Chapter 18 – Moisture, Precipitation, & Clouds Define all vocabulary for Chapter 18 Read each...

Chapter 18 – Moisture, Precipitation, & Clouds

• Define all vocabulary for Chapter 18• Read each night to review material

covered in class and to gain an understanding of material to be covered next class meeting

• Good Luck on the test to come

Chapter 1818

Moisture, Clouds, and Precipitation

Key ConceptsKey Concepts Which gas is most important for understanding atmospheric Which gas is most important for understanding atmospheric

processes?processes? What happens during a change of state?What happens during a change of state? How do warm and cold air compare in their ability to hold How do warm and cold air compare in their ability to hold

water vapor?water vapor? What is relative humidity?What is relative humidity? What can change the relative humidity of air?What can change the relative humidity of air?

VocabularyVocabulary Precipitation, latent heat, evaporation, condensation, Precipitation, latent heat, evaporation, condensation,

sublimation, deposition, humidity, saturated, relative humidity, sublimation, deposition, humidity, saturated, relative humidity, dew point, hygrometerdew point, hygrometer

18.1 Water in the Atmosphere

Precipitation is any form of water that falls from a cloud.

When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere.

Water vapor is the source of all condensation and precipitation.

Water vapor is 0-4% of the atmosphere

Water’s Changes of State

3 states of matter are solid, liquid, and 3 states of matter are solid, liquid, and gasgas

The process of changing state required The process of changing state required energy to be gained or lost. It is energy to be gained or lost. It is transferred in the form of heat.transferred in the form of heat.

Latent heat is energy added to matter Latent heat is energy added to matter that does not produce a temperature that does not produce a temperature change.change.

Solid - Liquid

Solid to LiquidSolid to LiquidMelting: when ice gains energy the molecules

move faster until they become a liquidFreezing: when liquid water loses energy the

molecules slow down until they become a solid

Liquid - Gas

Liquid to GasLiquid to GasEvaporation: when liquid water gains energy

the molecules move even faster until they become a gas

Condensing: when water vapor loses energy the molecules move slower until they condense into a liquid

BIOLOGY: The Water Cycle• The water cycle is essential to life on

land. Water evaporates from the oceans and from the leaves of plants. Some of the water vapor condenses and falls as precipitation over the land back to the oceans, it is drunk by animals and absorbed by plants through their roots.

BIOLOGY: The Water Cycle

• What would happen if water evaporated and condensed at temperatures not found on Earth’s surface?

• What effect would this have on life on Earth?

Solid - Gas

Solid to GasSolid to Gas Sublimation is the change from a solid directly to a gas

without a liquid state. Example: Dry ice

Deposition is the change from a gas to a solid without a liquid state.


Humidity is a general term for the amount of water vapor in air.

• Air is saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure.

• When saturated, warm air contains more water vapor than cold saturated air.

Saturation


• Relative humidity is a ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure.

• When the water-vapor content of air remains constant, lowering air temperature causes an increase in relative humidity, and raising air temperature causes a decrease in relative humidity.

Relative Humidity

Relative Humidity Varies with Temperature

Relative Humidity varies with temperature.


• Dew point is the temperature to which a parcel of air would need to be cooled to reach saturation.

Dew Point

• A hygrometer is an instrument to measure relative humidity.

Measuring Humidity

• A psychrometer is a hygrometer with dry- and wet-bulb thermometers. Evaporation of water from the wet bulb makes air temperature appear lower than the dry bulb’s measurement. The two temperatures are compared to determine the relative humidity.

Dew on a Spider WebFigure 4

Sling PsychrometerFigure 5

This psychrometer is used to measure both relative humidity and dew point.

18.2 Cloud Formation18.2 Cloud Formation Key Concepts

What happens to air when it is compressed or allowed to expand?

List four mechanisms that can cause air to rise. Contrast movements of stable and unstable air. What conditions in air favor condensation of water?

Vocabulary Dry adiabatic rate, wet adiabatic rate, orographic lifting,

front, temperature inversion, condensation nuclei.

18.2 Cloud Formation

• When air is allowed to expand, it cools, and when it is compressed, it warms.

Adiabatic Temperature Changes

• Dry adiabatic rate is the rate of cooling or heating that applies only to unsaturated air.

Expansion and Cooling

• Wet adiabatic rate is the rate of adiabatic temperature change in saturated air.

• Unsaturated air cools as it rises, at the constant rate of 10°C for every 1000 meters of ascent.

• Descending air encounters higher pressures, compresses, and is heated 10°C for every 1000 meters it moves downward.

• If a parcel of air rises high enough, it will eventually cool to its dew point. (condensation begins)

• As the air rises, latent heat of condensation will be released.

• The released latent heat works against the adiabatic cooling process.

• The wet adiabatic rate varies from 5–9°C per 1000 meters.

Cloud Formation by Adiabatic CoolingFigure 7

Rising air cools at the dry adiabatic rate until the air reaches the dew point and condensation (cloud formation) begins. As air continues to rise the latent heat released by condensation reduces the rate of cooling.


Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting.

• Orographic lifting occurs when mountains act as barriers to the flow of air, forcing the air to ascend.

Orographic Lifting

• The air cools adiabatically; clouds and precipitation may result.

Processes that Lift Air


• A front is the boundary between two adjoining air masses having contrasting characteristics.

Frontal Wedging

Orographic Lifting and Frontal WedgingFigure 8

A Orographic Lifting Frontal Wedging

Why does the warm air mass move upward over the cold air mass?


• Convergence is when air flows together and rises.

Convergence

• Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy.

Localized Convective Lifting

Convergence and Localized Convective LiftingCon’t Figure 8

Convergence Localized Convective Lifting

Stability


• Stable air tends to remain in its original position, while unstable air tends to rise.

Density Differences

• Air stability is determined by measuring the temperature of the atmosphere at various heights.

Stability Measurements

• The rate of change of air temperature with height is called the environmental lapse rate.

Stability


• A temperature inversion occurs in a layer of limited depth in the atmosphere where the temperature increases rather than decreases with height. A temperature inversion is the most stable condition.

Degrees of Stability

• When stable air is forced above the Earth’s surface, the clouds that form are widespread and have little vertical thickness compared to their horizontal dimension.

Stability and Daily Weather

Condensation


For any form of condensation to occur, the air must be saturated.

• Generally, there must be a surface for water vapor to condense on.

Types of Surfaces

• Condensation nuclei are tiny bits of particulate matter that serve as surfaces on which water vapor condenses when condensation occurs in the air.

18.3 Cloud Types and Precipitation18.3 Cloud Types and Precipitation Key Concepts

How are clouds classified? How are clouds and fogs similar and different? What must happen in order for precipitation to form? What controls the type of precipitation that reaches

Earth’s surface? Vocabulary

Cirrus, cumulus, stratus, Bergeron process, supercolled water, supersaturated air, collision-coalescence process

Cloud ChartA cloud is classified based on ________ & _________.

High Clouds

Middle Clouds

Low Clouds Multi-level Clouds

Altitude

Type, description, and precipitation



Types of Clouds

18.3 Cloud Types and Precipitation

Clouds are classified on the basis of their form and height.

The three basic forms are: -cirrus- cumulus- stratus

CirrusCirrus ((cirrus = a curl of

hair) They are high, white,

and thin. They can occur as

patches or as delicate veil-like sheets or extended wispy fibers that often have a feathery appearance.

CumulusCumulus (cumulus = a pile) clouds consist of

rounded individual cloud masses.

They have a flat base and the appearance of rising domes or towers. (cauliflower structure)

StratusStratus (stratum = a layer) clouds are best

described as sheets or layers that cover much or all of the sky.

There are no distinct individual cloud units.

Types of Clouds


High Clouds (above 6000m)• Cirrus clouds are high, white, and thin.• Cirrostratus clouds are flat layers of clouds.• Cirrocumulus clouds consist of fluffy masses.

All high clouds are thin and white and are often made up of ice crystals.

These clouds are not considered precipitation makers. (they may warn of approaching stormy weather)

Types of Clouds


Middle Clouds (from about 2000 to 6000 meters))

• Altostratus clouds create a uniform white to gray sheet covering the sky with the sun or moon visible as a bright spot.

• Infrequent light snow or drizzle may accompany these clouds.

• Altocumulus clouds are composed of rounded masses that are larger and denser cirrocumulus clouds.

Types of Clouds


Low Clouds (below 2000m)• Stratus clouds are best described as sheets or

layers that cover much or all of the sky. (May produce light precipitation.

• Nimbostratus clouds are the main precipitation makers.

• Stratocumulus clouds have a scalloped bottom that appears as long parallel rolls or broken rounded patches.

Cloud Classification

Clouds are classified according to form and height.

Types of Clouds


Clouds of Vertical Development• Some clouds do not fit into any one of the three

height categories mentioned. Such clouds have their bases in the low height range but often extend upward into the middle or high altitudes.

• They all are associated with unstable air. • Once upward movement is triggered, acceleration is

powerful, and clouds with great vertical range form.• The end result often is a cumulonimbus cloud that

may produce rain showers or a thunderstorm.

Fog


Fog is defined as a cloud with its base at or very near the ground.

• As the air cools, it becomes denser and drains into low areas such as river valleys, where thick fog accumulations may occur.

Fog Caused by Cooling

• When cool air moves over warm water, enough moisture may evaporate from the water surface to produce saturation.

Fog Caused by Evaporation

How Precipitation Forms


For precipitation to form, cloud droplets must grow in volume by roughly one million times.

• The Bergeron process is a theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water.

Cold Cloud Precipitation

The Bergeron Process

Ice crystals grow at the expense of cloud droplets until they are large enough to fall. The size of these particles has been greatly exaggerated.



Cold Cloud Precipitation• Supercooled water is the condition of water

droplets that remain in the liquid state at temperatures well below 0oC.

• Supersaturated air is the condition of air that is more concentrated than is normally possible under given temperature and pressure conditions.



Warm Cloud Precipitation• The collision-coalescence process is a theory

of raindrop formation in warm clouds (above 0oC) in which large cloud droplets collide and join together with smaller droplets to form a raindrop.

Forms of Precipitation


The type of precipitation that reaches Earth’s surface depends on the temperature profile in the lower few kilometers of the atmosphere.

• In meteorology, the term rain means drops of water that fall from a cloud and have a diameter of at least 0.5 mm.

Rain and Snow

• At very low temperatures (when the moisture content of air is low) light fluffy snow made up of individual six-sided ice crystals forms.

Forms of Precipitation


• Sleet is the fall of clear-to-translucent ice. Rain and Snow

• Hail is produced in cumulonimbus clouds.

• Hailstones begin as small ice pellets that grow by collecting supercooled water droplets as they fall through a cloud.

Largest Recorded HailstoneThe largest recorded hailstone fell over Kansas in 1970

and weighed 766 grams.

Precipitation ChartType Description

RainSnowSleetGlazeHail

Chapter 18 – Moisture, Precipitation, & Clouds Define all vocabulary for Chapter 18 Read each...

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