Download Water vapor

Atmospheric Moisture
(chapter 4)
Water vapor (p. 84-93)
Cloud formation (p. 93-98)
Cloud classification (p. 101-111)
Stability and clouds
1.
2.
3.
4.
1.
2.
3.
dry adiabatic processes (p. 70-74)
moist adiabatic processes (p. 98-101)
chinook (p. 122-123)
Precipitation (p. 112-121)
5.
1.
2.
3.
warm cloud precip growth
cold cloud precip growth
precip types
Water is the source of all life on earth.
The distribution of water is quite varied
Water vapor is not uniformly distributed in the atmosphere
Water vapor channel,
GOES 8
dry
humid
Water (and only water!) exists in all 3 phases on earth:
solid (ice), liquid or gas (water vapor)
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
1. the three phases of water
questions

Is water vapor present in the air at temperatures below the
boiling point?
Yes

Does evaporation occur from a pot of water, before it boils?
Yes
Water vapor is present in the atmosphere at any temperature.
open container, unsaturated air:
evaporation > condensation
saturation
closed container:
evaporation = condensation
dynamic
equilibrium
The maximum water vapor concentration increases
exponentially with temperature
Phase Changes:
Gain/Loss of Heat by the Air
Process
Condensation
Evaporation
Freezing
Melting
Deposition
Sublimation
Changes
From
vapor
liquid
liquid
ice
vapor
ice
Heat gained/lost
To
liquid
vapor
ice
liquid
ice
vapor
2500 J/g
-2500 J/g
333 J/g
-333 J/g
2833 J/g
-2833 J/g
note: the specific heat of water is 4.186 J/(g °C) [that is 1 cal/(g °C) ]
the hydrologic cycle and global climate
Evaporation (& melting) = cooling
Condensation (& freezing) = heating
This condensation heating is a major source of energy for the global
circulation.
Recall the equation R = H + LE
Evaporation uses energy (LE)
Pop quiz : sublimation is the transition from … to …
1.
2.
3.
4.
vapor
liquid
ice
liquid
 liquid
 vapor
 vapor
 ice
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 <)
the Hydrologic Cycle
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)
The Hydrologic Cycle
Rivers carry water from land to oceans. What does this imply?
 there must be more precip than evaporation over land
 net transport of water vapor from ocean to land
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.
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 ?
PW
1' '

 1 year  9 days
RT = reservoir content / flux =
40
40' '
P
year
 water is rapidly recycled though the atmosphere.
Residence time of H2O molecules in the hydrologic
cycle









Atmosphere
Rivers (speed ~1m/s)
Soil Moisture
Largest lakes
Shallow groundwater (speed ~1-10 m/d)
Mixed layer (~150m) of oceans
Global oceans (avg depth 4 km)
Deep groundwater
Antarctic icecap
9 days
2 weeks
2 weeks - 1 year
10 y
10s - 100s y
120 y
3000 y
up to 10,000 y
10,000 y - more
3. Water vapor variables
1.
The three phases of water


2.
3.
The hydrologic cycle
How to measure atmospheric
moisture
4.
How water vapor varies on Earth
5.
Humidity and human comfort





Mixing ratio
Precipitable water
Vapor pressure
Saturation vapor pressure
Relative humidity
Dewpoint
Wet-bulb temperature
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.
Terminology: mixing ratio and PW
Mixing ratio
Definition: the ratio of the mass of water vapor in a sample to the total
mass of the air
Units: g/kg
The mixing ratio is conserved.
Precipitable Water
The PW is the vertically integrated amount of mixing ratio.
(global mean = 1’’)
Units : water depth (mm or inches)
Saturation and temperature
The higher the temperature, the greater the number of water molecules the air can hold.
400
350
300
250
200
150
100
saturation vapor pressure, mb

50
Clausius-Clapeyron
equation
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
temperature, ºF
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.
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
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.
1013 mb
What is the boiling
point in Laramie?
Why is it different?
mb 400
350
300
250
200
150
100
50
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
212ºF (100 ºC)
saturation vapor pressure and boiling point
1013 mb
Laramie pressure: 780 mb
mb 400
350
300
Lower boiling point!
250
197 °F
200
150
100
50
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
212ºF (100 ºC)
Boiling Point
Boiling occurs when the SVP of escaping bubbles is greater than the total atmospheric pressure.
220
215
BP of water (°F)
210
205
200
195
190
185
180
-1000
1000
3000
5000
7000
9000
Height (feet)
11000
13000
15000
psychrometric chart
400
350
300
250
200
150
100
50
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Relative humidity
Wet bulb
temperature (°F)
Saturation vapor pressure (mb)
vapor pressure (mb)
dewpoint (°F)
psychrometric chart
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 holds
half the water vapor that it is capable
of holding; 100% RH means the air
holds all the water vapor it can.
Because of the temperature dependence
of the SVP, warm air has more water
vapor than cooler air for a given RH.
question
Exercise: T = 15ºC, e = 10.2 mb
determine the RH
(a) graphically
Exercise: T(b)
= 15ºC,
e = 10.2 mb
calculated
determine the RH
(a) graphicallyRH = 100 e/esat
(b) calculated RH = 100 e/esat
answer
Exercise: T = 15ºC, e = 10.2 mb
determine the RH
(a) graphically
Exercise: T(b)
= 15ºC,
e = 10.2 mb
calculated
determine the RH
(a) graphicallyRH = 100 e/esat
(b) calculated RH = 100 e/esat
RH = 100 e/esat
= 100 (10.2/17)
= 60 %
17
Relative humidity paradox:
an air parcel with less water vapor can have a higher RH
Solve this paradox!
inside
8
outside
4
18 F
70 F
temperature
water
vapor
content
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.
inside: 35% RH
8
outside: 80% RH
4
18 F
70 F
temperature
water
vapor
content
Explain the variation of surface RH on a quiescent day
dewpoint
Testing your understanding of relative humidity

Go to:
http://profhorn.meteor.wisc.edu/wxwise/relhum/rhac.html
Global RH
near ground level
http://ingrid.ldgo.columbia.edu/
RH and dewpoint temperature
RH=100%
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.
psychrometric
chart
11
Find the dewpoint
psychrometric chart
the dewpoint is:
Td = 8ºC
What is the dewpoint
depression?
8
the dewpoint depression is :
T - Td = 15-8 = 7ºC
Question: if the vapor pressure
increases, how does the
dewpoint change?
8
The dewpoint increases when the vapor pressure increases
new vapor
pressure e
Td
determine the
dewpoint
depression
(°F)
in Laramie
determine the
dewpoint
depression
in Laramie
now
Dew point temperatures
Mean July dew point
temperatures
Mean July noontime
relative humidities
Vapor pressure (mb)
January
Climatology of vapor pressure
at the surface
July
Frost point temperatures
saturation point relative to ice similar to dew
point but Tf < 0ºC
frost or rime forms on grass, trees …
Td < Tf … key in growth of ice crystals, and rainfall
400
350
over
water
vapor
pressure
(mb)
300
250
200
150
6
100
4
50
2
0
0
supercooled water
T T
ice
10 20 30 40 50 60 -40°C
70 80 90 100
150
d 110
f 120 130 1400°C
T
In a ‘mixed-phase’ cloud, supercooled droplets will evaporate and the
excess water vapor will deposit on ice crystals
Sub-saturated wrt water
Super-saturated wrt ice
‘Bergeron’ process of
precipitation formation
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
psychrometric chart
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
What units does it have ?





Dewpoint
Relative humidity
Wet-bulb temp
Precipitable water
Vapor pressure




mb
%
°C (or °F)
mm (or inches)
answer





Dewpoint
Relative humidity
Wet-bulb temp
Precipitable water
Vapor pressure




mb
%
°C (or °F)
mm (or inches)
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
precipitable water: mean distribution

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"
Water vapor near a
frontal disturbance
Synoptic variation in water vapor
2 Oct 2002, 18 Z
2 Oct 2002, 18:15 Z
2 Oct 2002, 18:00 Z
Synoptic variation –relative humidity
2 Oct 2002, 18:00 Z
Seasonal variations
Average near-surface vapor pressure (mb)
Which one is for January?
Which one for July?
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)
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
human thermoregulation
homeostasis
Apparent
temperature (or
heat stress index)
Apparent temperature extremes


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

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


Heat exhaustion: due to slow loss of fluid and salt through
perspiration



muscle cramps, especially in the legs, because of brief imbalances in body
salts.
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.

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
summary chapter 4a: water vapor

Water vapor concentrations vary a lot


There are many measures of atmospheric humidity:







hydrologic cycle
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