Lecture 07 February 10, 2010 Water in the Atmosphere: Part...

Lecture 07 February 10, 2010

Water in the Atmosphere: Part 1

About Water on the Earth: The Hydrological Cycle

Review 3-states of water, phase change and Latent Heat

Indices of Water Vapor Content in the Atmosphere

(1) precipitable water, (2) absolute humidity,

(3) relatively humidity, (4) vapor pressure of water

(5) specific humidity, (6) mixing ratio,

(7) dew point temperature, (8) frost point

Distributions of Water Vapor in the Atmosphere

Measuring Humidity

Introduction to Clouds Formation

Methods of Achieving Saturation

Two Cooling Processes

Uplifting Mechanisms

Types of Condensation & deposition (dew, frost, fog, cloud)

About Water on the Earth

Over 70% of the Earth is covered by water a water planet.

Water simultaneously exists in three states (solid, liquid, gas).

Water can shift between states very easily phase change.

Water has large specific heat, latent heat is energy associated with phase change, plays significant role in weather & climate.

The hydrologic cycle refers to the regular cycle of water through the Earth-atmosphere system.

Water is a variable gas ( 0–4% ), a greenhouse gas.

In the temperature range on Earth, adding water vapor to the atmosphere or lowering the air temperature in saturated air can lead to condensation or deposition.

Energy associated with phase change between different states (solid, liquid, gas) of a substance.

Atmospheric processes mainly involves latent heat associated with water (ice, liquid water, water vapor).

Latent heat of evaporation / vaporization: from liquid water towater vapor, energy is added to liquid water to raise its temperature, molecules gain more kinetic energy to break free into the atmosphere.

evaporative cooling: energy used to evaporate liquid water instead to raise body temperature (examples: sweat, wet surface).

this energy held by those escaped molecules as “latent” in the atmosphere, and released when the reverse process condensation(from water vapor to liquid water) occurs e.g., dew, fog, clouds, energy to fuel severe weather events such as hurricanes.

for the same net radiation gain by a surface, the presence of liquid water redirect some of this available energy as latent heat and reduce sensible heat (cooler surface temperature).

Latent Heat

Latent heat of fusion: melting of ice to liquid water

Reverse process: freezing, from liquid water to solid ice

Latent heat of evaporation = 7.5 times latent heat of fusion

Latent heat of sublimation: from solid ice to water vapor

Reverse process: deposition, water vapor solid ice

In addition to evaporative cooling, wet environments are cooler also because more water vapor in the atmosphere which absorbs more near-infrared insolation and reduces the amount to reach and heat Earth’s surface and less heating of the lower atmosphere by the Earth’s surface.

Globally, 21 units of latent heat are transferred from the Earth’s surface to the atmosphere, leaving only 8 units as sensible heat from the surface to the atmosphere.

Latent Heat

Precipitable Water

The amount of water in a column of air, measured in depth per unit area.

It cannot show vertical water variability, but does vary horizontally.

Global average

~ 25 mm

Deserts:

< 10 mm

Tropics

> 40 mm


Saturation over water surface: when evaporation and

condensation reach equilibrium. This occurs at normal

temperature on Earth.

Saturation over ice surface: equilibrium between

sublimation and deposition.

Saturation can occur whether dry air exists or not, so

the statement “air holds water” is not accurate.

Net evaporation Saturation

liquid waterliquid water liquid water

Vapor pressure

Dalton’s law: the total atmospheric pressure pequals the sum of partial pressures of all gases.

Partial pressure due to water vapor is called vapor pressure and denoted by e.

Both p and e may change with the air temperature.

The maximum e is called saturation vapor pressure, denoted by es .

es is greater in hotter air.

Indices of Water

Vapor Content in the

Atmosphere

Non-linear increase of saturation vapor pressure (es) with increasing temperature, same temperature increase leads greater increase in es at higher temperature.

es, 1

es, 2

T1

T2

T1 = T2 = 10 oC

But

es, 1 < es, 2


It is simply the density of water vapor.

It depends on temperature because at constant pressure, air expands or contracts when heated or cooled change air volume.

It depends on pressure because at constant temperature, air volume increases with increasing altitude and decreasing pressure.

Other problems:

How do you weigh the water vapor in the air?

How do you determine the volume of an air parcel?

Not widely used in free atmosphere: weather & climate

mass of water vapor (kg or g)

volume of air (m3)

Absolute humidity =


mv

mq = =

Specific humidity q (g kg-1)

Mixing ratio r (g kg-1)

mv = mass of water vapor

md = mass of dry air

m = mass of atmosphere

mv

mv + md

Mainly scientific application.

Both q and r independent of temperature and pressure because either water vapor mass nor dry air mass changes with volume of air when air expands or contracts thus, they are very useful in comparing water vapor in the air at locations of different temperatures and pressures

Saturation values are denoted by qs and rs

Only problem: unfamiliarity to the public.

mv

md

r =

Numerical values of q and r are

very close because mv << md

Read examples in textbook! p128

Relative humidity (RH) is the percentage of actual water vapor content in the air relative to that (maximum water vapor content) in saturated air.

Although RH is defined in many classical and modern

texts using the ratio e / es, International Meteorological

Organization in 1947 adopted the ratio r / rs . However,

the difference is very slight, thus:


e

esRH = x 100%

q

qs= x 100%

r

rs= x 100%

RH is a poor choice for comparing actual water content

in the air at different places with different temperature,

or same place at different time of the day or of the year

as temperature changes with time.

RH depends on air temperature. RH changes even though

actual water vapor content does not. This is because when

temperature changes, the maximum water vapor content at

saturation (es, qs and rs ) all change as indicated by the

change in the size of the open circles.

Daily RH changes mainly due to temperature change


Dew point temperature or dew point = the temperature to

which an air parcel needs to be cooled in order to reach

saturation (100% RH).

Dew point (temperature) = or < air temperature.

When air temperature < dew point, condensation occur.

Frost point = below 0 oC temperature to which an air parcel

needs to be cooled in order to reach saturation (100% RH).

When air temperature < frost point, deposition occur.

Simple to use, easy to interpret.

Mainly depends on actual water vapor content in the air.

Higher dew point, higher actual water vapor content in the air

Dew point

temperature

The brown and green parcels have the same RH (75%),

but different dew points (~14°C) vs (~26°C)

Dew point temperature is directly related to the

amount of water vapor in the air and is widely used

(local weather report and daily weather maps).

14 26 32

Distribution of

Water Vapor in the

Atmosphere

January, dew point

July, dew point

Water vapor in the

atmosphere either from local

evaporation or from horizontal

transport by advection of

moisture from other regions

(warm moist ocean).

Warm July yields higher

water vapor content in the

atmosphere than cold January

The blue arrows indicate the

direction of decreasing dew

point or water vapor content

in the atmosphere from

source regions (warm Gulf of

Mexico or Pacific).

Measuring Humidity

Sling

Psychrometer

Hair Hygrometer: hair length changes with

the relative humidity (human or horse hair)

Wet

Bulb Dry

Bulb

Hygrothermograph:

hair hygrometer &

bimetallic strip are

combined

Introduction to Clouds Formation

Water vapor condenses on condensation nuclei

suspended in the air (fog & cloud) in saturation

without condensation nuclei, super-saturation with

RH > 100% is required to produce fog & clouds.

Clouds are visible aggregates of minute droplets of

water or tiny crystals of ice.

Clouds are instrumental to Earth’s energy (radiation)

and moisture balances.

Most clouds form as air parcels are lifted & cooled to

saturation.


Temperature

Satu

rati

on

Sp

ec

ific

Hu

mid

ity

Adding water vapor

Cooling

Examples:

(1) condensation

on bathroom

mirror during

shower,

(2) precipitation fog

Example: clouds


Mixing cold

air with warm

moist air

Examples:

contrails behind

jetliners,

Steam fog

Two Different Cooling Processes

Diabatic process:

Heat is added or removed from an air parcel.

Air parcel cooled by surface, radiation fog.

More important for fog formation than for clouds.

Adiabatic processes:

No heat added or removed from an air parcel.

Cooling by expansion or warming by compressing.

Most important for clouds formation.

Adiabatic Cooling & Warming

DALR = dry adiabatic

lapse rate = 10 oC km-1

Adiabatic Cooling

Rising air cools at a consistent rate, called the dry

adiabatic lapse rate (DALR):

Unsaturated air

10oC/km (1000m)

If the cooling decreases the air temperature to the

dew point and below, the lapse rate changes to

the wet adiabatic lapse rate (WALR)

saturated air

varies: .4-9oC/km

These lapse rates apply no matter

what lifting mechanism is at work

ea = es

Note: Average environmental lapse rate

(ELR) is 6.5 °C km-1 (page 17 of textbook)

Condensation

release latent

heat warm

atmosphere to

offset adiabatic

cooling.

WALR = Wet

adiabatic lapse

rate is always

lower than the

dry adiabatic

lapse rate and

ranges between

4 and 9 °C km-1.

Adiabatic Cooling

Why is the wet adiabatic lapse rate different?

Because when condensation occurs after the dew

point is reached, latent heat is released. This heat is

added to the atmosphere, so the rate at which air

cools is offset by the heat added to the atmosphere.

The wet adiabatic lapse rate is always lower than the

dry adiabatic lapse rate.

9oC/km4oC/km

more

condensation

less

condensation

warmer, moist air

ex: Tropics

cooler, dry air

ex: Poles

SALR = saturated

adiabatic lapse

rate = the lapse

rate when

saturated air

parcel rises

SALR is not

constant, more

latent heat is

released by

warmer air to

offset adiabatic

cooling thus,

lower SALR rate,

than cooler air

Windward Side Leeward Side


(a) Orographic lifting: over mountains and hills

On the windward side of the barrier, air is displaced

toward higher altitudes, undergoes adiabatic cooling,

possibly to saturation, even rain

On the leeward side, descending air warms adiabatically

through compression leading to a rain-shadow

(b) Frontal WedgingWhen boundaries (fronts) between airmasses of unlike

temperatures migrate, warmer (less dense or lighter) air is pushed aloft

This results in adiabatic cooling and cloud formation

(c) Convergence Air mass is non-uniformly distributed over Earth

Air advects from areas of more mass to areas of less mass

Air moving into these low pressure regions converges

Stimulates rising motions and adiabatic cooling

(d) Convection Localized surface heating leads to local free convection

Vertical motions are stimulated from the surface upward resulting in towering clouds and a chance for intense precipitation over small spatial scales


Forms of Condensation & Deposition

Dew

Liquid condensation on surface objects

Diabatic cooling of surface air typically takes place through terrestrial radiation loss on calm, cool, clear nights

Surface air becomes saturated and condensation forms on objects acting as condensation nuclei

Frost

Similar to dew except that it forms when surface temperatures are below freezing

Deposition occurs instead of condensation

May be referred to as white or hoar frost

Dew Frost

Frozen DewOccurs when normal dew formation processes occur followed by a drop in temperature to below freezing

Causes a tight bond between ice and the surface

Forms dangerous “black ice” on roadways

FogSimply a surface cloud when air either cools to the dew

point, or moisture added, or when cooler air is mixed

with warmer moister air

Radiation Fog

Occurs when near surface air chills diabatically to

saturation through terrestrial radiation loss on clear

cool nights

Require a slight breeze to vertically mix air through a

shallow column

Advection Fog

Occurs when warm moist air moves across a cooler surface

Air is chilled diabatically to saturation

Common on the U.S. west coast as warm, moist air from the central Pacific advects over the cold California ocean current

Frequently develop near boundaries of opposing ocean temperatures, e.g.: off northeast US coast

Upslope Fog

The only fog developed through adiabatic cooling

Occur when air is advected over land surfaces which increase in elevation

A common occurrence in the Great Plains of the U.S. where warm, moist air advects from the Miss. River Valley towards the Rocky Mountains

Lecture 07 February 10, 2010 Water in the Atmosphere: Part...

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