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Chapter 15


Changes of state of water

A. Three states of matter

1. Solid

2. Liquid

3. Gas


B. To change state, heat must be

1. Absorbed, or

2. Released


C. Heat energy

1. Measured in calories - one calorie is the heat necessary to raise the temperature of one gram of water one degree Celsius

2. Latent heat

a. Stored or hidden heat

b. Not derived from temperature change

c. Important in atmospheric processes


D. Processes

1. Evaporation

a. Liquid is changed to gas

b. 600 calories per gram of water are added - called latent heat of vaporization

2. Condensation

a. Water vapor (gas) is changed to a liquid

b. Heat energy is released - called latent heat of condensation


Processes continued

3. Melting

a. Solid is changed to a liquid

b. 80 calories per gram of water are added - called latent heat of melting.

4. Freezing

a. Liquid is changed to a solid

b. Heat is released - called latent heat of fusion.


Processes continued

5. Sublimation

a. Solid is changed directly to a gas (e.g., ice cubes shrinking in a freezer)

b. 680 calories per gram of water are added

6. Deposition

a. Water vapor (gas) changed to a solid ( e.g., frost in a freezer compartment)

b. Heat is released


II. Humidity

A. Amount of water vapor in the air.

1. Water vapor adds pressure ( called vapor pressure ) to the air.

2. Saturated air is air that is filled with water vapor to capacity.

3. Capacity is temperature dependent - warm air has a much greater capacity


B. Measurements of humidity

1. Specific humidity

a. Quantity of water vapor in a given mass of air

b. Often measured in grams per kilogram


Measurements continued

2. Relative humidity

a. Ratio of the air’s actual water vapor content to it’s potential water vapor capacity, at a given temperature.

b. Expressed as a percent

c. Saturated air

1. Content equals capacity

2. Has a 100 percent relative humidity


Measurements continued

d. Relative humidity can be changed in two ways

1. Add or subtract moisture to the air

a. Adding moisture raises the relative humidity

b. Removing moisture lowers the relative humidity

2. Changing the air temperature

a. Lowering the temperature raises the relative humidity

b. Raising the temperature lowers the relative humidity.


Measurements continued

e. Dew point

1. Temperature at which the air is saturated and the relative humidity is 100 percent

2. Cooling the air below the dew point causes condensation

a. e.g., Cloud formation

b. Water vapor requires a surface to condense on


f. Instruments to measure humidity

1. Psychrometer

a. Compares temperatures of

1. Wet-bulb thermometer, and

2. Dry-bulb thermometer

b. If the air is saturated ( 100 percent relative humidity ) then both thermometers read the same temperature.

c. The greater the difference between the thermometer readings, the lower the relative humidity


III. Adiabatic heating / cooling

A. Adiabatic temperature changes occur when

1. Air is compressed

a. Motion of air molecules increases

b. Air will warm

c. Descending air is compressed due to increasing air pressure

2.Air expands

a. Air parcel does work on the surrounding air

b. Air will cool

c. Rising air will expand due to decreasing air pressure


B. Adiabatic rates

1. Dry adiabatic rate

a. Saturated air

b. Rising air expands and cools at 1 oC per meters

c. Descending air is compressed and warms at 1 oC per 100 meters.


Adiabatic rates continued

2. Wet adiabatic rate

a. Commences at condensation level

b. Air has reached the dew point

c. Condensation is occurring and latent heat is being liberated

d. Heat released by the condensing water reduces the rate of cooling

e. Rate varies from 0.5 oC to 0.9 oC per 100 meters


IV. Stability of air

A. Two types of stability

1. Stable air

a. Resists vertical displacement

1. Cooler than surrounding air

2. Denser than surrounding air

3. Wants to sink

b. No adiabatic cooling

c. Stability occurs when the environmental lapse rate is less than the wet adiabatic rate

d. Often results in widespread clouds with little vertical thickness

e. Precipitation, if any, is light to moderate


2. Unstable air

a. Acts like a hot air balloon

b. Rising air

1. Warmer than surrounding air

2. Less dense than surrounding air

3. Continues to rise until it reaches a altitude with the same temperatures.

c. Adiabatic cooling

d. Air is unstable when the environmental lapse rate is greater than dry adiabatic rate

e. Clouds are often towering

f. Often results in heavy precipitation

g. Conditional instability occurs when the atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parcel


B. Determines to a large degree

1. Type of clouds that develop

2. Intensity of the precipitation


V. Processes that lift air

A. Orographic lifting

1. Elevated terrains act as barriers

2. Results can be a rainshadow desert

B. Frontal are part of the storm systems called middle-latitude cyclones

C. Convergence where the air is flowing together and rising


VI. Condensation and cloud formation

A Condensation

1. Water vapor in the air changes to a liquid and forms dew, fog, or clouds

2. Water vapor requires a surface to condense on

a. Possible condensation surfaces on the ground can be the grass, a car window, etc.

b. Possible condensation surfaces in the atmosphere are tiny bits of particulate matter

1. Called condensation nuclei

2. Dust, smoke, etc.

3. Ocean salt crystals which serve as hygroscopic ("water seeking") nuclei


B. Clouds

1. Made of millions and millions of

a. Minute water droplets, or

b. Tiny crystals of ice


Clouds continued

2. Classification based on

a. Form ( three basic forms )

1. Cirrus - high, white, thin

2. Cumulus

a. Globular cloud masses

b. Often associated with fair weather

3. Stratus

a. Sheets or layers

b. Cover much or all of the sky


Clouds continued

B. Height

1. High clouds

a. Above 6000 meters

b. Types

1. Cirrus

2. Cirrostratus

3. Cirrocumulus

2. Middle clouds

a. 2000 - 6000 meters

b. Types ( alto as part of the name )

1. Altocumulus

2. Altostratus


Clouds continued

3. Low clouds

a. Below 2000 meters

b. Types

1. Stratus

2. Stratocumulus

3. Nimbostratus ( nimbus means " rainy " )

4. Clouds of vertical development

a. From low to high altitudes

b. Called cumulonimbus

c. Often produce

1. Rain showers

2. Thunderstorms


VII. Fog

A. Considered an atmospheric hazard

B. Cloud with its bas at or near the ground

C. Most fog form because of

1. Radiation cooling, or

2. Movement of air over a cold surface


D. Types of fog

1. Fogs caused by cooling

a. Advection fog - warm, moist air moves over a cool surface

b. Radiation fog

1. Earth’s surface cools rapidly

2. Forms during cool, clear, calm nights

c. Upslope fog

1. Humid air moves up a slope

2. Adiabatic cooling occurs


Types of fogs continued

2. Evaporation fogs

a. Steam fog

1. Cool air moves over warm water and moisture is added to the air

2. Water has a steaming appearance

b. Frontal fog, or precipitation fog

1. Forms during frontal wedging when warm air lifted over colder air

2. Rain evaporates to form fog


VII. Precipitation

A. Cloud droplets

1. Less than 10 micrometers in diameter

2. Fall incredibly slow

B. Formation of precipitation

1. Bergeron process

a. Temperature in the cloud is below freezing

b. Ice crystals collect water vapor

c. Large snowflakes form and

1. Fall to the ground as snow, or

2. Melt on their descent and form rain


Precipitation continued

C. Forms of precipitation

1. Rain and drizzle

a. Rain - droplets have at least a 0.5 mm diameter

b. Drizzle - droplets have less than a 0.5 mm diameter

2. Snow - ice crystals, or aggregates of ice crystals

3. Sleet and glaze

a. Sleet

1. Wintertime phenomenon

2. Small particles of ice

3. Occurs when

a. Warmer air overlies colder air

b. Rain freezes as it falls


Precipitation continued

b.Glaze, or freezing rain - impact with a solid causes freezing

c. Hail

1. Hard rounded pellets

a. Concentric shells

b. Most diameters range from 1 to 5 cm


Formation of hail

a. Occurs in large cumulonimbus clouds with violent up - and downdrafts

b. Layers of freezing rain are caught in up - and - down drafts in the cloud

c. Pellets fall to the ground when they become too heavy

d. Rime

1. Forms on cold surfaces

2. Freezing of

a. Supercooled fog, or

b. Cloud droplets


Measuring precipitation

1. Rain

a. Easiest form to measure

b. Measuring instruments

1. Standard rain gauge

a. Uses a funnel to collect and conduct rain

b. Cylindrical measuring tube measures rainfall in centimeters or inches

2. Recording gauge

2. Snow has two measurements

a. Depth

b. Water equivalent

1. General ratio is 10 snow units to 1 water unit

2. Varies widely

Back to: 348 Syllabus