moisture, clouds, and precipitation - lssc.edu · •environmental lapse rate greater than dry...

Moisture, Clouds,

and Precipitation

Chapter 12 Lecture Outline

Focus Question 12.1

• Why is latent heat important to our weather?

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 during phase change

– Important in atmospheric processes


• Three states of matter

1. Solid — ice

2. Liquid — water

3. Gas — water vapor

• To change state, heat must be either

absorbed or released


• Processes

• Evaporation

–Liquid is changed to gas

–540-600 calories per gram added

–Latent heat of vaporization

• Condensation

–Water vapor (gas) is changed to a liquid

– 540-600c of heat energy/gram is released

–Latent heat of condensation


• Processes

• Melting

–Solid is changed to a liquid

–80 calories per gram added

–Latent heat of melting

• Freezing

–Liquid is changed to a solid

–80c of heat energy/gram is released

–Latent heat of fusion


• 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

– 680c of heat energy is released

Water Phase Changes

../Animations/WaterPhasesppt.html

Focus Question 12.2

• If the temperature remains unchanged and

the amount of water vapor in the air

decreases, how does relative humidity

change?

Humidity: Water Vapor in the Air

• 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


• 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)


• Measuring humidity

• Relative humidity

–Expressed as a percent

–Saturated air

• Content equals capacity

• Has 100% relative humidity

• Dew point temperature

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

-Relative humidity can be

changed in two ways:

Relative humidity can be

changed in two ways 1. Adding moisture raises the

relative humidity

Removing moisture lowers the relative humidity

2. Changing the air temperature Lowering the temperature raises the

relative humidity

Raising the temperature lowers the relative humidity


Relative Humidity

change with

Temperature

Change

- It is “related” to T!

Dew point temperature

Cooling the air below

the dew point

causes

condensation e.g., dew, fog, or cloud

formation

Water vapor requires

a surface to condense

on

Dew Point Temp does

not change when the

air temp changes


• Two types of hygrometers are used to measure humidity:

1. 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

2. Hair hygrometer

• Reads the humidity directly

Humidity: H2O Vapor in the Air

Hair hygrometer—Reads the

humidity directly

Focus Question 12.3

• Describe the importance of adiabatic cooling

in cloud formation.

The Basis of Cloud Formation: Adiabatic

Cooling

• 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


Cooling • Adiabatic rates

• Dry adiabatic rate – Unsaturated air

– Rising air expands & cools at :

10°C/1000 m (5.5F/1000 ft.)

– Descending air compresses and warms at:

10°C/1000 m (5.5F/1000 ft.)

• Wet adiabatic rate – Begins at condensation level

– Air has reached the dew point

– Condensation is occurring and latent heat is being liberated

– Heat released by condensing water reduces cooling rate

– Rate varies from 5°C to 9°C/1000 m


Cooling

Focus Question 12.4

• How do orographic lifting and frontal wedging

force air to rise?

Processes That Lift Air

• Orographic lifting

• Elevated terrains act as barriers

• Result can be a rainshadow desert


• Frontal wedging

• Cool air acts as a barrier to warm air

• Fronts are part of middle-latitude cyclones


• Convergence

• Air flows together

and rises


• Localized convective lifting

• Unequal surface heating causes pockets of

air to rise because of their buoyancy

Focus Question 12.5

• How does atmospheric stability affect our

weather?

The Weathermaker: Atmospheric Stability

• Stability of air determines:

• Type of clouds that develop

• Intensity of the precipitation


• Types of stability

• Stable air

– Resists vertical displacement

• Cooler and denser than surrounding air

• Wants to sink

– No adiabatic cooling

– Widespread clouds with little vertical thickness

– Precipitation is light to moderate

– Absolute stability

• Environmental lapse rate less than wet adiabatic rate


• 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

Focus Question 12.6

• What is the function of condensation nuclei in

cloud formation?

Condensation and Cloud Formation

• 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.


• Clouds • Made of millions and millions of

– Minute water droplets, or

– Tiny crystals of ice

• Classification based on form

–Cirrus • High, white, thin

–Cumulus • Globular, puffy, cloud masses

• Vertically developed

–Stratus • Sheets or layers that cover much of the sky

• Horizontally developed


• 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”)


• Clouds of vertical development

–From low to high altitudes

–Called cumulonimbus

–Often produce rain showers and

thunderstorms

Focus Question 12.7

• Define fog.

Fog

• 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

• Advection fog

–Warm, moist air moves over a cool

surface

• Radiation fog

–Earth’s surface cools rapidly

–Forms during cool, clear, calm

nights

• 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

Focus Question 12.8

• Describe the 2 mechanisms that produce

precipitation.

How Precipitation Forms

• Cloud droplets

• < 20 micrometers (0.02 millimeter) in diameter

• Fall incredibly slowly

• Formation of precipitation – 2 ways

• 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


• Collision-coalescence process

–Warm clouds

–Large hygroscopic condensation

nuclei

–Large droplets form

–Droplets collide with other droplets

during their descent


http://www.youtube.com/watch?v=AYg4Y3mZ8cg

Focus Question 12.9

• Explain why snow can sometimes reach the

ground as rain, but the reverse does not

occur.

Forms of Precipitation

• Rain and drizzle

– Rain

• Droplets have at least a 0.5 mm diameter

– Drizzle

• Droplets have less than a 0.5 mm diameter

• Snow

– Ice crystals, or aggregates of ice crystals


• 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


• 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


• Rime

– Forms on cold surfaces

• Freezing of supercooled

fog

• Freezing of cloud droplets

Focus Question 12.10

• How is precipitation measured?

Measuring Precipitation

• Rain

– Easiest form to measure

– Measuring instruments

• Standard rain gauge –Uses a funnel to collect rain

–Cylindrical tube measures in cm or

inches

• Radar is also used to measure

the rate of rainfall


• Snow has two measurements:

1. Depth

2. Water equivalent

• General ratio is 10 snow units to

1 water unit

• Varies widely

moisture, clouds, and precipitation - lssc.edu · •environmental lapse rate greater than dry...

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