables hygrometer an instrument designed to measure the air s water vapor content RH The sensing part of the instrument can be hair a plate coated with carbon or an infrared sensor Dew and Frost on clear calm nights objects near the earth s surface cool rapidly by emitting infrared radiation and the air that comes in contact with these cold surfaces cool by conduction Eventually the air cools to the dew point This causes water vapor to condense upon them causing dew dew water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air frost a covering of ice produced by deposition on exposed surfaces when the air temperature falls below the frost point 1 When dew point is warmer than 32 F dew forms when surface is cooled below dew Dew or frost point temperature 2 When dew point is colder than 32 F frost forms through deposition vapor ice when surface is cooled below dew point temperature in dry weather air may freeze but no frost occurs haze fine dry or wet dust or salt particles dispersed through a portion of the atmosphere Individually these are not visible but cumulatively they will diminish visibility Wet haze particles are larger than dry more RH more haze fog a cloud at the earth s surface fog is formed two ways 1 By cooling air below its saturation point dew point 2 Evaporation and mixing water vapor is added to the air by evaporation and the moist air mixes with relatively dry air radiation fog fog produced over land when radiational cooling reduces the air temperature to or below its dew point in low lying areas because cold heavy air drains downhill aka valley fog factors that promote fog long nights fall and winter light breezes of less than five knots advection fog occurs when warm moist air moves over a cold surface and the air cools to below its dew point
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