Chapter 4: Moisture and Atmospheric Stabilityastrodee.net/ch_4_pp_17.pdfRelative humidity indicates how close air is to saturation rather than the actual quantity of water vapor in

Copyright © 2013 Pearson Education, Inc.

Chapter 4: Moisture and Atmospheric Stability

Lewis U Fall 2017


Movement of Water Through The Atmosphere

Hydrologic cycle:


Concept Check 4.1

2. The quantity of water lost to evaporation over the oceans is no equaled by precipitation. Why, then, does the seal level not drop?

The deficit is made up by runoff from the continents.


Water: A Unique Substance

Water:

Water is the only liquid on the surface of the Earth in large quantities.

It exists in all forms on Earth.

Ice (solid state) is less dense than liquid.

Water has a high heat capacity.

It has a unique ability to form hydrogen bonds (H2O).


Water has Hydrogen Bonds

Hydrogen Atoms (-)

Oxygen Atom (+) (-)

(+) (-)

(-)

(+)


Water’s Changes of State

Ice, liquid water, and water vapor:


Latent Heat

Latent heat of melting

Adding heat to melt ice does not

result in a temperature change.

Melting 1 gram of ice requires

80 calories.

The heat is used in breaking

the hydrogen bonds.

Latent heat of fusion Freezing 1 gram of water releases 80 calories. The heat goes into the environment and warms the environment.


Latent Heat Latent heat of vaporization

600 calories/gram is the energy

absorbed by water during evaporation.

Evaporation is a cooling process.

Latent heat of condensation

600 calories/gram is the energy

released by water during condensation.

Energy is released, which warms the

surrounding air.

Important part of weather


Latent Heat

Latent heat of sublimation

process that turns a solid to a gas.

(680 calories released)

Disappearing ice cubes in freezer are

a result of sublimation.

Latent heat of deposition

process of changing a vapor to a solid.

(680 calories absorbed)

Frost accumulating in a freezer is

a result of deposition.


Concept Check 4.2

2. Explain what happens as ice melts to become liquid water.

Melting causes some, but not all, of the hydrogen bonds to break. As a result, the water molecules in liquid water display a more compact arrangement than in ice, So water in its liquid phase is denser than it is in the solid phase.


3. Explain why evaporation is a cooling process.

When water evaporates, it absorbs heat energy from its surroundings in order to convert from liquid to vapor. This causes the surrounding temperature to drop.

*


Water Vapor in the Atmosphere

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Humidity: Water Vapor in the Air

Relative humidity indicates how close air is to saturation rather than the

actual quantity of water vapor in the air.

Dew point is the temperature to which air needs to be cooled to reach saturation.


Vapor Pressure and Saturation

Saturation:

Saturation is the equilibrium point

between evaporation and condensation.

Where the number of water molecules

leaving the surface is equal to the

number of molecules returning.

It increases with temperature.


Amount of water vapor required to saturate 1 kg of dry air. -10 0C 2.5 gm/kg 100C 7.5 gm/kg 250C 20 gm/kg 350C 34.5 gm/kg

Increases with temperature

*


5. Refer to Table 4-1 to answer this question. How much more water is contained in saturated air at a tropical location with a temperature of 400C compared to a polar Location with a temperature of -100C?

47 g/kg at 40°C compared to 2 g/kg at –10°C,

47 g/kg -------- = 23.5 2 g/kg

40°C air holds 23.5 times more water vapor than –10°C air.

*


Relative Humidity

Relative humidity is the ratio of the air’s

actual water vapor content

and amount of water vapor required

for saturation

at a specific temperature.


RH = ------------------------------ water vapor required for saturation


Relative Humidity

How relative humidity changes:

If water vapor is added, relative humidity goes up.

If water vapor is removed, relative humidity goes down.

100% relative humidity equals saturation.

A decrease in temperature equals an increase in relative humidity.


RH = ------------------------------ water vapor required for saturation


Ways to change Relative Humidity

Add/remove water vapor to parcel of air without changing the temperature Change the temperature of a parcel of without changing the amount of water vapor it contains Change both the temperature and amount of water vapor in the air parcel



Relative Humidity


Relative Humidity


Changing Relative Humidity by Adding Water Vapor


Changing Relative Humidity by Changing the Temperature

As the temperature of air increases Its ability to hold water vapor increases The relative humidity decreases

If the temperature decreases Its ability to hold water vapor decreases The relative humidity increases

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Variation in relative humidity due to change in temperature

low

low

high

high

Which cart is temperature Which chart is R.H.


Variation in relative humidity due to change in temperature

low

low

high

high

Which cart is temperature Which chart is R.H.


Dew-Point Temperature

Dew point:

The dew point is the temperature air is cooled to reach saturation.

Dew-point temperature is a measure of actual moisture content.


1. List three ways that relative humidity changes in nature.

2. Which measure of humidity, relative humidity or dew point best describes the actual quantity of water vapor in a mass of air? Why?

3. At what time of day will dew most likely form?


1. List three ways that relative humidity changes in nature.

First would involve daily changes in temperatures (daylight versus nighttime temperatures). Second would be temperature changes that result as air moves horizontally from one location to another. A third way would be temperature changes caused as air moves vertically in the atmosphere.


2. Which measure of humidity, relative humidity or dew point best describes the actual quantity of water vapor in a mass of air?

Dew point. High dew points imply very moist air and low dew points correlate with dry air.

*


3. At what time of day will dew most likely form?

Just before sunrise when the temperature is typically the lowest.


How is Humidity Measured?

Hygrometer:

A hygrometer measures moisture the content of air.

A hair hygrometer operates on the principle that hair changes length in proportion to changes in relative humidity.

A psychrometer consists of two identical thermometers;

one (dry thermometer) measures air temperature,

the other called the “wet bulb,” measures the dryness of air.

The larger the difference in temperatures between the wet and dry,

the lower the relative humidity (with a table).


SLING psychrometer


Chapter Problems Chapter 4 # 2 (pp 120)

Using the standard tables (appendix C Tables C-1 and C-2), Determine the relative humidity and dew point temperature if the dry-bulb thermometer reads 220C, and the wet-bulb thermometer reads 16 0C.


Relative Humidity

TD – Tw = 220C – 160C = 60 C depression

RH = 53 %


Dew-Point Temperature - 20C

TD – Tw = 220C – 160C = 60C depression


What region has the highest Relative Humidity?

Dew-point surface map



Physical processes in the atmosphere


Adiabatic Temperature Changes

An adiabatic process

is one in which no heat is transferred.

When air is compressed, it warms.

When air expands, it cools.


Let Us Consider an “AIR PARCEL”

An imaginary volume to assist us in simplifying the physical processes involved the start point for model-building. A volume of air enclosed in a thin balloon-like bubble, a few hundred cubic meters in volume. It acts independent of the surrounding air. No heat is transferred into or out of the parcel.



Adiabatic cooling

Cooling occurs when air

moves up and it expands

Unsaturated air

cools at a rate of

10° C/1000m

this is called the dry adiabatic rate.



Condensation is triggered

when air rises high enough

to cool down enough

to reach its saturation point

and clouds form.

This is called the

lifting condensation level.


Adiabatic Temperature Changes above the lifting condensation level

When air ascends above

the lifting condensation level,

the rate at which it cools

is reduced.

The slower rate of cooling

is called the wet adiabatic rate

which varies from 5° C/1000m

to 9° C/1000m .


2. Why does the adiabatic lapse rate of cooling change when condensation occurs?

*

3. Why is the wet adiabatic lapse rate not a constant?

1. Why does air expand as it moves upward through the atmosphere?


*

1. Why does air expand as it moves upward through the atmosphere?

Air expands as it moves upward through the atmosphere because pressure decreases with increasing elevation.


2. Why does the adiabatic lapse rate of cooling change when condensation occurs?

When a parcel of air ascends above the lifting condensation level, the rate of cooling is reduced to the wet adiabatic rate because the release of latent heat partially offsets the cooling due to expansion.

*


5. Why is the wet adiabatic lapse rate not a constant?

The wet adiabatic rate is not a constant figure because the amount of latent heat released depends on the quantity of moisture present in the air.


Air Wants to Stay Put

Air has a tendency to resist motion. To move a mass requires that work is done. To perform work energy must be available. So, how does AIR get lifted so that it will expand and cool, reach saturation and form clouds, then lift further so that precipitation occurs?


Processes That Lift Air

Orographic Lifting Frontal Wedging

Convergence



Orographic lifting

occurs when elevated terrains,

such as mountains,

act as barriers to the flow of air.

When air reaches the leeward side, much of its moisture has been lost.

Adiabatic cooling can generate clouds and copious precipitation.

Many of the wettest places in the world are located on windward mountain slopes.



Frontal wedging:

Masses of warm and cold air collide, producing fronts.

Cooler, denser air acts as a barrier over which the warmer, less dense air rises.



Convergence:

Converging horizontal air flow

results in upward movement.



Localized convective lifting:

Unequal surface heating causes localized pockets of air to rise.


1. Explain why the Great Basin of the western United States is dry. What term is applied to these locations?

3. Describe localized lifting.

*

2. How does frontal lifting cause air to rise?

Use diagrams to show your answer


2. Explain why the Great Basin of the western United States is dry.

What term is applied to such a situation?

The Great Basin lies on the leeward side of large mountains. As air ascends the windward slopes of the mountains, cloud formation and precipitation remove much of the water vapor. Farther on, as the air descends the leeward slopes, it is warmed by compression, making condensation and precipitation unlikely.

Deserts such as the Great Basin are often termed rain shadow deserts.



2. How does frontal lifting cause air to rise?

Frontal wedging (lifting) occurs when relatively warm (light) air rises over cooler (dense) air. The cooler air acts as a barrier over which the warm air is wedged aloft.


3. Describe localized lifting.

Localized convective lifting occurs because of unequal heating of the Earth’s surface. This causes warm pockets of air to form, which are less dense than the surrounding air, making them buoyant and causing them to rise.


To get “Weather” our air parcel needs to rise so that it will expand and cool reach the condensation level where condensation will occur.

So, How Do We Get Weather?


The Critical Weathermaker: Atmospheric Stability

Stability is the property of our air parcel

that describes its tendency to

remain in its original position

to rise (unstable)

to sink (stable)



The stability of the atmosphere is determined by measuring the air temperature at various heights. This gives us the “actual” rate at which temperature changes as we move up in the atmosphere.

The lapse rate of the air in which our “parcel” exists

environmental lapse rate


The temperature in the atmosphere is never the same

The dry and wet adiabatic lapse rates are constant. The environmental lapse rate is what “varies.”


2. Explain the difference between the environmental lapse rate and adiabatic cooling.

The environmental lapse rate refers to the temperature change of the atmosphere as altitude increases. It is variable and is determined by measurements (vertical soundings). Adiabatic cooling, on the other hand, applies only to the cooling by expansion that occurs in vertically ascending air.


Our air parcel can have three kinds of motion depending on The stability of the atmosphere






Three types of atmospheric stability

By measuring the parameters of the atmosphere meteorologists can predict what air parcels will do in the atmosphere when lifting occurs. Will the parcels - rise - sink - stay put It all depends on the stability of the atmosphere.


Our rising air parcel is always cooler and heavier then the atmosphere it is in. The air parcel will not rise by itself.

Absolute Stability

Atmosphere is stable


Absolute Instability

Our rising air parcel is always warmer and lighter then the atmosphere it is in. The air parcel will rise by itself.

Atmosphere is unstable


Extra

Conditional Instability

Our rising air parcel is at first cooler than the environment but after it is “forced up” it becomes warmer than the environment.

Will now rise on its own.

Atmosphere stable

Atmosphere unstable



Conditional instability is the most common type of atmospheric instability.

This situation prevails when moist air has an environmental lapse rate

between the dry and wet adiabatic rates.



End Chapter 4



Humidity is amount of water vapor in the air.

Absolute humidity is the mass of water vapor in a given volume of air.

The mixing ratio is the mass of water vapor in a unit of air compared to the

remaining mass of dry air.



Vapor pressure is defined as the part of the total atmospheric pressure

attributable to its water-vapor content.

pressure from water vapor

Pvapor = ----------------------------------------

total atmospheric pressure

Relative humidity indicates how close air is to saturation rather than the

actual quantity of water vapor in the air.

Dew point is the temperature to which air needs to be cooled to reach saturation.

air pressure

Total atmospheric pressure is sum of the pressures from all the gases


Concept Check 4.12

3. List four ways instability can be enhanced.

a) intense solar heating warming the lowermost layer of the atmosphere;

b) the heating of an air mass from below as it passes over a warm surface; c) general upward movement of air caused by processes such as orographic lifting, frontal wedging, and convergence; d) radiation cooling from cloud tops.


Concept Check 4.12

4. List three ways stability can be enhanced.

a) radiation cooling of Earth’s surface after sunset

b) the cooling of an air mass from below as it traverses a cold surface;

c) general subsidence within an air column.

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Chapter 4: Moisture and Atmospheric Stabilityastrodee.net/ch_4_pp_17.pdfRelative humidity indicates how close air is to saturation rather than the actual quantity of water vapor in