atmo 170 lecture mwf atmospheric moisturebrunke/atmo170a1-humidity.pdfmeasures of atmospheric...

ATMO 170Lecture MWF

Atmospheric Moisture

Not on Quiz 2

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.


• 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


• 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


• 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)


• 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

CCN￫Cloud Droplets￫Raindrops

Ahrens 6th ed.

100 times

100 times

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.


• 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.


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)


• 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)


• 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

ATMO 170A1Lecture MWF

Fog

Fog in a village of the Himalayas. Credit: Himalaya Valley Fog

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

Advection Fog Shrouds the Golden Gate

http://en.wikipedia.org/wiki/File:GGB_in_fog_2007_edit.jpg

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

Steam Fog Rising from Hot Asphalthttp://farm3.static.flickr.com/2651/3724731423_abe14b195d_o.jpg

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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