GEOG 1112: Weather and Climate

Atmospheric Moisture and Precipitation

Physical Properties of Water

• H2O molecule – O side (-) – H side (+)

• Hydrogen bonding – (+) bonded to (-)

• Liquid – flexible bond

• Ice – rigid hexagonal bond

• Surface tension – water molecules hold together

• Capillary action – upward movement of water through soil and plants

Thermal Properties of Water

• Water absorbs and releases latent heat, hidden energy stored in molecular bonds

• Heat absorbed when hydrogen bonds loosened or broken – melting & evaporation

• Heat released when hydrogen bonds strengthened – freezing & condensation

Three States of Water

GAS

LIQUIDSOLID

Depos

ition

Hea

t Rel

ease

dSu

blim

atio

nH

eat A

bsor

bed

Vaporization

Condensation

Heat R

eleased

Heat A

bsorbed

Heat Released

Heat Absorbed

Melting

Freezing

Hydrologic Cycle• Model illustrating how water is stored and moves

from one reservoirs on Earth

Humidity

• Concentration of water vapor in the air

• 3 types:

– Maximum Humidity

– Specific Humidity

– Relative Humidity

Maximum Humidity

• Max amt of water vapor a body of air can hold

• Depends on air temperature

• Warmer air can hold more water vapor

• Saturation – air with max amt of water vapor is saturated, can hold no more

Saturation Curve

Maximum Humidity risesdramatically with rising temperature

Specific Humidity

• The measurable amt of water vapor in a mass of air

• units g/kg (grams water vapor/kg air)

Relative Humidity

• Ratio of specific to maximum humidity – how close the air is to saturated

• RH (%) = (SH/MH) X 100

• Cooling an unsaturated body of air raises its relative humidity

• Cool body of air to point of saturation –

100% RH - this is Dew Point Temperature

Relative Humidity

Saturation

Water Vapor

Water Vapor

Water Vapor

Air Temperature Air

Temperature

Air Temperature

Daily Pattern of Humidity•Specific humidity constant

•As air warms, its water vapor capacity increases

•RH falls

•In evening, temp & vapor capacity will fall

•RH will rise

•At 100% RH, dew forms

Dew-Point Temperature• Dew-point temperature not really a

temperature, but a measure of moisture content

• When air temperature tries to decrease below the dew point, surplus water vapor is removed from the air by condensation

Dew Point TemperatureThink about a glass of ice water or your windshield in the morning

Humidity Examples

Adiabatic Processes• Rising air expands due to reduced pressure

• Thus, rising air cools

• Falling air compresses due to greater pressure

• Thus, falling air warms

• Bouyancy caused initially by differences in (near) surface temperature

• Less dense, warmer air rises, more dense, colder air sinks, after which…

• Ascending or descending air will undergo changes in temperature with no exchange of heat. This is an adiabatic process.

Adiabatic Lapse Rates• Near surface, air usually unsaturated (< 100% RH)

• Unsaturated rising air (< 100% RH) cools at DRY Adiabatic Lapse Rate (10ºC/1000m elevation)

• Cooling air may reach dew point temp (100% RH) – condensation begins – heat is released

• Rising air =100% RH cools at WET Adiabatic Lapse Rate 6ºC/1000m elevation – less due to heat released by condensation

Adiabatic Cooling

Adiabatic Processes  • Dry adiabatic rate (DAR)

– Also called the Dry Adiabatic Lapse Rate (DALR)

– 10 C°/ 1000 m

– 5.5 F°/ 1000 ft

• Lifting Condensation Level (LCL) is reached, then…

• Moist adiabatic rate (MAR)– Also called the Wet Adiabatic Lapse Rate (WALR)

– 6 C°/ 1000 m

– 3.3 F°/ 1000 ft

Atmospheric Stability  • Stable and unstable atmospheric conditions

– Involves a parcel of air and its surrounding environment in the atmosphere

• Stable atmosphere:– A parcel of air is discouraged from rising– Kind of weather normally associated?

• Unstable atmosphere:– A parcel of air is encouraged to rise– Kind of weather normally associated?

 Examples of Stability

 Unstable AtmosphereParcel of air is encouraged to rise

 Examples of Stability

 Stable AtmosphereParcel of air is discouraged from rising

Atmospheric Stability  

• For example:– We measure and find the ELR to be

12 Cº/ 1000 m– We know the DAR is 10 Cº/ 1000 m.– We know the MAR is 6 Cº/ 1000 m.– If ELR (12) > DAR (10) > MAR (6) then?– If ELR > DAR > MAR = UNSTABLE

Precipitation

• Forms within clouds from either water droplets or ice crystals

• When droplet or crystal is heavy enough, it falls to earth as precipitation

Condensation Nuclei

• Pure water droplets are uncommon

– Homogeneous nucleation

• Hygroscopic aerosols

– Dust, salt, pollution, ash

• Heterogeneous nucleation

Moisture Droplets

Precipitation Types• Rain – large, unfrozen water droplets

• Snow – ice crystals that do not melt before they hit ground

• Sleet – rain that refreezes before hitting ground

• Freezing Rain – rain that freezes on impact with ground

• Hail – ice crystals that are repeatedly drawn up into a violent thunderstorm, growing each time

Rain and Snow

Cloud Formation & Classification

• Clouds – visible masses of suspended, minute water droplets or ice crystals

• Two conditions for cloud formation :– Air must be saturated– Small airborne particles of dust, Condensation

Nuclei, must be present

Fog• Fog forms when surface air is saturated

• How it forms :

– Radiation fog – cool or cold air is trapped at the surface – Temperature Inversion, in deep valleys or over snowy/icy surfaces

– Advection fog – warm air flows over a cooler surface – air cools to saturation

– Sea fog – cool marine air contacts colder ocean water – Calif coast

– Evaporation fog – cold air moves over warmer water body

Fog Types

Radiation fog at Blue Mts Natl Park, Australia

Sea fog across the Golden Gate, San Francisco, CA

Evaporation Fog

Cloud Classification• Categories of clouds:

– Cirrus – thin, wispy, made of ice crystals; highest altitude

– Altus – middle altitude clouds

– Stratus – layer-like gray sheets that cover most or all of sky; lowest altitude

– Cumulus – individual, puffy clouds with a flat, horizontal base; any altitude

– Nimbo- or -nimbus → precipitation

Cloud Types and Identification  

Cumulonimbus Development

Air Masses & Fronts• Air Mass – Large body of lower atmosphere with

uniform conditions of temp & moisture• Source Regions based on 2 criteria:

– Moisture Content• c – continental – dry• m – maritime – moist

– Latitude• A or AA – arctic or antarctic• P – polar – 50-60º N or S• T – tropical – 20-35º N or S• E - equatorial

Air mass source regions for North America

Precipitation Processes

• Precipitation driven by uplift in atmosphere

• 4 Types of Lifting Mechanisms:

– Convectional – warm bubbles of rising air

– Orographic – air forced up & over mts

– Frontal – air masses collide, driving air up

– Convergent – low pressure centers or troughs

Lifting Mechanisms

Convectional Uplift

• Stable air– Upper troposphere warmer– Low ELR (≤ 6ºC)– Hinders strong convection

• Unstable air– Cold upper troposphere– High ELR (> 10ºC)– Drives strong convection

Local Heating and Convection

Figure 8.7

Orographic Uplift

• Air flows up & over a natural barrier

• On windward side, air cools at DAR to dewpoint – clouds form – cooling at WAR

• Precipitation follows to top of windward side

• Air descends leeward side, warming at DAR

• Leeward side drier & warmer - Rainshadow

Orographic Precipitation

Convergent Lifting

Frontal Lifting

• Fronts: named after attacking air mass

• Remember: cold air is denser, heavier

• Cold Fronts– Cold air forces warm air aloft– 400 km wide (250 mi)

• Warm Fronts– Warm air moves up and over cold air– 1000 km wide (600 mi)

FrontsBoundaries between air masses at surface

Cold Front

Warm Front