GEOG 1112: Weather and Climate Atmospheric Moisture and Precipitation

Physical Properties of Water H 2 O 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 Deposition Heat Released Sublimation Heat Absorbed Vaporization Condensation Heat Released Heat Absorbed 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 rises dramatically 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 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 Temperature Think 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 Atmosphere Parcel of air is encouraged to rise

Examples of Stability Stable Atmosphere Parcel 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)

Fronts Boundaries between air masses at surface Cold Front Warm Front