Weather and Climate - Atmospheric and Oceanic Sciences

aos.wisc.edu

Weather and Climate - Atmospheric and Oceanic Sciences

Notes

• Enter forecasts this week for TUS (Tucson,

AZ) before 6PM

• You may be removed from the contest (by

the managers) if you consistently make

forecasts that are ‘extreme’

• Continue to fill out forecasting logs (for

extra credit)

• Reading: Martin: begin Section 3


Weather and Climate

(Lecture 11)

Instructor: Prof. Michael C. Morgan

Teaching Assistant: Dianna N. Nelson


During deposition, water changes

phase from _____ to _____.

A. vapor, solid

B. vapor, liquid

C. solid, vapor

D. liquid, vapor


Which of the following is a process that

involves the absorption of latent heat?

A. Deposition

B. Condensation

C. Sublimation

D. Freezing


How does a moist surface in the morning

affect the high temperature for the day?

A. Raises it

B. Lowers it

C. Doesn’t have any affect


During evaporation, water changes

phase from _____ to _____.

A. vapor, solid

B. vapor, liquid

C. solid, vapor

D. liquid, vapor


The average albedo of the Earth is…

A. 10%

B. 30%

C. 70%


Energy Balance

• The earth and its atmosphere combined

send off to space just as much energy

as they receive.

96

117


Water in the Atmosphere

• Water comprises 0-4% of the earth’s

atmosphere

• Water is important to the earth’s energy

budget because of:

– Latent heat release

– Greenhouse gas

• Water has a high heat capacity


Ideal (Perfect) Gas Law

Gas constant (J/(kg K))

p = ρ RT

pressure (Pa)

density kg/m 3

Temperature (K)


Phase changes that release latent heat

•Condensation

Vapor => Liquid

•Freezing

Liquid => Solid (Ice)

• Deposition

Vapor => Solid (Ice)


Phase changes that absorb latent heat

• Evaporation

Liquid => Vapor

• Melting

Solid (Ice) => Liquid

• Sublimation

Solid (Ice) => Vapor


Phase Changes

• If you leave a glass of water sitting out for

many days, what happens to the water

level in the glass?

….does it go up?

…..does it go down?

…..does it remain constant?


Phase changes

• What happens if you cover a glass of

water with plastic wrap and let it sit out

for many days?

…….does the water level go up?

…….does it go down?

…….does it stay the same?

When evaporation balances condensation……..

the air is said to be saturated with water vapor.


What determines the rate of

evaporation?

•Wind

• Temperature of the Water

What determines the rate of

• Air temperature

condensation?


Some vapor

condenses into

clouds, and then falls

as precipitation

Hydrologic Cycle

Wind transports water vapor

to other regions

Precipitation lands

on ground and in

water ways.

Water evaporates

back into vapor

Water evaporates from

oceans

Water runs off land into oceans


Hydrologic Cycle

Precipitation lands on

ground and in water ways.

Water evaporates back

into vapor

Some vapor condenses

into clouds, and then falls

as precipitation

Wind transports water

vapor to other regions

Water runs off land into

oceans

Water evaporates from

oceans


Humid air weighs more than dry air.

A. True

B. False


Water vapor is a gas in the

atmosphere, just like Nitrogen and

Oxygen


Weight of Dry Air

• Oxygen (O 2 )

– Molecular weight = 32

– % by volume = 21%

– 32 x .21 ~ 6.72

• Nitrogen (N 2 )

– Molecular weight = 28

– % by volume = 78%

– 28 x .78 ~ 21.84

• Dry air

– Molecular weight ~ 21.84 + 6.72 = 28.56


Weight of Moist Air

• Water Vapor (H 2 O)

– Molecular weight = 18

– % by volume = 4%

– 18 x .04 ~ .72

• Dry Air

– Molecular weight = 28.56

– % by volume = 95%

– 28.56 x .95 ~ 27.132

• Moist Air

Moist air weighs

less!!!!

– Molecular weight ~ 27.132 + .72 = 27.852


How can we measure water

vapor in our atmosphere?


Vapor Pressure


Relative Humidity

• The relative humidity can be changed by

two methods:

1. Change the air’s water vapor content

2. Change the temperature

• Relative humidity does NOT give you an

idea of the actual amount of water vapor

in the air.


Relative Humidity

RH = (w/w s ) x 100%

=(e/e s ) x 100%

• Example: e= 12 mb, e s = 48mb

– RH = 25%


Which has drier air?

• The Sahara Desert

– Air temp = 35 o C

– Dew point = 5 o C

– Relative humidity = 16%

• Antarctica

–Air Temp = -2 o

C

–Dew Point = -2 o

C

– Relative humidity = 100%


Relative Humidity vs. Specific Humidity


What is the relative humidity in

your home like?

• Do you think it is greater or less than that

of a desert?

• Where does the air in our homes come

from?


Moisture Variables

• Absolute measures: depend only on the

moisture content

• Relative measures: depend on both the

water content and the temperature


Relating Variables

T => e s

T d => e


What does it mean when

people say that the weather in

Arizona isn’t bad because it is

a dry heat?


Humidity and People

• Evaporation is a cooling process for

human skin.

• The less humid the air is, the easier it is

for sweat to evaporate.

• The skin can become as cool as the wet

bulb temperature.


Heat Index

• This index is used by the NWS.

• Combines ambient temperature with relative humidity.

• Determines an “apparent” air temperature, or what the

temperature “feels like” to the average person.


Measuring Humidity

• Hygrometers measure

humidity.

– Hair Hygrometer

– Electric Hygrometer

– Infrared Hygrometer

– Dew Cell

– Dew Point Hygrometer

– Sling Psychrometer

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