CH. 17 – MOISTURE

Changes of State of Water

Heat = energy Adding heat to an object makes

atoms vibrate fasterHeat ≠ temperature!Temperature = average of all

molecular motion

Change of State of Water

Latent heat – energy added to or released by substance w/out changing temperature

Latent = “hidden”Ex: Beaker w/ ice and water

Humidity

Describes amount of water vapor in air

- water vapor sits in spaces between dry air gases

Saturation = spaces filled to capacity w/ water vapor

Dry Air Analysis (consistent worldwide)

Nitrogen = 78%Oxygen (O2) = 21%

Argon = <1%Carbon Dioxide = 0.035%TOTAL 100%

Water Vapor Capacity Table

Temperature dependent

Hotter air = molecules moving farther apart = more space for water vapor

Relative Humidity

- measures how close air is to saturation w/ water vapor

- expressed as a percentage- is temperature dependent!

Relative Humidity

Mixing ratio - actual amount water vapor by

weight in one kg of air

Examples of humidity problems

Temp (oC) M.R. (g) R.H.1. 25 4 ?2. 10 3.5 ?

True or False?

Air with a higher relative humidity has more water vapor in it than air with a lower relative humidity?

January Temperatures

50% RH in Siberia30% RH in Australia

Relative Humidity

Question:If temp. increases w/out changing

the mixing ratio, what happens to relative humidity?

Factors that change relative humidity

1) Change in air temperatureAssuming no change in mixing

ratio:Increase temp. = __________ RHDecrease temp. = _________ RH

Rel. hum. vs. temp

Factors that change relative humidity

2) Change amount of water vapora) Add water vapor through ____________b) Remove water vapor by ____________

Dew Point

Temperature at which air is saturated without changing the mixing ratio

- air has to ________to reach dew point

Examples of humidity problems

Temp (oC) M.R. (g) R.H. 1. 25 4 20%2. 10 3.5 50%What is dew point of Example 2?

Condensation in the atmosphere

On surface = dewAbove surface = fogAloft = clouds

Necessary conditions for condensation

1) Saturated Air (air at dew point)2) Condensation Nuclei

Ex: dust, salt, smoke, ash“hygroscopic particles”

Saturated air

Two methods for cooling air to dew point:

1) Environmental Lapse Rate- air is cooler the further you are

from Earth’s surface- rate changes daily

Cooling to Dew Point: Method #2

2) Adiabatic Temp. Change- caused by changing air’s volume* No heat (energy) is added or

removed from the air!

Adiabatic Temperature Changes

Air expands = temp. decreasesEx: aerosol spray can

Air compresses = temp. increasesEx: bicycle tire pump heating up

Cloud Formation

As air rises, it expands & coolsDry adiabatic rate = cooling of

unsaturated air- constant 10o C per km

Condensation

Latent heat released by water vapor

Ex: In summer, can “sweats”- latent heat warms can

Cloud Formation

Wet adiabatic rate = cooling of saturated air

- always < 10o C per kmVaries depending on amount of

condensation in cloud(latent heat warms surrounding

air)

Review cloud forming process:

1. Air rises2. Air expands3. Air cools adiabatically

Review cloud forming process:

4. Air reaches dew point- altitude varies based on how

much water vapor is in rising air5. Condensation begins

Review cloud forming process:

* Key to cloud formation is to get large amounts of air to rise!

Mechanisms forcing air to rise:

1) Orographic Lifting - physical barrier to wind

Clouds form on windward side of mountain

As air descends, it compresses & heats up adiabatically

Orographic Lifting

“Rain shadow” deserts on leeward side of mountains

Ex: Basin & Range in Nevada

Mechanisms forcing air to rise:

2) (hot air rises) LocaConvective lifting lly, caused

by intense heating on summer afternoons

- forms cumulus clouds

Mechanisms forcing air to rise:

3) ConvergenceSurface air moves from high

pressure to low pressure (wind)

Mechanisms forcing air to rise:

4) Frontal WedgingCool air acts as a barrier forcing

warmer air to riseRegional cause of clouds

Frontal Wedging

Major factor in Midwest- warm moist air (Gulf of Mexico)- cold dry air (Canada)

Fog

Cloud with its base at or very near the ground

Ex: Radiation fog (ground fog)- fairly clear nights w/ high relative

humidity

Radiation Fog

Air above ground cools rapidly to dew point

- after sunrise, fog evaporates from ground up (“lifts”)

Air Stability

Causes difference in cloud development

Ex: gray layered clouds vs. thunderheads

Stable Air

Air resists vertical displacement- air at surface is colder than

temp. of surrounding air- Force air to riseEx: Orographic lifting

Unstable Air

Air that does not resist vertical displacement

- air at surface is warmer than surrounding air

- begins and continues to rise on its own (ex: “convective lifting”)

Weather Conditions

Stable Air:a) Characteristically sheet-like

layered clouds (stratus)Ppt.- producing = nimbostratusb) light to moderate ppt. of long

duration

Stratus clouds

Weather with stable air:

c) Other phenomena:- poor visibility & haze- fog, sleet, glaze- inversions (smog alerts)

Inversions

Weather Conditions

Unstable Air:a) Clouds w/ considerable vertical

developmentb) Short episodes of heavy ppt. (cumulonimbus clouds)

Cloud of vertical development

c) Good visibility except during rain

Weather w/unstable air

d) Extreme weather phenomena:- lightning & thunder- hail- tornadoes- hurricanes