The atmosphere’s water

Atmospheric Moisture and Precipitation

• Properties of Water

• The Hydrosphere and the Hydrologic Cycle

• Humidity

• The Adiabatic Process

• Clouds

• Precipitation

• Air Quality

Main points for today

• Water’s properties are due to its polar nature

•Water has a very high heat capacity and high heat of fusion

• Warm air can contain more water vapor than cold air

• Relative Humidity and Dew Point Temperature are the most

common ways to express how much water is in the air

•The Environmental Lapse rate is the observed change in air

temperature with change in elevation

• An Adiabatic Process is the expansion or contraction of an air

mass without heat loss or gain

Main points for today (cont)

• Clouds are classified by altitude and shape

• Precipitation can occur because of 1) convection, 2)

orographic lifting, and/or 3) frontal processes

• Air Quality

majority of water occurs as ocean saltwater

remaining water (only 2.8%) is accounted for by glaciers and ice sheets,

and groundwater

only 0.001% occurs in the atmosphere

The Hydrosphere and the Hydrologic Cycle

Figure 4.2, p. 121

the global water

balance

constantly

cycles between

these reservoirs

while the

atmosphere

contains

relatively little

water, it is

responsible for

the largest flow

The Hydrosphere and the Hydrologic Cycle

Figure 4.3, p. 122

Properties of Water• Physical States

– only natural substance that occurs naturally in three states on the earth’s surface

• Heat Capacity

– Highest of all common solids and liquids

• Surface Tension

– Highest of all common liquids

• Latent Heat of Fusion

– Highest of all common substances

• Compressibility

– Virtually incompressible as a liquid

• Density

– Density of seawater is controlled by temperature, salinity and pressure

– Liquid has maximum density at +4oC; solid phase has lower density!

Properties of Water (cont’)

• 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%

Molecular Structure of Water

Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.

water moleculeice

Molecular Structure of Water

Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.

ice

Water can exist

in three states -

solid (ice), liquid

(water), and gas

(water vapor)

Water exists in

the air in the form

of water vapor,

clouds, fog, and

precipitation

Three States of Water

Figure 4.1, p. 121

Energy associated with phase change

80 calories 100 calories 540 calories!

Why does it take so much energy to

evaporate water?

• In the liquid state, adjacent water molecules attract

one another

– “-” charge on O attracted to “+” charge on H

– we call this hydrogen bonding

• This same hydrogen bond accounts for surface

tension on a free water surface

– column of water “sticks together”

Water vapor pressure

• Molecules in an air parcel all contribute to pressure

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

pressure

• The VAPOR PRESSURE, e, is the pressure exerted by water

vapor molecules in the air

– similar to atmospheric pressure, but due only to the water vapor

molecules

– often expressed in mbar (2-30 mbar common at surface)

Water vapor saturation• Water molecules move

between the liquid and gas phases

• When the rate of water molecules entering the liquid equals the rate leaving the liquid, we have equilibrium

– The air is said to be saturated with water vapor at this point

– Equilibrium does not mean no exchange occurs

How do we express the amount of water vapor in

an air parcel?

• Absolute humidity

– mass of water vapor/volume of air (g/m3)

– changes when air parcel volume changes

• Specific humidity

– mass of water vapor/mass of air (g/kg)

• Mixing ratio

– mass of water vapor/mass of dry air (g/kg)

• Specific humidity and mixing ratio remain constant as long as water vapor is not added/removed to/from air parcel

• Dew point temperature– the temperature at which the air parcel would be saturated

Expressing the water vapor pressure

• Relative Humidity (RH) is ratio of actual vapor pressure to saturation vapor pressure

– 100 * e/eS

– Range: 0-100% (+)

– Air with RH > 100% is supersaturated

• RH can be changed by

– Changes in water vapor content, e

– Changes in temperature, which alter eS

Humidity Facts

• Humidity is the amount of water vapor in the

atmosphere

• Warm air can hold much more than cold air

• Cold dry air can have close to 0%

• Warm tropical air may have 4-5%

• Two ways to describe humidity (specific humidity and

relative humidity)

Specific Humidity

the actual quantity

of water vapor in

the air

expressed as

grams of water

per kilogram of air

(g/kg)

used to describe

the water content

of large air

masses, and how

it varies by latitude

Figure 4.5, p. 124

Relative Humidity

• a measure of the amount of water vapor

present in air relative to the maximum amount

that the air can hold at a given temperature (%)

• e.g. if relative humidity is 50%, then it contains

1/2 the amount of water vapour it could hold at

a given temperature

• relative humidity decreases as temperature

increases

Relative Humidity and Temperature

if no water vapour is added or removed from the air mass,

then relative humidity decreases as temperature increases

Figure 4.7, p. 125

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Dew Point Temperature

• as air is cooled it eventually becomes saturated (100% relative humidity)

• the temperature of saturation is called the dew point temperature

• if cooling continues, condensation begins and dew forms

Relationship between eS and T

• The saturation vapor pressure of water increases with temperature

– At higher T, faster water molecules in liquid escape more frequently causing equilibrium water vapor concentration to rise

– We sometimes say “warmer air can hold more water”

• There is also a vapor pressure of water over an ice surface

– The saturation vapor pressure above solid ice is less than above liquid water

Water vapor saturation and atmospheric

pressure; at what Temperature does water boil

Dewpoint Temperatures

• Dewpoint temperature is a measure of the water vapor content of the air• It is not a measure of temperature!

The Adiabatic Process• when a gas expands, its volume increases and its

pressure and temperature decrease

• air temperature change solely as a result of air

expansion or contraction is a result of the adiabatic

process

• the adiabatic lapse rate is used to quantify how the

temperature of air decreases as it rises or increases

as it ascends

• lapse rates differ for dry (unsaturated) and wet

(saturated) air masses

Dry Adiabatic Lapse Rate

Dry Adiabatic

Lapse Rate

(DALR)

- decrease in

temperature

with altitude:

10˚C/1000m

Figure 4.10, p. 127

Wet (Saturated) Adiabatic Lapse Rate

• ranges from 4 to 9 degrees C per 1000

meters

• varies because it depends on temperature,

pressure and water vapour content

• less than the DALR because as water

condenses it releases latent heat, so the

temperature decrease is less

Wet Adiabatic Lapse Rate

as a parcel of air rises, it cools and becomes saturated at the dew point

dew point lapse rate (1.8 degrees per 1000 meters) means that the dew point of the air parcel decreases as the air rises

when it reaches its dew point, condensation occurs (lifting condensation level)

Figure 4.10, p. 127

Water vapor is distributed

throughout the atmosphere

• Generally largest amounts are found close to

the surface, decreasing aloft

– Closest to the source - evaporation from ground,

plants, lakes and ocean

– Warmer air can hold more water vapor than colder

air

Stability & Instability

A rock, like a parcel of air, that is in stable equilibrium will return to its original position when pushed.

If the rock instead accelerates in the direction of the push, it was in unstable equilibrium.

Stability in the atmosphere

Stable Unstable Neutral

If an air parcel is displaced from its original height it can:

Return to its original height - Stable

Accelerate upward because it is buoyant - Unstable

Stay at the place to which it was displaced - Neutral

An Initial

Perturbation

Vertical Motion and Temperature

Rising air

expands, using

energy to push

outward against its

environment,

adiabatically

cooling the air

A parcel of air

may be forced to

rise or sink, and

change

temperature

relative to

environmental air

“Environmental Lapse rate”

• The lapse rate is the change of temperature with height in the atmosphere

• There are two kinds of lapse rates:

– Environmental Lapse Rate (~ 4°/1000 m)

• What you would measure with a weather balloon

– Parcel Lapse Rate

• The change of temperature that an air parcel would experience when it is displaced vertically

• This is assumed to be an adiabatic process (i.e., no heat exchange occurs across parcel boundary)

The Adiabatic Process• when a gas expands, its volume increases and its

pressure and temperature decrease

• air temperature change solely as a result of air

expansion or contraction is a result of the adiabatic

process

• the adiabatic lapse rate is used to quantify how the

temperature of air decreases as it rises or increases

as it ascends

• lapse rates differ for dry (unsaturated) and wet

(saturated) air masses

Dry Adiabatic Lapse Rate

Dry Adiabatic

Lapse Rate

(DALR)

- decrease in

temperature

with altitude:

10˚C/1000m

Figure 4.10, p. 127

Wet (Saturated) Adiabatic Lapse Rate

• ranges from 4 to 9 degrees C per 1000

meters

• varies because it depends on temperature,

pressure and water vapour content

• less than the DALR because as water

condenses it releases latent heat, so the

temperature decrease is less

Wet Adiabatic Lapse Rate

as a parcel of air rises, it cools and becomes saturated at the dew point

dew point lapse rate (1.8 degrees per 1000 meters) means that the dew point of the air parcel decreases as the air rises

when it reaches its dew point, condensation occurs (lifting condensation level)

Figure 4.10, p. 127

Conditionally unstable air

• What if the environmental lapse rate falls between the moist and dry adiabatic lapse rates?

– The atmosphere is unstable for saturated air parcels but stable for unsaturated air parcels

– This situation is termed conditionally unstable

• This is the typical situation in the atmosphere

Clouds

• Made up of water droplets and/or ice particles

• form when air is saturated AND contains

particles (condensation nuclei) e.g. dust, salts

• water can remain in liquid state below freezing

(supercooled) to as low as -12°C (10°F)

high (eg. cirrus)

middle (eg. altocumulus)

low (eg stratus, cumulus)

Clouds types - classified by altitude

thunder cloud (cumulonimbus)

extends from low to high

Figure 4.11, p. 128

Fog

• cloud layer at or close to the Earth’s surface

• radiation fog forms at night when air near the ground falls below the dew point temperature

• advection fog forms when warm moist air moves over a cool surface

• sea fog forms when cool marine air comes in contact with cold ocean currents

Precipitation

Precipitation formation requires:

• growth of droplets in clouds

• ice crystal process - ice particles act as freezing nuclei

• coalescence process - large droplets collide with smaller ones and coalesce

• produced in clouds well below the dew point temperature

• usually near cloud tops

• all precipitation begins as frozen water

• if it reaches the ground in liquid form -rain, drizzle (small drops)

Precipitation

Types of Precipitation:

• freezing rain (ice crystals freeze onto a frozen surface)

• snow (ice crystals have not melted)

• sleet (ice crystals melt as they fall)

• hail (melting and refreezing crystals that form in thunder clouds)

Precipitation: three mechanisms

Convectional

Orographic

Frontal (cyclonic)

1.) Convectional

warm air rises

cools to dew point

temperature - clouds

form

latent heat release

adds energy and

increases updraft

may produce

thunderstorms

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2.) Orographic lifting

air rising over a highland

intercepting slope =

windward slope (wetter)

change in temperature

alters the humidity

leeward

slope (drier)

(rain shadow)

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where air masses with different temperatures come together

warm air lifted by cold dense air along a weather front

leads to frontal precipitation

3.) Frontal (cyclonic) precipitation

Cold air Warm air

Thunderstorms are

intense convectional

storms associated

with massive

cumulonimbus clouds.

They may produce

heavy rains, hail,

thunder, lightening,

and intense

downdrafts

(microbursts) which

may create hazards

for humans

Figure 4.21, p. 137

Air Quality

Air pollutants are undesirable gases, aerosols, and particulates injected into the atmosphere by human and natural causes

Figure E4.1, p. 143

Smog over China

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Cloud type summary

High Clouds• High clouds

– White in day;

red/orange/yellow at sunrise

and sunset

– Made of ice crystals

– Cirrus

• Thin and wispy

• Move west to east

• Indicate fair weather

– Cirrocumulus

• Less common than cirrus

• Small, rounded white puffs

individually or in long rows

(fish scales; mackerel sky)

– Cirrostratus

• Thin and sheetlike

• Sun and moon clearly

visible through them

• Halo common

• Often precede

precipitation

Cirrus

Cirrus

Cirrus Display at Dawn

Cirrocumulus

Cirrocumulus

Cirrocumulus at Sunset

Cirrostratus

Cirrostratus with Halo

Contrails

Middle Clouds

• Altocumulus

– <1 km thick

– mostly water drops

– Gray, puffy

– Differences from cirrocumulus

• Larger puffs

• More dark/light contrast

• Altostratus

– Gray, blue-gray

– Often covers entire sky

– Sun or moon may show through dimly

• Usually no shadows

Altostratus

Alto Stratus Castellanus

Altocumulus

Altocumulus

Alto Cumulus Radiatus

Alto Cumulus

Alto Cumulus Undulatus

Low Clouds

• Stratus

– Uniform, gray

– Resembles fog that does not reach the ground

– Usually no precipitation, but light mist/drizzle possible

• Stratocumulus

– Low lumpy clouds

– Breaks (usually) between cloud elements

– Lower base and larger elements than altostratus

• Nimbostratus

– Dark gray

– Continuous light to moderate rain or snow

– Evaporating rain below can form stratus fractus

Stratus fractus

Looking down on an eastern

Atlantic stratus deck

Stratiform cloud layers

Stratocumulus cloud streets

Stratus undulatus

Stratus

A Layer of Stratocumulus

Cloud viewed from above

Vertically

“developed”

clouds• Cumulus

– Puffy “cotton”

– Flat base, rounded top

– More space between cloud elements than stratocumulus

• Cumulonimbus

– Thunderstorm cloud

– Very tall, often reaching tropopause

– Individual or grouped

– Large energy release from water vapor condensation

Cumulonimbus with Pileaus caps

Cumulonimbus Clouds Spawn Tornadoes

The Earth’s Hydrologic Cycle