intro to meteorology course 4a(1)

7/23/2019 Intro to Meteorology Course 4a(1)

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The role of water in the

atmosphere

Terms and definitions for

atmospheric moisture

The impact of moisture

on comfort

Formation of dew, frost,haze, fog, and clouds

Classification of fog and

cloud types

Humidity, Condensation, and CloudsHumidity, Condensation, and Clouds


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Distribution of Water on EarthDistribution of Water on Earth

1.3109 km32.8107 km3

8.1106 km3

0.2106 km3

1.3104 km3

0.6103 km3

Total hydrosphere 1.4109 km3

OceansGlaciers

Ground water

Lakes/rivers

Atmosphere

biosphere

(=97.3%)

(=2.7%)(=0.001%)

The atmosphere contains only ~ 1 week supply of precipitation!

Oceans

Glaciers

Groundwater

Lakes/rivers

atmosphere


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Most abundant trace gas; Large variability:

0 4%

Large amounts are found close the surface, decreasing quickly aloft.

Properties of WaterProperties of Water

Winter Summer

Altitude [hPa]Altitude [hPa]

5 km

1.5 km

Global distribution of H2Oin the northern hemisphere

[g H2O/ kg air]


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Most abundant trace gas; Large variability: 0 4%


Source: evaporation from the ocean and land. Sink: precipitation.


Hydrologic Cycle

Evaporation & transpiration by plants liquid water into atmospheric vapor.

Condensation converts water vapor back to a liquid droplet, which maythen fall as precipitation to ground or surface water supplies.


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Surface Tension: highest of all common liquids



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Surface Tension: highest of all common liquids Heat Capacity: highest of all common solids and liquids (1 cal g-1 C-1)


c


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Surface Tension: highest of all common liquids Heat Capacity: highest of all common solids and liquids (1 cal g-1 C-1)

Most important greenhouse gas

Radiative Properties:

- transparent to visible wavelengths

- virtually opaque to many infrared wavelengths

- large range of albedo possible


water 10 % (daily average)

Ice 30 to 40%

Snow 20 to 95%

Cloud 30 to 90%


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Structure of Water

Water's unique molecular structure and hydrogen bonds enable

all 3 phases to exist in earth's atmosphere.



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Structure of Water

Also: hydration possible

Hydration

H2O permanent dipol: H2O taken

up by ions

particle

growth

increasingwater content

of particles



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Water on earth exists in all 3 phases


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Phase changes of water:



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Describing atmospheric moisture

Atmospheric water vapor can

be defined in different ways,

including

absolute humidity v,

specific humidity q,

mixing ratio r,

vapor pressure e,

dew point temperature Td, relative humidity RH

Atmospheric MoistureAtmospheric Moisture


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Absolute Humidity v, Specific Humidity q, Mixing ratio r

Specific humidity =

mass of water vapor/total mass of air

Specific humidity q is not affected

by changes in parcel volume.

Vmvv =

a

v

m

mq=

Mixing ratio = mass of watervapor/mass of dry air

usually, q r

d

v

m

mr =

Absolute humidity =

mass of water vapor/volume of air

For a given mass of water vapor

in an air parcel, the absolute

humidity v

changes as the

parcel volume changes

(e.g., lifts or descends).


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Vapor pressure e:

Air molecules all contribute to air pressure p

Each subset of molecules (e.g., N2, O2, H2O) exerts a partial pressure

The vapor pressure, e, is the pressure exerted by H2O vapor molecules in air

the larger the vapor pressure is, the more H2O vapor molecules in air

2-30 mb common at surface


= q qa pp


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In general: water molecules move between the liquid and gas phases


Unsaturated air

Saturation vapor pressure es:


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In general: water molecules move between the liquid and gas phases

Saturated air (e = es):

for every water molecule evaporated into the air, a vapor molecule wouldcondense to liquid water (equilibrium)

in this case: water vapor pressure (e) = water saturation vapor pressure (es)

The air can not hold more water vapor


Unsaturated air Saturated air



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Condensation, Evaporation, Saturation Vapor Pressure (SVP)

Number of molecules escaping particle = number of molecules entering particle

in this case: partial gas pressure = saturation vapor pressure (SVP)

partial gas pressure > SVP SVP > partial gas pressure


EQULIBRIUM

What happens, if there is no equilibrium?


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Note: Water saturation vaporpressure decreases with altitude




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Relationship between temperature and saturation vapor pressure:

Clausius-Clapeyron equation

Saturation vapor pressure es increases exponentially with temperature

At higher T, faster water molecules in the liquid escape more frequently

causing saturation water vapor amount to rise

We sometimes say: Warmer air can hold more water vapor

es [kPa] = saturation vapor pressure

L [J Kg-1] = latent heat of vaporization or deposition

e0 [kPa] = 0.611 kPa at T0=273.15 K

Rv

[J K-1 kg-1] = gas constant for water vapor

T [K] = temperature

= )

11(exp

0

0TTR

Lee

V

s



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Relationship between temperature and saturation vapor pressure:

Clausius-Clapeyron equation


Note:

Es(water) > Es(ice) at all times!

this difference is reason for rain-

drop growth in cumulus clouds

Evaporation

condensation

partial gas pressure > SVP

SVP > partial gas pressure

partial gas pressure > SVP SVP > partial gas pressure


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Dew-Point Temperature Td:

Temperature to which air must becooled to become saturated

(at constant pressure and water vapor content).

Always: Td T

Difference between relative humidity (RH) and dew-point temperature Td:

RH: measure of how close the air is to saturation,

Td : measure of the airs actual moisture content.

The higher Td

, the more water vapor in the air.

Dew point depression: T- Td

The largerthe dew point depression is, the drierthe air is, or the air is

farther away from saturation


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Average surface dew-point temperature (F)

January July


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Td [K] = dew-point temperature

L [J Kg-1

] = latent heat of vaporization or depositione0 [kPa] = 0.611

Rv [J K-1 kg-1] = gas constant for water vapor

T0 [K] = 273.15

1

00 )ln(

1

=

e

e

L

R

TT

sV

d


= )

11

(exp 00

dVs TTR

L

ee

Dew-point temperature Td as function of saturation vapor pressure esThe higher Td, the more water vapor in the air.


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Relative humidity (RH):

The ratio of the actual amount of water vapor in the air compared to the

maximum amount of water vapor the air can hold.

RH [%] = relative humidity

es, qs, s, rs = pressure, specific humidity, absolute humidity,

mixing ratio of water vapor at saturation vapor

pressure

100100100100 ===ssss r

r

q

q

e

eRH



25/43

Relative Humidity Trends

RH indicates air parcel proximity to saturation.

Higher RH does not necessarily mean more water vapor in the air

RH increases by adding more wateror dropping T.

Dew point Td is the temperature at which saturation occurs (i.e. RH=100%).


Td ?

RH=100% ?


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A common misconception:

Air with high RH must have a greater water vaporcontent than air with lower RH.

Which place has more water vapor in the air?

International Fall, Minnesota:

T= - 10oC and RH = 100%

or

Phoenix, Arizona:T= 20oC and RH = 30%


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14.0020

10.0015

7.0010

5.005

3.500

2.00-10

0.75-20

Saturation mixing ratio rs(g/kg)

Temperature

(C)

International Fall:

T = -10 oC; rs = 2 g/kg,

RH = 100% = r / rs

Actual mixing ratio r:

r = RH x rs = 1 x 2 g/kg

= 2 g/kg

Phoenix:

T = 20 oC; rs = 14 g/kg,

RH = 30% = 0.3 = r / rs

Actual mixing ratio r:

r = RH x rs= 0.3 x 14 g/kg

= 4.6 g/kg


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Specific Humidity vs. Saturation


Which environment has

higher water vapor inthe air:

Desert air or polar air?

While RH may be higher

in polar air (or duringwinter-time), more wateris actually absorbed in

desert air (or during

summer).


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Warm air can absorb more vapor than cold

air, so for a given parcel of air, specific

humidity declines from its highest in the

tropics to its lowest in the colder poles.

Desert air is far from saturated, cold polar

air nears saturation.


Specific Humidity(latitudinal distribution)

Relative Humidity(latitudinal distribution)


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Relationship between

specific humidity and relative

humidity and its dependanceon air temperature



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Exercise:

a chilly morning in Houston:

Temperature: 10oC; RH = 100%.

In the afternoon, the air warms to 30oC

Assumption:

Same air mass has been in the area, i.e. themoisture content in the air has not changed

much from the morning to the afternoon.

Would RH increase or decrease in the afternoonfrom the morning?

What would be the RH in the afternoon?


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emorning = RHmorning x es = 100% x es

What is the vapor pressure, e,

in the morning?

at 10C, es = 12 mb

100=

se

eRHHouston morning:

T=10C; RH=100%


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emorning = RHmorning x es = 12 mb


in the morning?

at 10C, es = 12 mb

100=

se

eRHHouston morning:

T=10C; RH=100%


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at 30C, es = 42 mb


emorning = RHmorning x es = 12 mb


in the morning?

at 10C, es = 12 mb

RHafternoon = (emorning/es afternoon) x 100

= (12 mb / 42 mb) x 100

= 28%

100=

se

eRH

In the afternoon:

Houston morning:

T=10C; RH=100%


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Sources of Moisture


Why is the southwest coast of the US hot and dry,while the Gulf coast is hot and moist?

Both are adjacent to large bodies of water

Both experience onshore wind flow on a regular basis


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Sources of Moisture


CoolerPacific waters create

lowerhumidities along the W coast

warmerGulf waters generate high

humidity along the SE and E coast.


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Relative Humidity and Comfort

Unsaturated air (low RH) may absorb more water from the evaporation of

human sweat.

The departure of fast moving, and by definition higher temperature, water

molecules into the vapor phase cools the human skin (effect increases with

decreasing RH).

Air close to saturation (high RH) lowers cooling of human skin due to less

evaporation.



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Heat Index & Safety


Human perception of

temperature is distinct

from measured air

temperature, and is

particularly different at

higher RH when the human

body is less efficient at

sweating and self-cooling.

On hot days, fans that

move saturated air away

from the skin help humans

avoid unwanted heat

syndromes.

Apparent temperature

[Heat Index] takes into

account ambient air

temperature with RH.


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Heat Index & Safety



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Sling Psychrometer (also aspirated Psychrometer available)

Wet bulb temperature indicates how cool a surface will become by

evaporating water into the air. When compared with the dry bulb, or regular air temperature (T), it

indicates relative humidity (RH) and dew point (Td).

Measurement of Atmospheric MoistureMeasurement of Atmospheric Moisture

Why necessary to whirl or aspirate a psychrometer?


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Hair & Other Hygrometers

Human and horse hair becomes roughly 2.5% shorter as RH drops from

100% to 0% (used as the principle of the hair hygrometer). Other hygrometers are based on electrical resistance (humidity changes

resistance), infrared absorption (by water vapor), and dew point

condensation (cooling surface of a mirror until condensation forms).

Measurement of Atmospheric MoistureMeasurement of Atmospheric Moisture

http://catalogue.museogalileo.it/multimedia/Hygrometer.html

S f i t i bl


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Summary of moisture variables

Vapor pressure the pressure exerted by the water vapor moleculesin a given volume of air

Specific humidity the ratio of mass of water vapor in a given

air parcel to the total mass of air in the parcel

Mixing ratio - the ratio of mass of water vapor in a given air

parcel to the total mass of dry air in the parcel

Relative humidity The ratio of the amount of water vapor in

the air compared to the amount of required for saturation

Dew point temperature - Temperature to which air must becooled (at constant pressure and constant water vapor content)

to become saturated

Wet bulb temperature - The lowest temperature that can be

obtained by evaporating water into the air.

Absolute humidity the density of water vapor which is the ratio

of mass of water vapor and the volume of air.

SUMMARYSUMMARY


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SUMMARY

1. Saturation exists when the number of water molecules evaporating from a

liquid equals the number condensing.

2. In our atmosphere, condensation occurs primarily when the air is cooled.

3. Absolute humidity describes the mass of water vapor in a fixed volume of air,

or the water vapor density.

4. The airs actual (water) vapor pressure is an indication of the airs water vapor

content.

5. Relative humidity, expressed as a percent, does not tell us how much water

vapor is actually in the air, rather it tells us how close the air is to being

saturated.

6. Without changing the airs water vapor content, as air cools the relativehumidity increases, and as air warms the relative humidity decreases.

7. The dew-point temperature is a good indicator of the airs water vapor content.

High dew points indicate high water vapor content and vice versa.

8. When the air temperature and dew point are close together, the relative

humidity is high; when they are far apart, the relative humidity is low.

9. Summertime is generally more humid in the eastern half of the United States

because of the air flow off the warm Gulf of Mexico.

10. High relative humidity in hot weather can make us feel it is hotter than it

actually is by retarding the evaporation of perspiration

intro to meteorology course 4a(1)

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