atmospheric moisture (chapter 4)
DESCRIPTION
Atmospheric Moisture (chapter 4). Water vapor (p. 84-93) Cloud formation (p. 93-98) Cloud classification (p. 101-111) Stability and clouds dry adiabatic processes (p. 70-74) moist adiabatic processes (p. 98-101) chinook (p. 122-123) Precipitation (p. 112-121) warm cloud precip growth - PowerPoint PPT PresentationTRANSCRIPT
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Atmospheric Moisture(chapter 4)
1. Water vapor (p. 84-93)2. Cloud formation (p. 93-98)3. Cloud classification (p. 101-111)4. Stability and clouds
1. dry adiabatic processes (p. 70-74)2. moist adiabatic processes (p. 98-101)3. chinook (p. 122-123)
5. Precipitation (p. 112-121)1. warm cloud precip growth2. cold cloud precip growth3. precip types
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Water is the source of all life on earth.
The distribution of water is quite varied
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Water vapor is not uniformly distributed in the atmosphere
dry humid
Water vapor channel,GOES 8
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Water (and only water!) exists in all 3 phases on earth:
solid (ice), liquid or gas (water vapor)
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topics
1. The three phases of water
2. The hydrologic cycle
3. How to measure atmospheric moisture
4. How water vapor varies on Earth
5. Humidity and human comfort
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1. the three phases of water
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questions
Is water vapor present in the air at temperatures below the boiling point?
Does evaporation occur from a pot of water, before it boils?
Yes
Yes
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open container, unsaturated air:evaporation > condensation
Water vapor is present in the atmosphere at any temperature.
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dynamic equilibrium
closed container:evaporation = condensation
saturation
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The maximum water vapor concentration increases exponentially with temperature
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Phase Changes:Gain/Loss of Heat by the Air
Process Changes Heat gained/lost From ToCondensation vapor liquid 2500 J/g Evaporation liquid vapor -2500 J/g Freezing liquid ice 333 J/g Melting ice liquid -333 J/g Deposition vapor ice 2833 J/g Sublimation ice vapor -2833 J/g
note: the specific heat of water is 4.186 J/(g °C) [that is 1 cal/(g °C) ]
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the hydrologic cycle and global climate
Evaporation (& melting) = coolingCondensation (& freezing) = heating
This condensation heating is a major source of energy for the global circulation.
Recall the equation R = H + LE
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Evaporation uses energy (LE)
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Pop quiz : sublimation is the transition from … to …
1. vapor liquid2. liquid vapor3. ice vapor4. liquid ice
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2. The hydrological cycle …some team work
Sketch the hydrologic cycle Do not look at your textbook Draw land, oceans, etc, and show how water is moved around
Write down your guesses of what fraction of the global water is held … In the oceans ( ..%) In ice caps & glaciers ( ..%) In lakes & rivers ( ..%) In the atmosphere ( ..%)
Also guess what the global mean rainfall is (inches per year) Over the ocean, evaporation … precipitation (fill in > or <) Over land, evaporation … precipitation (fill in > or <)
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the Hydrologic Cycle
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answer
The oceans contain 97.5% of the earth's water,
Ice accounts for 2.1%
Lakes and rivers is 0.3%
the atmosphere less than 0.001%.
if all the water vapor in the global atmosphere were to condense and rain out at once, you ‘d have 1’’ of rain (precipitable water)
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The Hydrologic Cycle
there must be more precip than evaporation over land net transport of water vapor from ocean to land
Rivers carry water from land to oceans. What does this imply?
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The Hydrologic Cycle
• Water vapor links the surface to the atmosphere
• Water vapor concentrations are extremely variable
• The hydro cycle is closely tied to atmospheric circulation patterns.
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Relative Storage
The global mean annual precipitation P is about 40’’, i.e. about 40 times the precipitable water PW
What is the average residence time of water vapor in the atmosphere ?
RT = reservoir content / flux =
water is rapidly recycled though the atmosphere.
daysyear
yearP
PW940
1''40
''1
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Residence time of H2O molecules in the hydrologic cycle
Atmosphere 9 days Rivers (speed ~1m/s) 2 weeks Soil Moisture 2 weeks - 1 year Largest lakes 10 y Shallow groundwater (speed ~1-10 m/d) 10s - 100s y Mixed layer (~150m) of oceans 120 y Global oceans (avg depth 4 km) 3000 y Deep groundwater up to 10,000 y Antarctic icecap 10,000 y - more
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3. Water vapor variables Mixing ratio Precipitable water Vapor pressure Saturation vapor pressure Relative humidity Dewpoint Wet-bulb temperature
1. The three phases of water
2. The hydrologic cycle
3. How to measure atmospheric moisture
4. How water vapor varies on Earth
5. Humidity and human comfort
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The mean vertical distribution of mixing ratio in the atmosphere
Mixing ratio: grams of water vapor per kg of air
Nearly half the total water in the air is between sea level and about 1.5 km above sea level. Less than 5-6% of the water is above 5 km, and less than 1% is in the stratosphere.
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Terminology: mixing ratio and PW
Mixing ratioDefinition: the ratio of the mass of water vapor in a sample to the total mass of the airUnits: g/kgThe mixing ratio is conserved.
Precipitable WaterThe PW is the vertically integrated amount of mixing ratio.(global mean = 1’’)Units : water depth (mm or inches)
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Saturation and temperature
The higher the temperature, the greater the number of water molecules the air can hold.
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temperature, ºF
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ratio
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Clausius-Clapeyronequation
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Terminology: Vapor Pressure
The partial pressure of a given sample of moist air that is attributable to the water vapor is called the vapor pressure.
Units: mb
Distinguish the actual from the saturation vapor pressure.
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Pop quiz: vapor pressure is …
a) the weight of water vapor
b) the partial pressure of water vapor molecules in the air
c) the concentration of water vapor, expressed as a mass of water vapor per mass of air
d) the density of water vapor, expressed as a mass of water vapor per volume of air
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Terminology: Saturation Vapor Pressure
The vapor pressure necessary to saturate the air is the saturation vapor pressure. Saturation vapor pressure increases rapidly with temperature: the value at 90°F is about double the value at 70°F.
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mb
212ºF (100 ºC)
1013 mb What is the boiling point in Laramie?
Why is it different?
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saturation vapor pressure and boiling point
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212ºF (100 ºC)
1013 mb
Laramie pressure: 780 mb
Lower boiling point!
197
°F
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Boiling Point
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-1000 1000 3000 5000 7000 9000 11000 13000 15000
Height (feet)
BP
of w
ater
(°F
)Boiling occurs when the SVP of escaping bubbles is greater than the total atmospheric pressure.
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psychrometric chart
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Relative humidity
Wet bulb temperature (°F)
Saturation vapor pressure (mb)
vapor pressure (mb)
dewpoint (°F)
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psychrometric chart
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Terminology: Relative Humidity
Definition: RH = actual vapor pressure (%) saturation vapor pressure
RH is a relative variable: it relates the actual amount to the amount that would saturate the air.
Example: 50% RH means the air holdshalf the water vapor that it is capableof holding; 100% RH means the airholds all the water vapor it can.
Because of the temperature dependenceof the SVP, warm air has more watervapor than cooler air for a given RH.
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questionExercise: T = 15ºC, e = 10.2 mb determine the RH(a) graphically(b) calculated
RH = 100 e/esat
Exercise: T = 15ºC, e = 10.2 mb determine the RH(a) graphically(b) calculated RH = 100 e/esat
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answerExercise: T = 15ºC, e = 10.2 mb determine the RH(a) graphically(b) calculated
RH = 100 e/esat
Exercise: T = 15ºC, e = 10.2 mb determine the RH(a) graphically(b) calculated RH = 100 e/esat
RH = 100 e/esat
= 100 (10.2/17) = 60 %
17
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Relative humidity paradox:an air parcel with less water vapor can have a higher RH
temperature
water vapor content
18 F 70 F
4
8
Solve this paradox!
outside
inside
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Relative humidity paradox:the air outside is colder and has less water vapor than the air inside. But because the
svp is so much lower at lower temperature, the RH is higher outside.
temperature
water vapor content
outside: 80% RH
inside: 35% RH
18 F 70 F
4
8
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Explain the variation of surface RH on a quiescent day
dewpoint
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Testing your understanding of relative humidity
Go to: http://profhorn.meteor.wisc.edu/wxwise/relhum/rhac.html
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Global RH
http://ingrid.ldgo.columbia.edu/http://ingrid.ldgo.columbia.edu/
near ground level
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RH and dewpoint temperature
RH=100%
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Dew point temperatures
The dew point temperature is the temperature the air would have if it were cooled, at constant pressure and water vapor content, until saturation.
The difference between the actual temperature and the dew point is called the dew point depression, a measure of relative humidity.
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psychrometricchart
Find the dewpoint11
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psychrometric chart
the dewpoint is:Td = 8ºC
What is the dewpoint depression?
8
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T - Td = 15-8 = 7ºC
the dewpoint depression is :
Question: if the vapor pressure increases, how does the dewpoint change?
8
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new vapor pressure e
Td
The dewpoint increases when the vapor pressure increases
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determine the dewpoint depression
(°F)in Laramie
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determine the dewpoint depressionin Laramie
now
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Dew point temperatures
Mean July dew pointtemperatures
Mean July noontimerelative humidities
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Vapor pressure (mb)
January
July
Climatology of vapor pressure at the surface
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Frost point temperatures
saturation point relative to ice similar to dew point but Tf < 0ºC
frost or rime forms on grass, trees …
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Td < Tf … key in growth of ice crystals, and rainfall
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vapor pressure (mb)
supercooled water ice
over water
0°C-40°C TTd Tf
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In a ‘mixed-phase’ cloud, supercooled droplets will evaporate and the excess water vapor will deposit on ice crystals
Sub-saturated wrt waterSuper-saturated wrt ice
‘Bergeron’ process of precipitation formation
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Terminology: Wet Bulb Temperature
The wet-bulb temperature is the temperature an air parcel would have if it were cooled to saturation by evaporating water into the parcel.
Sling psychrometer
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psychrometric chart
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wet-bulb, dewpoint, and saturation
The wet-bulb temperature is the lowest air temperature that can be achieved by evaporation (e.g. evaporative coolers).
Normally Td < Tw < T
At saturation (RH=100%), Td = Tw = T
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What units does it have ?
Dewpoint Relative humidity Wet-bulb temp Precipitable water Vapor pressure
mb % °C (or °F) mm (or inches)
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answer
Dewpoint Relative humidity Wet-bulb temp Precipitable water Vapor pressure
mb % °C (or °F) mm (or
inches)
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4. Water vapor variability in the atmosphere
Where do you expect more water vapor ?
Latitude Season Land/sea Location relative to a cold front Day/night Elevations/mountain barriers Upwind distance from shore
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Less PW in colder regions
PW low also in deserts
The most vapor-rich region is in the western equatorial Pacific, above the "oceanic warm pool"
precipitable water: mean distribution
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Water vapor near a frontal disturbance
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Synoptic variation in water vapor
2 Oct 2002, 18 Z
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2 Oct 2002, 18:15 Z
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2 Oct 2002, 18:00 Z
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2 Oct 2002, 18:00 Z
Synoptic variation –relative humidity
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Seasonal variations
Which one is for January?
Which one for July?
Average near-surface vapor pressure (mb)
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Global Water Vapor Trends
Some increase in atmospheric water vapor (0-2 mm of PW per decade). Regions that have experienced most warming also have seen the most moisture increase. Also, global warming is evident mostly in terms of the dawn minimum
temperature. What causes the decrease in nocturnal cooling? Also, on average, global rainfall amounts have increased ~5% in the last 40
years evidence for an increase in hydrological cycling rate.
Changes in PW between 1988-2002 according to 4 different estimates. The dashed lines are trend lines. The seasonal variability has been removed. (source: Trenberth et al. 2005, Climate Dynamics, 741-758)
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5. Humidity and human comfort
Warm air feels even hotter when the relative humidity is high
The reason … thermoregulation becomes difficult when high humidity prevents the evaporation of sweat.
Remember … evaporation = cooling
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human thermoregulation homeostasis
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Apparent temperature (or
heat stress index)
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Apparent temperature extremes
Category Apparent temperature Dangers Extreme danger >130 F Heat stroke imminent Danger 105 – 130 F Heat exhaustion likely Extreme caution 90 - 105 F Heat cramps, exhaustion
possible Caution 80 – 90 F Exercise more fatiguing
than usual.
Heat cramps: due to exercising in hot weather muscle cramps, especially in the legs, because of brief imbalances in
body salts. Heat exhaustion: due to slow loss of fluid and salt through
perspiration lead to dizziness and weakness during a heat wave body temperature might rise, but not above 102 F.
Heatstroke: upset of the body's thermostat Body temperature rise to 105 F or higher. Symptoms are lethargy, confusion and unconsciousness.
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Pop quiz: under hot conditions (T = 104ºF true temperature), human homeostasis can be maintained by all of the following except one. Which
one?
A: perspiration;
B: exposure to wind, which increases perspiration rate;
C: increased sensible heat flux, by wearing less clothing;
D: reduced exposure to radiation
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summary chapter 4a: water vapor
Water vapor concentrations vary a lot hydrologic cycle
There are many measures of atmospheric humidity: vapor pressure (hPa) mixing ratio (g/kg) dewpoint (F or C) relative humidity (%) wet-bulb temp (F or C) precipitable water (mm)
These variables can be related to each other using a psychrometric chart