Humidity, Condensation, Clouds, and Fog or Water in the Atmosphere

Humidity, Condensation, Clouds, and Fog or Water in the Atmosphere

Where the Water Exists on Earth

! Water molecules move in bothdirections between liquid andgas.Only water molecules areshown in the air.OPEN TOP! More molecules move fromliquid to gas than from gas toliquid, and thus there is a netloss of liquid – evaporation.! Eventually liquid water will dryup, unless replaced by asource such as precipitation.! Some water molecules thatleave the liquid move awayfrom the surface because thecontainer is open.

What affects evaporation rate?! Wind speed- The stronger the wind, the more rapid theevaporation.- Why? - Stronger wind over surface blowsaway vapor molecules from nearsurface, so they will not re-enter enter liquidsurface.- Example - Clothes dry out faster in thewind, we feel colder in the wind withwet clothes

! Temperature of the water– The higher the water temperature, themore rapid the evaporation.- Why? - The higher temperature means thatwater molecules in the liquid are movingfaster, and will escape to form vapor.- Example - wet clothes dry faster in the sun

! The humidity of the air (humidity = amountof water vapor in the air)– The higher the humidity, the slower theevaporation.- Why? - The more water molecules in theair, the more that go back to the liquid.- Example - clothes dry faster and yourperspiration evaporates quickly in dryclimates

Now Saturate the Air! Put a lid on container so nowater-vapor molecules blowaway! Water molecules evaporate fromthe liquid until enoughaccumulate in the air so thatthey move back to the liquid atthe same rate.! At this point we say that the airis saturated with water vapor – ithas reached its capacity.

The Amount of Water Vapor theAir Can Hold Depends onTemperature! The amount of water vapor that may beevaporated into air is directly proportional tothe air temperature. That is, the higher thetemperature, the more water vapor the aircan hold.

For Example! Allow liquid water in a closed container toevaporate until the air is saturated.! Warm the air by 20 F, and evaporation will beginagain because the air is no longer saturated. Atthe new saturation, when evaporation stops, twicethe original amount of water will be in the air.! Warm the air another 20 F, and the water in the airwill double again when saturation is reached andevaporation stops.

So! For about every 20 F change intemperature, the capacity of the airfor water vapor changes by a factorof two

Condensation to Form Fog,Clouds, Dew – How it Happens inthe Atmosphere! Condensation occurs mainly when air is cooledand its capacity for water vapor is decreased.! In the atmosphere, water vapor condenses onparticles of solid material rather than in the openair.! These particles may be natural or man-made, made, andare called condensation nuclei.! Thus, the condensation that begins each clouddroplet or fog droplet takes place on a solidparticle.

Humidity –Any way of specifying theamount of water vapor in air

1. Vapor PressureConcept - Each gas in the atmosphere contributesits own partial pressure (part of the total). Say thetotal pressure is 1000 millibars, , and 1% of the airis water vapor.nitrogen (78%) – partial pressure = 780 mboxygen (21%) – partial pressure = 210 mbwater vapor (1%) – partial pressure = 10 mb(Saturation vapor pressure = vapor pressure if theair is saturated)

How saturationvapor pressuredepends ontemperature – ittakes more watervapor to saturatethe air at highertemperatures!!!!

2. Relative HumidityRelative humidity tells us how close the air is tobeing saturatedorRH =water vapor content x 100water vapor capacityRH = actual vapor pressure x 100saturation vapor pressure

Relative Humidity (cont.)! If air contains ½ the amount required forsaturation the RH = 50%! If air is saturated, RH= 100%! How does RH change? – two ways– Change the temperature (capacity for watervapor)– Change the amount of water vapor in the air

Change of RH that results from dailytemperature change

Relative humidity and human comfort! The body cools itself through- evaporation from the skin- evaporation from the respiratory track- radiation- conduction and convection! When the body is under heat stress (large sources of heatfrom metabolism or external sources), evaporation ofperspiration from the skin is most important coolingmechanism.! But, evaporation slows down when air is close to saturation(high RH).! Thus, we are uncomfortably hot when the RH is high.

3. DewpointThe temperature to which you have to cool theair to produce saturation

Fog! Air becomes saturated and droplets form oncondensation nuclei! There is no physical difference between afog and a cloud, except that we tend to thinkof a fog as being near the ground! There are different names for a fog,depending on how it forms (that is, whatcauses the saturation)

Fog types

Radiation fog! The surface of Earth cools at night becauseinfrared radiation is emitted.! The cool surface cools the air near it.! If the air temperature cools to the dewpointtemperature, a radiation fog forms.

Radiation fog (continued)! These fogs tend to form- in low areas (e.g., valleys) because the cool airnear the ground drains downhill.- over moist surfaces (e.g., river valleys) becausethe dew point is higher.- when the sky is clear and air is dry, because moreinfrared energy escapes to space rather thanbeing absorbed and radiated back to Earth- when the overall airmass is moist, and thus thedewpoint is high- when nights are long in the winter, allowing morecooling- when the winds near the surface are relatively light

Advection fog! When air moves horizontally (advects(advects) ) overa cold surface, the air near the surface cools! If the air cools to the dewpoint,condensation takes place and an advectionfog forms.

Advection fog (continued)! These fogs tend to form- when air moves over a cold water surface- when the air has a high relative humidityto begin with

Mixing fog! Say two volumes of air are unsaturated buthave a high relative humidity.! When they mix, the resulting mixture may besaturated.

Mixing Fog (examples)! Your breath on a cold day! Condensation trails from jets! Exhaust from automobiles! “Steam fog” over a warm water surface! Fog forming over snow that is melting on thehighway

Fog climatology – average number of days a yearwith heavy fog

Cloud Types









“Fair-weather” cumulus

Cumulus congestus


Lenticular or wave clouds

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