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Moisture, Clouds, and Precipitation

Chapter 12 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Summarize the six processes by which water changes from one state of matter to another.

For each, indicate whether energy is absorbed or released.

Focus Questions 12.1

Three states of matter

Solid—ice

Liquid—water

Gas—water vapor

To change state, heat must be either absorbed or released

Water’s Changes of State

Heat energy

One calorie is the heat necessary to raise the temperature of one gram of water one degree Celsius

Latent heat

Stored or hidden heat

Not derived from temperature change

Heat exchanged between water and surroundings

Important in atmospheric processes

Water’s Changes of State

Processes

Melting

Solid is changed to a liquid

80 calories per gram added

Latent heat of melting

Freezing

Liquid is changed to a solid

Heat is released

Latent heat of fusion

Water’s Changes of State

Processes

Evaporation

Liquid is changed to gas

600 calories per gram added

Latent heat of vaporization

Condensation

Water vapor (gas) is changed to a liquid

Heat energy is released

Latent heat of condensation

Water’s Changes of State

Processes

Sublimation

Solid is changed directly to a gas

680 calories per gram of water are added

Deposition

Water vapor (gas) changed to a solid

Heat is released

Water’s Changes of State

Write a generalization relating air temperature and the amount of water vapor needed to saturate air.

Focus Question 12.2

Humidity—amount of water vapor in the air

Saturated air

Air filled to capacity with water vapor

Capacity is temperature dependent

Warm air has a much greater capacity

Water vapor adds pressure

Vapor pressure

Humidity: Water Vapor in the Air

Measuring humidity

Mixing ratio

Mass of water vapor in a unit of air compared to the remaining mass of dry air

Measured in g/kg

Relative humidity

Ratio of the air’s actual water vapor content compared with the amount of water vapor required for saturation

(at that temperature and pressure)

Humidity: Water Vapor in the Air

Measuring humidity

Relative humidity

Expressed as a percent

Saturated air

Content equals capacity

Has 100% relative humidity

Relative humidity can be changed in two ways

Changing the air temperature

Lowering the temperature raises the relative humidity

Dew point temperature

Temperature to which a parcel of air would need to be cooled to reach saturation

Humidity: Water Vapor in the Air

Two types of hygrometers are used to measure humidity:

Psychrometer

Compares temperatures of wet-bulb thermometer and dry-bulb thermometer

Greater difference = lower relative humidity

If air is saturated, both thermometers read the same temperature

Electric hygrometer

Reads the humidity directly

Humidity: Water Vapor in the Air

Describe adiabatic temperature changes.

Explain why the wet adiabatic rate of cooling is less than the dry adiabatic rate.

Focus Questions 12.3

Adiabatic temperature changes

Air is compressed

Motion of air molecules increases

Air warms

Descending air is compressed

Air expands

Air parcel does work on the surrounding air

Air cools

Rising air expands

The Basis of Cloud Formation: Adiabatic Cooling

Adiabatic rates

Dry adiabatic rate

Unsaturated air

Rising air expands & cools at 1°C/100 m

Descending air compresses and warms at 1°C/100 m

Wet adiabatic rate

Cloud formation begins at condensation level

Air has reached the dew point

Condensation is occurring and latent heat is being liberated

Sensible heat released by condensing water reduces cooling rate

Rate varies from 0.5°C to 0.9°C/100 m

The Basis of Cloud Formation: Adiabatic Cooling

List and describe the four mechanisms that cause air to rise.

Focus Question 12.4

Orographic lifting

Elevated terrains act as barriers

Result can be a rainshadow desert

Processes That Lift Air

Frontal wedging

Cool air acts as a barrier to warm air

Fronts are part of middle-latitude cyclones

Processes That Lift Air

Convergence

Air flows together and rises

Processes That Lift Air

Localized convective lifting

Unequal surface heating causes pockets of air to rise because of their buoyancy

Processes That Lift Air

Describe how atmospheric stability is determined.

Compare conditional instability with absolute instability.

Focus Questions 12.5

Stability of air determines:

Type of clouds that develop

Intensity of the precipitation

Stable air

Resists vertical displacement

Cooler and denser than surrounding air

Wants to sink

Unstable air

Warmer than surrounding air

Wants to rise

The Weathermaker: Atmospheric Stability

Types of stability

Environmental lapse rate—how temperature above parcel changes with height

Stable air

No adiabatic cooling

Widespread clouds with little vertical thickness

Precipitation is light to moderate

Absolute stability

Environmental lapse rate less than wet adiabatic rate

The Weathermaker: Atmospheric Stability

Absolute instability

Acts like a hot air balloon

Rising air

Warmer and less dense than surrounding air

Rises until it reaches altitude with same temperature

Adiabatic cooling

Environmental lapse rate greater than dry adiabatic rate

Clouds are often towering

Conditional instability

Atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parcel

The Weathermaker: Atmospheric Stability

Name and describe the 10 basic cloud types, based on form and height.

Contrast nimbostratus and cumulonimbus clouds and their associated weather.

Focus Questions 12.6

Condensation

Water vapor changes to a liquid and forms dew, fog, or clouds

Water vapor requires a condensation surface

On the ground

Grass, a car window, etc.

In the air are tiny bits of particulate matter called condensation nuclei

Dust, smoke, ocean salt crystals, etc.

Condensation and Cloud Formation

Clouds

Made of millions and millions of

Minute water droplets, or

Tiny crystals of ice

Classification based on form

Cirrus

High, white, thin

Stratus

Sheets or layers that cover much of the sky

Cumulus

Globular cloud masses

Nimbus – major producer of precipitation

Condensation and Cloud Formation

Clouds classified based on height

High clouds

Above 6000 m

Cirrus, cirrostratus, cirrocumulus

Middle clouds

2000 to 6000 m

Altostratus and altocumulus

Low clouds

Below 2000 m

Stratus, stratocumulus, and nimbostratus (nimbus means “rainy”)

Condensation and Cloud Formation

Clouds of vertical development

From low to high altitudes

Called cumulonimbus

Often produce rain showers and thunderstorms

Condensation and Cloud Formation

Identify the basic types of fog.

Describe how each forms.

Focus Questions 12.7

Fog is a cloud with its base at or near the ground

Considered an atmospheric hazard

Most fogs form because of

Radiation cooling, or

Movement of air over a cold surface

Fog

Fogs caused by cooling

Radiation fog

Earth’s surface cools rapidly

Forms during cool, clear, calm nights

Advection fog

Warm, moist air moves over a cool surface

Upslope fog

Humid air moves up a slope

Adiabatic cooling occurs

Fog

Evaporation fogs

Steam fog

Cool air moves over warm water

Water has a steaming appearance

Frontal fog, or precipitation fog

Forms during frontal wedging when warm air lifted over colder air

Rain evaporates to form fog

Fog

[insert Figure 12.25 here]

Describe the Bergeron process.

Explain how it differs from the collision-coalescence process.

Focus Questions 12.8

Cloud droplets

<20 micrometers (0.02 millimeter) in diameter

Fall incredibly slowly

Formation of precipitation

Bergeron process

Temperature in the cloud is supercooled

Ice crystals collect water vapor

Large snowflakes form and fall to the ground or melt and turn to rain

How Precipitation Forms

Formation of precipitation

Collision-coalescence process

Warm clouds

Large hygroscopic condensation nuclei

Large droplets form

Droplets collide with other droplets during their descent

How Precipitation Forms

Describe the atmospheric conditions that produce sleet, freezing rain (glaze), and hail.

Focus Question 12.9

Rain, drizzle, and mist

Rain

Droplets have at least a 0.5 mm diameter

Drizzle

Droplets have less than a 0.5 mm diameter

Mist

Smallest droplets able to reach the ground

Snow

Ice crystals, or aggregates of ice crystals

Forms of Precipitation

Sleet and glaze

Sleet

Small particles of ice in winter

Occurs when warmer air overlies colder air

Rain freezes as it falls

Glaze, or freezing rain

Impact with a solid causes freezing

Forms of Precipitation

Hail

Hard rounded pellets

Concentric shells

Most diameters range from 1 to 5 cm

Formation

In large cumulonimbus clouds

Layers of freezing rain are caught in violent up- and down-drafts

Pellets fall when they become too heavy

Forms of Precipitation

Rime

Forms on cold surfaces

Freezing of supercooled fog

Freezing of cloud droplets

Forms of Precipitation

List the advantages of using weather radar versus a standard rain gauge to measure precipitation.

Focus Question 12.10

Rain

Easiest form to measure

Measuring instruments

Standard rain gauge

Uses a funnel to collect rain

Cylindrical tube measures in cm or inches

Tipping-bucket gauge

Two compartments capable of holding 0.025 cm each

Bucket fills and tips, then other bucket begins to fill

Each bucket tip is recorded on a graph

Radar also used to measure rain

Measuring Precipitation

Snow has two measurements:

Depth

Water equivalent

General ratio is 10 snow units to 1 water unit

Varies widely

Measuring Precipitation