Discussion

Water and Atmospheric Moisture

I. Introduction - (What causes precipitation?): One of the fundamental questions we will answer in the next few sections is " What causes precipitation?" This is a complex and involved process, but involves some major, important steps. Start at the bottom…

II. Distribution of Water on Earth: See chart in book; notice that most water (by far) is in the oceans. The main point here is that oceans have an enormous effect on Earth’s climate and weather.

III. Changes of Phase of Water: Water exists in three states, and routinely changes between these states. This is called “change of phase.” Heat energy (called latent heat) is either absorbed or released during the changes of phase of water.

Change of Phase of Water

A. Hydrogen Bonds: The hydrogen atoms in water molecules are naturally attracted to each other by a phenomenon called hydrogen bonding. Ice has incredibly strong hydrogen bonds which progressively break as melting and evaporation occur. Keep in mind that it takes energy to break these bonds and keep them broken.

B. Latent Heat (or “Hidden” Heat): Latent heat is the energy which breaks the bonds. It holds the water and vapor molecules apart.

1. Melting and Evaporation (called cooling processes):

-Hydrogen Bonds break during these processes!

-The water must absorb latent heat in order to break the bonds and keep them broken. We say that the latent heat is “hidden” in the water in this process.

-Because the heat energy must be “taken” from the air or land in order to break the hydrogen bonds, the air or land cool down.

-For this reason these processes are called “cooling processes”.

2. Condensation and Freezing (called warming processes - sometimes):

-Hydrogen Bonds formed during these processes!

-The water must release (or let go of) the latent heat in order to form the bonds.

-Since the energy is “given” to the air or land, warming occurs.

-For this reason these processes are called “warming processes”.

IV. Humidity: Defined as the water-vapor content of the atmosphere.

A. Saturation (filled to capacity w/ water vapor): Air has a finite (limited) capacity to hold water vapor. When air is filled to capacity with water vapor, it is said to be saturated.

B. Relationship Between Saturation & Temperature: The most important thing to remember is that warm air holds more water vapor at when it is saturated than cold air!!

As Temperature Increases................ capacity to hold water vapor increases. Warm air can hold more water vapor.

As Temperature Decreases............... capacity to hold water vapor decreases. Cold air can hold less water vapor.

C. Specific Humidity: An important measure of water vapor content. Measured as grams of water vapor per kilogram of air. This is an actual (absolute) measure of water vapor in the air.

D. Relative Humidity: Relative humidity is the percentage of the observed (actual) amount of water vapor in the air compared to the maximum it could hold when saturated.

E. . Calculating Relative Humidity:

Observed Specific Humidity (what it’s actually holding)	x 100	= % Relative Humidity
Maximum Specific Humidity (what is could hold when saturated)

PROBLEM (calculating relative humidity) Use the graph and formula provided on the previous page to calculate the relative humidity in the three scenarios below. Assume that the observed specific humidity is 10 grams H2O vapor per kilogram of air in each scenario (this value is already written into the problem). Start by using the temperatures provided with the graph on the previous page to determine the maximum specific humidity for each problem. Then calculate the relative humidity.

Scenario #1	Scenario #2	Scenario #3
37(C (99(F)	24(C (75(F)	13(C (56(F)
10 gm/kg	X 100	10 gm/kg	X 100	10 gm/kg	X 100
?		?		?
= ? %	= ? %	= ? %

1. Relationship Between Relative Humidity & Temperature (see problem above)

(As the temperature increases Relative Humidity decreases. This is because the capacity increases while the specific humidity does not.

( As the temperature decreases Relative Humidity increases toward 100%.

2. Dew point (measured in degrees Celsius): The temperature at which the relative humidity reaches 100% (temperature at which air is saturated) is called the dew point. High dew points mean moist air, while low dew points mean dry air…

V. Atmospheric Stability (Adiabatic Heating and Cooling)

A. Physics Review: "An important principle of physics is that when a gas is allowed to expand, its temperature drops. Conversely, when a gas is compressed, its temperature increases."

B. Dry Adiabatic Heating & Cooling (NO clouds present; rate is + or - 10C°/1000m or 5.5F° /1000 ft.)

C. Moist Adiabatic Heating and Cooling (clouds present)

(NOTE: Moist Adiabatic Rate (MAR) = approximately 6C°/1000m or 3.3F° /1000 ft)

(Similar to the process described above.

(As the parcel rises and cools, it eventually reaches the level of condensation.

(The process of condensation will release some latent heat.

(The result is that the rising parcel continues to cool, but not quite as fast (because latent heat is being added to the parcel through condensation).

Special Review

Normal Lapse Rate: … is the average temperature decrease of the surrounding air as altitude increases (6.4C°/1000m or 3.5F°/1000ft) Environmental Lapse Rate: … is the actual/observed temperature decrease of the surrounding air as altitude increases Moist and Dry Adiabatic Rates: … ONLY refers to temperature changes inside ascending and descending air parcels

D. Stability of Air: To rise or not to rise; that is the question… (if you are a parcel)

1. Stable Conditions: Occurs when a rising parcel of air is colder (and more dense) than the surrounding air. It will resist rising (and often sink!), which means that clouds will generally not form.

2. Unstable Conditions: Occurs when a rising parcel of air is warmer (and less dense) than the surrounding air. This causes air parcels to actively rise! This is called buoyancy (the tendency of something to rise in a fluid)!!!

3. Three Basic Causes of Unstable Conditions: Includes almost any condition where warm, moist air is forced or pushed upward.

1. warm, moist air moving across a warm surface

2. warm, moist air pushed into and over a mountain

3. warm, moist air colliding with cold dry air

Stability Problem #1: Calculate the values for surrounding air temperature assuming that the environmental lapse rate is 12C(/1000m. Then calculate the temperature of the parcel using the dry adiabatic rate (see previous notes). Are these stable or unstable conditions??? Why???

Stability Problem #2: Calculate the values for surrounding air temperature assuming that the environmental lapse rate is 5C(/1000m. Then calculate the temperature of the parcel using the dry adiabatic rate (see previous notes). Are these stable or unstable conditions??? Why???

Sample Questions: Questions similar to these will be on your exam. As you study you should anticipate how I might use these questions to create new questions on the same concepts.

1	What is the role of latent heat in the change of phase of water from solid to liquid to gas? A. it stretches the water molecules B. it breaks the hydrogen bonds C. it stabilizes the electrons D. it solidifies the oxygen bonds
2	A parcel of air at 50% relative humidity is holding 4 grams H20 / kilogram of air. What could the parcel hold if it were completely saturated? A. 4g/kg B. 5g/kg C. 8g/kg D. 16g/kg E. 24g/kg
3	A parcel of air at 25% relative humidity is holding 2 grams H20 / kilogram of air. What could the parcel hold if it were completely saturated? A. 2g/kg B. 4g/kg C. 6g/kg D. 8g/kg E. 10g/kg
4	Fill the blank in this sentence: In unstable conditions, a parcel of air will ___________________ than the surrounding air. A. fall because it is colder B. fall because it is warmer C. rise because it is colder D. rise because it is warmer
5	In what units is dew point measured?
6	What will cause instability (unstable conditions)?
7	Assume a parcel of air at 2000 meters elevation is 20(C. Now assume that this parcel is forced upward 1000 meters to 3000 meters elevation. If the surrounding air temperature is 11(C at 3000 meters elevation, are these conditions stable or unstable? (Hint: There are no clouds present in this scenario.) A. Stable, because the parcel is warmer than the surrounding air. B. Unstable, because the parcel is warmer than the surrounding air. C. Stable, because the parcel is cooler than the surrounding air. D. Unstable, because the parcel is cooler than the surrounding air.
8	Assume a parcel of air at 1000 meters elevation is 30(C. Now assume that this parcel is forced upward 2000 meters to 3000 meters elevation. If the surrounding air temperature is 4(C at 3000 meters elevation, are these conditions stable or unstable? (Hint: There are no clouds present in this scenario.) A. Stable, because the parcel is warmer than the surrounding air. B. Unstable, because the parcel is warmer than the surrounding air. C. Stable, because the parcel is cooler than the surrounding air. D. Unstable, because the parcel is cooler than the surrounding air.
9	Possible Essay Question: As air rises it expands and cools. Explain this process more fully.
10	Possible Essay Question: Condensation results from cooling the air to the dew point. Explain this process more fully.

GOOD NEWS!!! The practice questions above - plus many, many more – can be found in the online practice quizzes discussed in your syllabus and in class. You can take each quiz multiple times, and each time you will get some new questions. Once submitted, the quizzes are graded automatically, with the correct answers provided immediately. This is a great way to prepare for the exams!!!

When there is a net surplus of radiation at the surface of Earth, energy (called latent heat) is available to evaporate water. It is this water vapor which eventually becomes precipitation.

Lifting is a critical step in the precipitation process. In nature there are three basic lifting mechanisms:

Orographic Lifting (air is pushed up a mountain)

Frontal Lifting (cold air pushes up warm air)

Convectional Lifting (air heats, expands, and rises)

As air is cooled, its capacity to hold water vapor decreases, and its relative humidity increases. Eventually, the temperature drops to the dew point, the air becomes saturated, and the relative humidity reaches 100%. At this point condensation occurs (vapor becomes liquid).

Adiabatic cooling occurs when a parcel of air is lifted upward. The lifting parcel expands due to a decrease in the surrounding atmospheric pressure. This results in decreased molecular collisions, decreased friction and decreased temperature of the parcel.

Not all clouds produce precipitation, but if conditions are right raindrops, hail, or snowflakes will form and fall.

PRECIPITATION

F O R M A T I O N

CONDENSATION

(Cloud Formation)

A D I A B A T I C

C O O L I N G

L I F T I N G

E V A P O R A T I O N

Water at Earth’s Surface

ICE

melting

freezing

condensation

evaporation

sublimation

sublimation

LIQUID WATER

WATER VAPOR

Fig. 5-12, pg 155 (Christopherson, 6th ed.)�Fig. 5-9, pg 147 (Christopherson, 7th ed.)�Fig. 5-10, pg 151 (Christopherson, 8th ed.)

CRITICAL CONCEPT REVIEW

As air is cooled, its capacity to hold water vapor decreases, and its relative humidity increases. Eventually, the temperature drops to the dew point, the air becomes saturated, and the relative humidity reaches 100%. At this point condensation occurs (vapor becomes liquid).

CRITICAL CONCEPT REVIEW

Adiabatic cooling occurs when a parcel of air is lifted upward. The lifting parcel expands due to a decrease in the surrounding atmospheric pressure. This results in decreased molecular collisions, decreased friction and decreased temperature of the parcel. (Remember that adiabatic heating is the reverse of this process)

Some natural force initiates uplift.

SURFACE

25(C

1000m

3000m

2000m

0m

Surrounding Air

Temperature

Temperature

of Parcel

25(C

Some natural force initiates uplift.

SURFACE

25(C

1000m

3000m

2000m

0m

Surrounding Air

Temperature

Temperature

of Parcel

25(C