Some unusual properties of water
• Boiling points and freezing points of polarized hydrogen compounds typically decrease as molecule size decreases. H2O has an incredibly high boiling point and freezing point for its molecular weight.
• Cohesion is an intermolecular force between like molecules. It is why liquids can hold themselves together as a drop. H2O is very cohesive because of the
molecule’s polarity.
Strong cohesion is why H2O can easilyform and maintain drops.
Some unusual properties of water
• Substances typically become less dense when heated and more dense when cooled. H2O is rare in that its solid state floats on its liquid state.When H2O cools below 4°C, it becomes LESS dense.
Link
Some unusual properties of water
• H2O has a very high latent heat of vaporization, 540 cal/gm, for its molecular weight. Its value
rivals that of metals.
H2O vapor flows from the tropics to the poles, where it later condenses, warming the
surrounding air. Phase changes of water play a major role transferring heat poleward
earthguide.ucsd.edu
Some unusual properties of water
• H2O is unusual in that it can exist as liquid, solid or gas at the temperatures and pressures that characterize Earth.This property of H2O greatly facilitates the formation of precipitation and hence production of fresh water on Earth.
Hydrological Cycle
85% of water vapor in atmosphere evaporates from oceans
Ahrens Fig. 4.1
85%15%
50%
50%
Precipitation is the ultimate source of fresh water
Precipitation that falls over land 50% is runoff to the ocean50% evaporated/transpired back to atmosphereWater vapor molecules reside in atmosphere for 7-10 days
1 week
Hydrological Cycle
Atmospheric Sciences deals with the aerological branch of the water cycle.– sources/sinks of atmospheric water– transport of water in the atmosphere– phase changes of water in atmosphere– precipitation
Measures of Water VaporTotal Pressure and Partial Pressure
• “In a mixture of gases (such as air), each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature.[1] ” From en.wiki.org
• “The total pressure of a mixture of gases is the sum of the partial pressures of each individual gas in the mixture.” From en.wiki.org
Dalton’s Law of Partial Pressures
Humid air is a mixture of dry air (N2, O2, trace gases) and lighter H2OPressure Moist Air = Partial Pressures of N2+O2+H2O+(Other Trace Gases)
partial pressureof water vapor
LinkLink
209 mb + 781 mb + 9.7 mb + 12.8 mb + 0.5 mb = 1013.0 mb
Partial Pressure of Water Vapor
• The partial pressure of H2O vapor is termed
Vapor Pressure
for short
en.wikipedia.org
Saturation Vapor Pressure
• Partial pressure of water vapor has an upper‐limit above which it cannot exceed.
• The limiting value is termed Saturation Saturation Vapor Pressure
• Depends strongly on temperaturehigher temp higher SVP
en.wikipedia.orgen.wikipedia.org
saturationvapor pressure
en.wikipedia.org
What is Saturation?
Saturation is a state of equilibrium where Evaporation Rate = Condensation Rate (It is not Evaporation = Condensation = 0)
dry air
SVP
chemwiki.ucdavis.edu
SVP of Ice and Liquid• Matter can exist simultaneously in all of its phases
• But SVP over ice is less than SVP over liquid. Why?
en.wikipedia.org
gasice
chemwiki.ucdavis.edu
gasliquid
en.wikipedia.org
Bonds between molecules are stronger in ice than liquid.
Thus, at the same temperature fewer molecules can escape from ice than liquid.
Hence, SVP over ice is lower than the SVP over liquid.
SVP ice SVP liquid
SVP and Temperature
SVP almost doubles for every 10oC warming
Supercooled water droplets can exist down to temps as cold as -40oC
For temps below 0oC, SVP runs 10%-20% lower over ice than liquid
Ahrens 6th ed
12
23
42
Summary
Vapor pressure is a measure of actual water vapor content
Saturation vapor pressure (SVP) is a measure of potential water vapor capacity strong temperature dependencehigher over liquid than ice at the same temp
Measures of Atmospheric Moisture Relative Humidity (RH)
RH changes by either changing the water vapor content or temperature
Air with a RH=100% is said to be saturatedat which point condensation occurs
RH= Water Vapor ContentWater Vapor Capicity
RH(%)= Vapor PressureSaturation Vapor Pressure 100
Relative Humidity Temperature Sensitivity
RH is very sensitive to air temperature. Can vary radially even if VP remains constant
Because of its sensitivity to temperature change, meteorologists prefer other measures for water vapor
Ahrens
Vapor Pressure Constant
Measures of Atmospheric Moisture Relative Humidity (RH)
Consider air that starts saturated at 0oCTemp VP SVP RH0oC 6 mb 6 mb 100%
10oC 6 mb 12 mb 50%20oC 6 mb 22 mb 26%30oC 6 mb 40 mb 14%40oC 6 mb 70 mb 8%
(See Appendix B of Ahrens)
Measures of Atmospheric Moisture Dew Point (Td)
• Dew Point (Td)Temperature to which air must be cooled at constant pressure to become saturated.
Higher Td => higher water vapor content
Excellent indicator of the water vapor contentDoes not vary as temperature changesRather insensitive to modest pressure changes
2°C less per 100 mb decrease, or2°C less per km increase in altitude
Reason Td is plotted on surface weather maps
Measures of Atmospheric Moisture Dew Point Ranges for Arizona
Vapor Pressure Dew Point24 mb 20oC12 mb 10oC6 mb 0oC3 mb ‐10oC
Rule of Thumb: Vapor pressure almost doubles for every Td increase of 10°C
Water Vapor Applet
Wet Bulb Temp - Lowest temp to which air can be cooled by evaporation of water into it.
Higher than Dew Point - H2O vapor is added to unsaturated air which raises its dew point.
Measured with sling psychrometer
Measures of Atmospheric Moisture Wet Bulb Temperature (Tw)
dry thermometerwet bulb
Ahrens, Fig 4.9
Measures of Atmospheric Moisture Wet Bulb Temperature (Tw)
Wet bulb temperature lies about 30-40% of the difference from Td to T for AZ surface conditions.
Wet bulb temperature gives the maximum possible cooling of a swamp cooler…or thunderstorm downdraft.Month Temperature Wet Bulb Dew PointJune 100oF 65oF 37oF July 100oF 75oF 63oF
Heat Index
Humidity reduces the rate at which sweat evaporates. Thus, the cooling rate is lowered.
Record MAX
Rocky PtJuly MAX June MAX Ahrens 6th ed.
Summary: Moisture
• Water vapor mostly comes from the evaporation of sea water 85%
• H2O molecules reside in atmosphere for 7-10 days
• Humid air is any air that contains H2O molecules.
Less dense than dry air since the molecular weight of H2O is less than N2 and O2.
Summary: Moisture
• Air has a saturation level for water vapor, beyond which it cannot get bigger
• Saturation level depends on air temperature
• Actual water vapor content of air can be quantified by several measures
Vapor Pressure, Relative HumidityDew Point, Wet-Bulb
Summary: Moisture Measures
• Saturation Vapor Pressure (SVP) ‐ Highest possible partial pressure for water vapor
• Vapor Pressure ‐ Actual partial pressure of water vapor
• Relative Humidity – Ratio of
actual content (VP)
saturation value (SVP)
Summary: Moisture Measures
• Dew Point (DP) - temperature to which air must be cooled at constant pressure to become saturated. Plotted on WX maps.
• Wet Bulb Temperature - lowest temperature to which air can be cooled by evaporation of water into the air. Higher than DP since water vapor is added to air.
Temp ≥ Wet Bulb Temp ≥ Dew PointOnly Equal When R.H.=100%
ATMO 170A1Lecture MWF
Condensation: Fog and Cloud Formation
Foggy Sunrise over Vancouver, B.C. January 18, 2009 Blair Kent http://jaksview3.files.wordpress.com/2009/01/cypressview.jpg
Condensation
Condensation is the change of phase from vapor to liquid.Warming of surrounding air is realized.Latent heat of vaporization for H2O is large. 540 cal/gm
Cloud Condensation Nuclei
Small, airborne particles provide a surface on which water vapor can condense to produce cloud droplets.
Without such particles, RH>100% would be needed to produce clouds.
Such surfaces are called Cloud Condensation Nuclei (CCN)
CCN are small, light and stay suspended for days.
Cloud Condensation Nuclei
• SourcesDust, volcanic ash, smoke, soot, salt, sulfate particles
• Concentrations1,000-10,000 per cc Near the surface Over continents Over megacities
Ahrens, Meteorology Today, 5th Ed.
Cloud Condensation Nuclei
Water-SeekingSaltSulfuric acidNitric acid
Water-RepellingOilsGasolineWaxes
Condensation can occur on hygroscopic CCN for RH<100% Accounts for Haze when the RH approaches 60-80%
Ahrens, Meteorology Today, 5th Ed.
Morning Haze over Lake Annecey, France
http://www.fotothing.com/photos/64b/64b04ad23a6ee44bc58efeea1b79dfa9.jpg
Condensation
When air barely becomes supersaturated from either cooling or the addition of water vapor, water condenses onto CCN to form small cloud droplets
Diameters of cloud droplets are 2-20 microns (10-100 times smaller than human hair)
Concentrations are 50-1,000 droplets per cc
Summary: Condensation
CondensationCan occur by cooling or moistening of airCloud Condensation Nuclei (CCN)
Permit condensation at RH’s < 100%Small airborne particles (<0.2 to 1 microns) Can
stay suspended for weeksResponsible for haze at RH < 100%
What is the correct relationship between Temperature, Dew Point Temperature and Wet
Bulb Temperature?
a) Dew Point ≥ Wet Bulb Temp ≥ Tempb) Temp ≥ Wet Bulb Temp ≥ Dew Pointc) Temp ≥ Dew Point ≥ Wet Bulb Tempd) Wet Bulb Temp ≥ Dew Point ≥ Temp
(Only one selection allowed)
Summary: Moisture Measures
• Saturation Vapor Pressure (SVP) ‐ Highest possible partial pressure for water vapor
• Vapor Pressure ‐ Actual partial pressure of water vapor
• Relative Humidity – Ratio of
actual content (VP)
saturation value (SVP)
Summary: Moisture Measures
• Dew Point (DP) - temperature to which air must be cooled at constant pressure to become saturated. Plotted on WX maps.
• Wet Bulb Temperature - lowest temperature to which air can be cooled by evaporation of water into the air. Higher than DP since water vapor is added to air.
Temp ≥ Wet Bulb Temp ≥ Dew PointOnly Equal When R.H.=100%
Is the dew point higher or lower than the glass temperature?
1. Yes2. No3. Insufficient information. Must
know air temperature too.
http://www.123rf.com/
The graph shows temperature and dew point for past 24 hours. If we did the experiment for conditions at 7:30 am today would water still condense on the glass?
1) Yes2) No3) Insufficient Info
http://www.fotothing.com/photos/64b/64b04ad23a6ee44bc58efeea1b79dfa9.jpg
Haze at Sunrise near Ashville NC
link
What is Fog?
Fog ‐ a cloud at the groundThere are four basic types of fogRadiation FogAdvection FogUpslope FogSteam Fog
Combinations exist (Radiation‐Advection)
Radiation/Ground Fog Weather
Clear nights: IR loss to form inversion
Weak winds: no mixing of warm
air from aloft
Low‐level moisture: little cooling needed
to hit dew point.
Radiation Fog over Iowa
http://www.extremeinstability.com/
Fog type that forms over the lower deserts
Valley Fog Conditions
• Valley fog is a deep (1,000 ft thick), long-lived radiation fog that occurs under persistent high pressure in cool season
• Sun is too weak to evaporate fog completely during day
• Fog persists until a winter storm with strong winds blows outthe cold air that is trapped in the valley
• Occurs in Central Valley of California, Valleys of Appalachian Mts
Radiation/Valley Fog
Jameison Valley, New South Wales, Australia http://www.blackheathweather.com/generalphotos/Jameisonvalley03PSa.jpg
Advection Fog Weather
• Warm, humid air flows over colder ground.
• Air cools to its dew point and fog forms.
• Occurs over Central US in winter as warm moist air from Gulf of Mexico flows inland, especially over snow covered ground.
• Occurs along West Coast during spring/summer when warm air from mid-Pacific flows over cold waters of California Current along coast.
https://www.ec.gc.ca/meteo‐weather/
Advection Fog
http://www.meted.ucar.edu/mesoprim/dynfog/media/graphics/ca_dynfog.jpg
Cold Water
Warm Water
Wind
Upslope Fog
• Occurs along windward slopes of mountain ranges• Air cools to its dew point as it rises under easterly flow (Air
always cools as it rises as we will soon understand.)• Fog drifts up the slope. • Widespread upslope fog common over Upper Great Plains
https://www.ec.gc.ca/meteo‐weather/
Upslope Fog Satellite
http://apollo.lsc.vsc.edu/classes/met130/notes/chapter5/graphics/upslope_sat.gif
Wind
Upslope Fog
http://lh4.ggpht.com/-oiee7XikZTQ/RpgLE5VscdI/AAAAAAAAATo/Kq59iGzZeII/s720/IMG_0347.JPG
Chasm Trail Longs Peak, CO looking eastward toward the
Upper Plains
Why You See Your Breath on a Cold Winter Night
Condensation can occur if moist warm air and cold dry air mix.Temp RH SVP VP
Air 0oC 20% 6 mb 1 mb(clear)
Breath 36oC80% 63 mb 55 mb(clear)
50‐50 18oC140% 20 mb 28 mb(cloud)
90‐10 4oC 90% 8 mb 6 mb(clear)
50‐50 mix condenses into a cloud that quickly evaporates as your exhalation is diluted further
Ahrens older ed.
Breath
Air
50-50
90-10No Fog
Side
Fog Side
Steam Fog Conditions
• Occurs when cold air flows over much warmer water• Cold air mixes with warm, moist air next to water surface• Mixture condenses into fog, a couple of feet above surface• Common off of East Coast of US during winter Over
lakes, ponds and rivers during early fall
Williams, The Weather Book, p68
http://media.trb.com/media/photo/2011-01/216881120-24150101.jpg
Steam Fog
Bitter cold January air over Lake Michigan
Why You See Your Breath on a Cold Winter Night
Condensation can occur if moist warm air and cold dry air mix.Temp RH SVP VP
Air 0oC 20% 6 mb 1 mb(clear)
Breath 36oC80% 63 mb 55 mb(clear)
50‐50 18oC140% 20 mb 28 mb(cloud)
90‐10 4oC 90% 8 mb 6 mb(clear)
50‐50 mix condenses into a cloud that quickly evaporates as your exhalation is diluted further
Ahrens older ed.
Breath
Air
50-50
90-10No Fog
Side
Fog Side
Precipitation/Frontal Fog Weather
• A type of Steam Fog• Occurs when warm rain falls into colder unsaturated air• Warmer, rain drops evaporate as they fall• Added water vapor increases dew point to saturate the air• Most common in the shallow layer of cold air, just ahead of warm
fronts (rarely occurs in AZ)
Williams, The Weather Book, p68
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