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

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

dry humid

Water vapor channel,GOES 8

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?

Does evaporation occur from a pot of water, before it boils?

Yes

Yes

open container, unsaturated air:evaporation > condensation

Water vapor is present in the atmosphere at any temperature.

dynamic equilibrium

closed container:evaporation = condensation

saturation

The maximum water vapor concentration increases exponentially with temperature

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) ]

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

Evaporation uses energy (LE)

Pop quiz : sublimation is the transition from … to …

1. vapor liquid2. liquid vapor3. ice vapor4. liquid 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

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?

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 ?

RT = reservoir content / flux =

water is rapidly recycled though the atmosphere.

daysyear

yearP

PW940

1''40

''1

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

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

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 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)

Saturation and temperature

The higher the temperature, the greater the number of water molecules the air can hold.

0

50

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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

temperature, ºF

satu

ratio

n va

por

pres

sure

, mb

Clausius-Clapeyronequation

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.

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mb

212ºF (100 ºC)

1013 mb What is the boiling point in Laramie?

Why is it different?

saturation vapor pressure and boiling point

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0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

mb

212ºF (100 ºC)

1013 mb

Laramie pressure: 780 mb

Lower boiling point!

197

°F

Boiling Point

180

185

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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.

psychrometric chart

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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 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.

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

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

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

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

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

http://ingrid.ldgo.columbia.edu/http://ingrid.ldgo.columbia.edu/

near ground level

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.

psychrometricchart

Find the dewpoint11

psychrometric chart

the dewpoint is:Td = 8ºC

What is the dewpoint depression?

8

T - Td = 15-8 = 7ºC

the dewpoint depression is :

Question: if the vapor pressure increases, how does the dewpoint change?

8

new vapor pressure e

Td

The dewpoint increases when the vapor pressure increases

determine the dewpoint depression

(°F)in Laramie

determine the dewpoint depressionin Laramie

now

Dew point temperatures

Mean July dew pointtemperatures

Mean July noontimerelative humidities

Vapor pressure (mb)

January

July

Climatology of vapor pressure at the surface

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

2

4

6

vapor pressure (mb)

supercooled water ice

over water

0°C-40°C TTd Tf

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

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

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

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

2 Oct 2002, 18:00 Z

Synoptic variation –relative humidity

Seasonal variations

Which one is for January?

Which one for July?

Average near-surface vapor pressure (mb)

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

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.

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 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