Chapter 7Water & Atmospheric

Moisture

Hydrological Cycle

Hydrogen Bonding

Credit: http://bio.winona.msus.edu/berg/ILLUST/H-bond5.gif

Water is a polar or charged molecule

+ charge on the hydrogen side & a

- charge on oxygen

hydrogen bonding is the attraction between oxygen

and hydrogen atoms on adjacent

molecules

polar covalent bonds

Phenomena due to hydrogen bonding

surface tension – cohesive force that pulls a surface closer together – water has highest

surface tension

capillary action - ascent of water up a tube thanks to the ‘pull’ of water molecules on one

another

Phenomena due to hydrogen bonding

Solvent – Water is an excellent solvent as it can electrically “surround a salt”

http://www.brooklyn.cuny.edu/bc/ahp/SDPS/SD.PS.water.html

720 calories required -->

<-- 720 calories released

Latent Heat Exchanges in the Environment

Show water phases shockwave

Measures of Humidity humidity - refers to water vapor

in airrelative humidity - ratio of the water vapor

content of air to the maximum possible water vapor content at that temperature

specific humidity - mass of water vapor (g) divided by the mass of air (kg) – independent of changes in pressure,

volume, and temperature

dew point temperature - temperature at which air is saturated with water vapor

water vapor capacity is set by air temperature

Saturation Vapor Pressure vs T

0

10

20

30

40

50

60

70

80

-20 -15 -10 -5 0 5 10 15 20 25 30 35 40Temperature (C)

SVP

(mba

r)

How much water can air hold?

saturation vapor pressure is the maximum amount of water air can

holdrule of thumb - slope is ~ 1.5

mbar/°C in typical ambient range

think of this like the amount of sugar you can dissolve in coffee or tea before it precipitates

out

air in the tropics can hold about 10x as much water vapor as air at the

poles

the saturation vapor density (or pressure) roughly doubles for

every 10 C increase

Figure Credit: “Earth’s Climate” by W.

Ruddiman

Specific &

Relative Humidity

RH = (SH/max SH)*100

RH = (20/28)*100 = ~71%

Diurnal Variations in Relative Humidity

temperature & RH can be anti-correlated

-if the specific humidity is

constant & daily heating occurs

Dew Point

if dew point is close to the ambient air

temperature,

relative humidity is very

high

Saturation Vapor Pressure vs T

0

10

20

30

40

50

60

70

80

-20 -15 -10 -5 0 5 10 15 20 25 30 35 40Temperature (C)

SVP

(mba

r)

Tdew

Dew Point Temperature

air is saturated when its water vapor content equals its water holding capacity

(RH = 100%)this temperature is the dew

point RH < 100%

Measuring Relative Humidityhair absorbs water

vapor and this increases its length

think about how your hair gets in

very humid climates

this system works so well that it was used for over 200

years!

Measuring Relative Humidity IIsling psychrometer

the principles of sling psychrometry

2 thermometers side by sideone is a ‘dry-bulb’

thermometer which measures ambient

temp.other is a ‘wet bulb’ thermometer that is wetted by a wick -

twirling drives evaporation and

depression of temp.

would the wet bulb temperature be

more depressed in a dry or a wet environment?

Atmospheric Stabilitythink of a packet or parcel of air to describe a mass with certain humidity and

temperature characteristics

stability refers to the tendency of an air

parcel to rise or sink

air rises because pressure

decreases with height -

rising air expands and does work, releasing

heatThis is called

adiabatic heating

DAR – Dry Adiabatic Rate

DAR – Dry Adiabatic Rate

Different kinds of lapse rates

Air temperature (Tair)

Altit

ude

(km

)

DAR ~ -10°C/km

MAR ~ -6°C/km

Avg lapse rate ~ -6.5°C/km

this is the environmental lapse rate

we discussed

condensation releases heat (~2500 J/g at

0°C)

DAR - dry adiabatic rateMAR - moist adiabatic

rate

environmental lapse rate (ELR) - the local, background change in air temperature with

heightunstable air - describes an air parcel that is more buoyant than its surrounding air (will

rise) stable air - describes an air parcel that is the same temperature or colder than

surrounding air (will fall or stay at the same elevation)

moist adiabatic rate (MAR) – the rate at which saturated air temperature

decreases with height

dry adiabatic rate (DAR) – the rate at which dry air temperature decreases with

height

we think of three ‘regimes’ of

stability

rising parcel cools faster than surrounding

air (it stays more dense)

rising parcel cools more slowly than surrounding

air (it stays more buoyant)

surrounding air cools at 12 °C/km

buoyancy>gravityUnstable air

surrounding air cools at 5 °C/km

buoyancy<gravityStable air

surrounding air cools at 7 °C/km

buoyancy<gravity if DAR

buoyancy>gravity if MAR

Conditionally unstable air

Shockwave of stability

Precipitation across the Sierras

Credit: S. Booth, Sierra College http://geography.sierra.cc.ca.us/booth/Physical/chp6_precip/oro_precip.jpg

what about temperature gradients?

Elev

atio

n (k

m)

lapse rate type

25 C

15 C

9 C

19 C

29 C

DAR MAR DAR DAR

1 km

2 km

air is compressed and is heated during descent

air temperature on the east side is greater

cross section of the Sierras

DAR

DARMAR

Elev

atio

n (k

m)

Temperature

LCL

LCL = lifting condensation level - the height at which an air parcel reaches 100% RH and

condensation occurs

Cloud formation

how do clouds form?

what are clouds?aggregations of suspended microscopic

water droplets and ice crystals

air must be saturated with water vapor (RH = 100%) and there must be

microscopic nuclei for the vapor to condense onto

a typical raindrop is 2 mm in diameter, or ~a tenth of an inch

How much does a cloud weigh?

this cloud weighs ~1000 metric tons (106 kg)

how much energy was released when water vapor condensed into all these

cloud droplets?condensation releases 585 calories/g or

~2500 J/gThis is 2.5 trillion Joules!!

(for comparison, a 100 W light bulb consumes only 360,000 Joules in an

hour)

raindrop formation

snow formation

Cloud Types and Identificationclouds are flat (stratus),

puffy (cumulus), or wispy (cirrus)

horizontal, layered clouds are stratiformvertically developed

clouds are cumuliform

high, wispy clouds are cirroform

altocumulus clouds

stratus clouds

Cirrus clouds

Cumulus clouds

Cumulonimbus clouds

photo of a cumulonimbus

cloud taken from the space

shuttle

high level winds shear the tops of these clouds, giving them an

anvil appearance

Fog types and fog formationfog is clouds in contact with the

surface

there are, of course, different kinds of fog, but the basic mechanism of fog formation is the

same

if you are right in the middle of a cloud/fog bank, what is the relative

humidity?ok, what does this tell you about the ambient temperature and the dew point

temperature?fog is confined to a restricted layer - this

is because it is ‘capped’ by a thermal inversion

Advection Fogthis is an especially common type of fog

along the CA coastSanta Barbara locals refer to the early summer fog banks as the “June gloom”

the name gives a big hint about how this kind of fog is formed - remember that

advection is simply horizontal mixing of air in response to pressure gradients

think about what would set up a horizontal pressure gradient between the

coast and offshore air

along the CA coast, most of the fogbanks are advection fog - as the central valley

warms up, air rises and a pressure gradient is establishedwarm,

moist air offshore is pulled to the coast and fog

condenses over the

cold waters of

the CA current

Evaporation Fog

this fog forms when cold air overlies a warm body of water, like this lake in the

Sierras

the warm lake is evaporating water which then comes into contact with the cold air and

condensation occurs

Upslope Fogthis fog forms when warm, moist air is uplifted

by a mountain - it cools to its dew point and condenses tropical montane cloud forests occur where this

condensation occurs

Radiation Fog

this kind of fog forms over land that undergoes nighttime radiative cooling,

bringing the overlying air to its dew point

Fog Harvesting for Water by Human Communities

the idea is to strip water droplets out of fog banks and collect the water in containers

for later human usethe amount of

water that can be harvested is

surprisingly large

Plants Also Harvest Fog for WaterRedwood trees get much of their water from

fog