Water cycle in the environment

Role of water in the atmosphere

• Clouds and precipitation (weather)

• Impact on radiative balance

• Impact on thermodynamic processes and vertical stability of the atmosphere through evaporation and condensation.

• Cleaning of the atmosphere

• Participation in chemical processes.

Water cycle in the environment

type volume (km3) share (%)

surface water 230 580 0.0171

underground water

8 406 720 0.625

ice 29 190 000 2.15

water vapor in the atmosphere

12 900 0.001

oceans 1 321 890 000

97.2

sum 1 359 700 000

100.0

Obieg wody w przyrodzie

Moisture Variables

• There are numerous ways to quantify the amount of water vapor in the air.

• 1. Vapor pressure e - partial pressure of water vapor. Also a saturation vapor pressure. This is the fundamental way to measure the amount of vapor.

• 2. Vapor density ρv - defined by equation of state for vapor. Also called absolute humidity

parcel sizeH2O vaporcontent

absolute humidity

2 m3 10 g 5 g/m3

1 m3 10 g 10 g/m3

with the same amount of water vapor in the parcel of air, an increase in volume decreases the absolute humidity, while an decrease in volume increases the absolute humidity

The absolute humidity changes as air rises and descends

• 3. mixing ratio r - mass of water vapor per unit of dry air. Usually expressed in g/kg, but most correct when unitless (i.e. kg/kg). Typical range for globe = 0-25 g/kg.

• 4. specific humidity q – mass of water vapor per unit mass of moist air,

epe

.ep

eRR

mm

rv

d

d

v

6220

pe

.eRRp

eRR

mmm

qvdv

d

vd

v

6220)1(

where: Rd=287.05 J·kg-1·K-1 – gas constant for dry airRv=461.51 J·kg-1·K-1 – gas constant for water vapor

parcel weight

H2O vaporweight

specifichumidity

1 kg 1 g 1 g/kg

1 kg 1 g 1 g/kg

The absolute humidity and mixing ration do

not change as air rises and descends

• The amount of water vapor the atmosphere can ‘hold’ is limited. This limit is given by the saturation vapor pressure, which is an exponential function of temperature only.

• When SVP is equaled or exceeded, vapor moves to the liquid phase and latent heat is released. The possibility of supersaturation exists, but is only observed in atmosphere to a small degree

Reaching Saturation

• There are several different processes that a parcel of air may undergo in order to reach saturation

• These processes define temperatures which can be used to indicate the amount of moisture in the air

What happens at saturation?

• Though not entirely obvious, water begins to condense.

• As vapor condenses, latent heat is released• Lapse rate of parcel changes dramatically.

phase changes of water

• evaporation / condensation (Lp=2462·103 J ·kg-1)

• icing / melting (Lt = 334·103 J ·kg-1)

• sublimation / resublimation (Ls=2834·103 J ·kg-1)

specific heat of water: 4·103 J ·kg-1

Water vapor pressure

The total pressure inside the parcel of air is equal to the sum of pressures of the individual gases. The partial pressure of water vapor is

called the actual vapor pressure e.

Actual vapor pressure is measured in hPa, kPa or mb.

The greatest values of actual vapor pressure are observed near equator, where they can reach 40 hPa. On average at 2 m 20 hPa.

Low values of actual vapor pressure are observed are observed near the poles in winter. Usually it doeas not exceed 2 hPa. Extremely low values were noted on Syberia in Vierchojansk, where during winter the actual vapor pressure can drop even to 0.03 hPa

Relative Humidity

• 5 Relative Humidity RH --Ratio of mixing ratio to its saturation value. Expressed in percent

• This is what many instruments measure

%100Ee

f

Relative Humidity

• most common measure, but not as useful as many others

• Function of two variables – temperature and the amount of water vapor

Virtual Temperature

• 6 Virtual Temperature TV – temperature of dry air having the same density as that of a sample of moist air at the same pressure

Tv = (1 + 0.61q) · T

Dewpoint

• 7 Dewpoint Td - temperature to which moist air must be cooled, holding p and w constant, in order to reach saturation wrt

water.

• ws at dewpoint = w of moist air.

wpAB

pwTT dd /ln,

Water vapor in the atmosphere

Earth surface is a main source of water vapor in the atmosphere. That is why the greatest amount of water vapor is observed in the lower troposphere.

Water vapor content near the surface amounts on average 0.2 % near the poles and 2.5% near the equator.

In some cases in can reach even 4%

Changes of water vapor content with the height

Above boundary layer the water vapor content decreases with the height ezponentialy (faster than other components of air).

At altitude 13-20 km in temperate region the content od water vapor is about 10-6 (mass of water vapor per unit of mass of dry air).

Above 20 km water vapor content increases slightly with the heightnand at 23-50 km the pearl clouds are observed build from ice crystals.

Above 70 km water molecules break up because of solar radiation at wavelenghts 0.1657 m and water vapor content decreases to 10-8 ..

Trace amount of water vapor are observed up to 90 km.

Saturation water vapor pressure

For each temperature there is an amount of water vapor saturated the air – E.

If water vapor pressure becomes greater than saturation water vapor pressure then the condensation occurs. The condensation nucleus are necessary. So sometimes it can happen that e > E.

Saturation water vapor pressure is:- greater for concave surfaces than for the flat

ones - lower for roztworów soli than for pure water

Saturation water vapor pressure

Clausius-Clapeyron equation describes the impact of temperature on saturation water vapor pressure

where E0=6.11 hPa, T0=273 oK.

Ratio L/Rv is different for supercooled water (5423oK) and ice (6139oK) (because of differences in heat of evaporation and heat of sublimation).

that is why saturation water vapor presssure is greater for supercooled water than for ice.

TTR

LEE

v

11exp

00

Mixing ratio and specific humidity at saturation

and

rr

q

1

qq

r

1

Because usually r,q < 0.04 than r q

Absolute humidity of saturated air

Other characteristic of humidity

Saturation deficit, d, it is a difference between the saturation vapor pressure maksymalnym at given temperature E and actual pressure e:

d = E – e

Dew point temperature, (or Td), temperature to which it is necessary to cool the air to saturate it relative to flat surface.

Dew point deficit, , the difference between given temperature T and the dew point temperature Td :

= T – Td

Precipitable water

Precipitable water is the total content of water in the atmosphere in the of the air. It is equal to the layer of water if all the water from the atmosphere condense near the surface. It is measured in kg·m-2 or w mm.

Annual course of precipitable water over Central Poland

Mean values of precipitable water in the period 1958-2003 in [kg/m2] ([mm])

Precipitable water

styczeń

lipiec

Daily course of relative humidity is oposite to the course of temperature with one maximum and one minimum.

Dobowy przebieg różnych charakterystyk wilgotności na stacji Łódź-Lublinek – wartości

uśrednione z 74 letnich dni z pogodą radiacyjną

In daily course of vapor pressure and specific and absolute humidity two maximas and two minimas can be distinguished.

Ee

f

• morning minimum is caused by temperature

• afternoon minimum is caused by convection. This minimum is not observed at the seaside and in the mountains.

Daily humidity course

Annual course of humidity

Annual course of water vapor pressure (as well as absolute humidity, specific humidity and mixing ratio) is parallel to the annual course of temperature; in summer the content of water vapor is the greatest and in winter the smallest. Is is caused by the relation of E (saturation vapor pressure) on temperature.

Annual course of the relative humidity is oposite to the course of temperature. But in monsun regions the relative humidity is much greater during summer than in winter. It is related to different features of air mases approaching these regions in winter and summer.

EvaporationEvaporation takes place were the body change phase from liquid to gasous. It happens in each temperature. The evaporation from plants is called transpiration. Potential evaporation (or evaporative capacity) is the maximum possible evaporation at given temperature, not restricted by the amount of water.

Actual evaporation is an amount of water which really evaporate.

The rate of evaporation Fw is measured in kg·m-2·s-1 or mm·day-1 and is:- proportional to saturation deficit (E-e),- opposite proportional air pressure p,- relate to shape of surface of evaporating body (coefficient A), - related to wind speed (function f(v))

)(vfpeE

AFw

CLOUDS

A cloud is a visible aggregate of tiny water droplets or ice crystals suspended in the air.

criterions of classification

composition

way of developing

appearance

Composition:

Liquid clouds are built from water droplets only. They develop in above 0°C temperatures as well as at temperatures slightly lower than 0°C .

Ice clouds are composed from ice crystals only. They exist in temperatures below -40°C .

Mixed clouds are composed from both water droplets and ice crystals. They exist in temperatures below -0°C but above -40°C .

Appearance:

According to International Cloud Classification ten

principal cloud froms are divided into four groups.

Each group is identified by the height of the cloud's

base above the surface: high clouds, middle clouds

and low clouds . The fourth group contains clouds

showing more vertical than horizontal development.

This system was introduced by Abercromby and

Hildebrandsson who expanded the original Howard

classification.

Cirrus (Ci)

Cirrostratus (Cs)

Cirrocumulus (Cu)

Altostratus (As)

Altocumulus (Ac)

Nimbostratus (Ns)

Stratocumulus (Sc)

Stratus (St)

Cumulus (Cu)

Cumulonimbus (Cb)

Cloud types

high clouds

middle clouds

low clouds

clouds with vertical development

Approximate height of cloud base

polar temperate tropical

high clouds 3 - 8 5 - 13 6 - 18

middle clouds 2 - 4 2 - 7 2 - 8

low clouds < 2 < 2 < 2

Cloud types

CIRRUS

thin wispy cloud blown by high winds into long streamers called mares' tailes. They are build from ice crystals only. They do not give any precipitation.

CIRROCUMULUS

Appear as small, founded, white puffs that may occur individually or in long rows. When in rows they have a rippling appearance that distinguishes them from the silky look of cirrus and sheetlike look of cirrostratus.

CIRROSTRATUS

The thin, sheetlike high clouds that often cover the entire sky. They are so thin that the moon or sum can be seen through them. They are composed from ice crystals, so refract the light passing through them producing a halo.

ALTOSTRATUS

Middle level cloud. It is a grey or blue-grey (never white) cloud that covers oten the entire sky. In thinner parts the sun (or moon) can be dimly visible (translucidus). If the cloud is thick (opacus) than the sun light could not be seen through the cloud.

ALTOCUMULUS

Middle clouds that appear as gray, puffy masses sometimes rolled out in parallel waves or bands.

The sky can be seen between individual particles of the cloud.

NIMBOSTRATUS

Dark gray "wet " – looking cloud layer associated with more or less continuous precipitation. The intensity of precipitation is usually low to moderate. The base of the cloud is usually difficult to define

STRATUS

A uniform grayish cloud that often covers the entire sky. Its base is very low over the groud sometimes resembling the fog. It gives no precipitation or drizzle

nebulosus (St neb – foggy, murky) – foggy, uniform curtain, without any details;

fractus (St fra – frayed)– cloud with unregular shape and frayed appearance

STRATOCUMULUS

A low lumpy cloud layer. It appears in rows , in patches or as rounded masses with blue sky visible betwen individual cloud elements. The color of stratocumulus ranges from light to dark gray.

the processes of saturation:• evaporation, • cooling• mixing

Clouds appear when air becames supersaturated

Usually it happens when air mass ascends and cool.s

Ascending air mass expands and its temperature drops.

When temperature is decreasing, its relative humidity is increasing up to the moment the air becames saturated (relative humidity =100%)

When rising of air parcel continues part of water vapor condences or resublimates.

Water vapor condences on small particles called aerosol. If the aerosol is hydroscopic than condensation or resublimation c an happen at saturation lower than 100%

Convective clouds

Orographic clouds

condensation level

moist air

dry air

condensation level 2

moist air

dry air

condensation level 3

Orographic clouds

500-1000 km

Frontal clouds

Ci

Cs

As

Ns

Convective clouds made by convergence

Fogs

radiative

orographic

advective

from evaporation

Many Deserts Are Located Where a High Percentage of the Precipitation Moisture Originates Over Land – These

Areas are Not Supplied With Much Moisture

Climate feedbacks

• Water vapor feedback

• Ice/snow albedo feedback

• Cloud feedback

Surfacetemperature

AtmosphericH2O

Greenhouseeffect

(+)

Water vapor feedback

Positive feedback loop

Snow/ice albedo feedback

Surfacetemperature

Snow and icecover

Planetaryalbedo

(+)

Another positive feedback loop

What about clouds?

Altitude

10 km

Cumulus/stratus clouds

Cirrus clouds(Thin)

(Thicker)

Some reflection

More reflection

What about clouds?

Temperature

Altitude

10 km

Low and warm

Cumulus/stratus clouds

Cirrus clouds

High and cold

TsTwTc

Ts4

Tw4

Tw4

Tc4

Tc4

What about clouds?

• Cumulus and stratus clouds– Low and warm– Small greenhouse effect– Big effect on albedo These clouds cool the climate

• Cirrus clouds High and cold Large greenhouse effect Smaller effect on albedo These clouds warm the climate

Cloud feedback

• Most models predict that cloudiness should increase as the climate warms– If low clouds increase the most, then the feedback will

be negative– If high clouds increase the most, then the feedback

will be positive

• The balance of evidence suggests that cloud feedback is negative. However, this is highly uncertain, as clouds are sub-grid-scale in size and are therefore difficult to model.

: orbit-net.nesdis.noaa.gov/arad/ gpcp/maps/frontmap.gif