Chapter 7Chapter 7

Atmospheric Moisture and Precipitation

Atmospheric Moisture & PrecipAtmospheric Moisture & Precip

• Physical properties of water

• Hydrosphere & Hydrological Cycle

• Humidity

• Evaporation

• Adiabatic processes

• Cloud Formation and Classification

• Precipitation

Atmospheric Moisture & PrecipAtmospheric Moisture & PrecipObjectivesObjectives

• Understand energies associated with phase changes of water

• Follow the hydrologic cycle, noting when processes result in precipitation/evaporation

• Distinguish between types of humidity and how nature modifies humidity

• Evaluate natural processes that change amount and rate of evaporation

• Relate pressure and temperature changes of an air parcel to the adiabatic process

• List ingredients needed for cloud formation

• Demonstrate how precipitation is produced via the 4 air lifting mechanisms

Physical Properties of WaterPhysical Properties of Water

• H20 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 WaterThermal 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

Latent Heat

Three States of WaterThree States of Water

HydrosphereHydrosphere

Hydrologic CycleHydrologic Cycle• Model illustrating how water is stored and moves

from one reservoirs on Earth

El Nino ______ temperatures in spring/summer and _____ temps in winter, respectively.

a) decreases, increases b) doesn't change, increasesc) increases, doesn't change d) increases, increasese) increases, decreases

Answer: E

HumidityHumidity• Concentration of water vapor in the air

• 3 types:

• __________________

• __________________

• __________________

Maximum HumidityMaximum 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 CurveSaturation Curve

Maximum Humidity risesdramatically with rising temperature

Specific HumiditySpecific Humidity

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

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

Relative HumidityRelative 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

Daily Pattern of HumidityDaily 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

Sling PsychrometerSling Psychrometer• Spin to measure humidity• Wet-bulb thermometer - evaporation• Dry-bulb thermometer - no evaporation• Big wet-dry difference - low humidity• Small wet-dry difference high humidity

EvaporationEvaporation

• Evaporation - Liquid water to water vapor

• Transpiration – water flows thru leaf pores in plants to atmosphere

• Evapotranspiration – combination of evaporation and transpiration

• Evapotranspiration rate depends on :

• Net radiation - higher in sunshine

• Air temperature - higher in warmer temps

• Relative humidity – higher in lower humidity

The temperature at which an air mass reaches water vapor saturation.

a) absolute humidity b) maximum humidity c) dew pointd) specific humidity e) relative humidity

Answer: C

Adiabatic ProcessesAdiabatic Processes

• Rising air expands due to reduced pressure

• Thus, rising air cools

• Falling air compresses due to greater pressure

• Thus, falling air warms

© 2007, John Wiley and Sons, Inc.

Adiabatic ProcessesAdiabatic Processes

Adiabatic Lapse RatesAdiabatic 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 5ºC/1000m elevation – less due to heat released by condensation

Adiabatic CoolingAdiabatic Cooling

Adiabatic Processes

To make the humidity of an air mass increase without changing the amount of moisture present, you would

a) Increase the temperature of the air massb) decrease the temperature of the air massc) Decrease the specific humidity of the air massd) increase the dew point temperature of the air masse) all of the above

Answer: B

Which one of the following statements about relative humidity is incorrect?

a) ratio of specific humidity max humidity; how close air is to saturated b) RH (%) = (SH/MH) x 100 c) Cooling an unsaturated body of air raises its relative humidityd) Cool an air parcel to point of saturation – 100% RH – this is dew pt temperaturee) all of the above statements are trueAnswer: E

Cloud Formation & ClassificationCloud Formation & Classification

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

• Three conditions for cloud formation :

• Air must be saturated or near saturation

• Air must be rising/cooling

• Small airborne particles of dust, condensation nuclei, must be present

Water Vapor Cloud Cover (visible)

Water Vapor & CloudsWater Vapor & CloudsClouds occur in regions with a high density of water vapor

FogFog• 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

Fog TypesFog Types

Radiation fog at Blue Mts Natl Park, Australia

Sea fog across the Golden Gate, San Francisco, CA

Cloud ClassificationCloud Classification• 3 Categories of clouds:

• Cirrus – thin, wispy, made of ice crystals

• Cumulus – individual, puffy clouds with a flat, horizontal base

• Stratus – layer-like gray sheets that cover most or all of sky

• Nimbo- or -nimbus → precipitation

Cloud TypesCloud Types

PrecipitationPrecipitation

• Forms within clouds from either water droplets or ice crystals

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

Precipitation TypesPrecipitation Types• Rain – large, unfrozen water droplets

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

• Sleet – rain that freezes 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

Precipitation ProcessesPrecipitation Processes• Precipitation driven by uplift in atmosphere

• 4 Types:

• 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

Precipitation ProcessesPrecipitation Processes

Convectional UpliftConvectional Uplift• Stable air

• Upper troposphere warmer

• ____Low___ ELR (≤ 5ºC)

• Hinders strong convection

• Unstable air

• Cold upper troposphere

• High ELR (> 10ºC)

• Drives strong convection

Unstable Air -Strong Convection - Precipitation

Stable Air – Little Convection - No Precipitation

Convectional UpliftConvectional Uplift

Convectional Precipitation

Orographic UpliftOrographic 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 leeward side, warming at DAR

• Leeward side drier & warmer - Rainshadow

Uplift of air that occurs when bubbles of warm air rise within an unstable body of air is convectional uplift.

Orographic UpliftOrographic Uplift

Rainshadows & Deserts Rainshadows & Deserts in Californiain California

Orographic Uplift

-Moisture from the Pacific Ocean is cut off by the mts of Calif including Coastal Ranges, Sierra Nevadas and San Bernadino Mts

-Internal valleys are dry, driest to the south