Physical Geography by Alan Arbogast Chapter 7 Atmospheric Moisture and Precipitation Lawrence McGlinn Department of Geography State University of New York - New Paltz © 2007, John Wiley and Sons, Inc.

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 © 2007, John Wiley and Sons, Inc.

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 © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Three States of Water GAS LIQUID SOLID Deposition Heat Released Sublimation Heat Absorbed Vaporization Condensation Melting Freezing © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Hydrosphere © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Hydrologic Cycle Model illustrating how water is stored and moves from one reservoirs on Earth © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Humidity Concentration of water vapor in the air 3 types: Maximum Humidity Specific Humidity Relative Humidity © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. 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 © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Saturation Curve Maximum Humidity rises dramatically with rising temperature © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Specific Humidity The measurable amt of water vapor in a mass of air units g/kg (grams water vapor/kg air) © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. 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 © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Relative Humidity Saturation Water Vapor Air Temperature © 2007, John Wiley and Sons, Inc.

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 © 2007, John Wiley and Sons, Inc.

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 © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Dew Point Temperature Think about a glass of ice water or your windshield in the morning © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Humidity Examples © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Evaporation 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 temp – higher in warmer temps RH – higher in lower humidity © 2007, John Wiley and Sons, Inc.

Physical Geography by Alan Arbogast Chapter 7 Atmospheric Moisture and Precipitation Lawrence McGlinn Department of Geography State University of New York - New Paltz © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. 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. © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. 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 5ºC/1000m elevation – less due to heat released by condensation © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Adiabatic Cooling © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. 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 © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc.

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 © 2007, John Wiley and Sons, Inc.

Unstable Atmosphere Parcel of air is encouraged to rise  Examples of Stability  Unstable Atmosphere Parcel of air is encouraged to rise © 2007, John Wiley and Sons, Inc. Figure 7.20

Stable Atmosphere Parcel of air is discouraged from rising  Examples of Stability  Stable Atmosphere Parcel of air is discouraged from rising © 2007, John Wiley and Sons, Inc. Figure 7.20

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 © 2007, John Wiley and Sons, Inc.

Distribution of Water Vapor January © 2007, John Wiley and Sons, Inc.

Distribution of Water Vapor July © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Precipitation Forms within clouds from either water droplets or ice crystals When droplet or crystal is heavy enough, it falls to earth as precipitation © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Condensation Nuclei Pure water droplets are uncommon Homogeneous nucleation Hygroscopic aerosols Dust, salt, pollution, ash Heterogeneous nucleation © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Moisture Droplets © 2007, John Wiley and Sons, Inc. Figure 7.20

© 2007, John Wiley and Sons, Inc. Precipitation 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 © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Rain and Snow © 2007, John Wiley and Sons, Inc.

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 © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. 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 – dole air moves over warmer water body © 2007, John Wiley and Sons, Inc.

Fog Types Radiation fog at Blue Mts Natl Park, Australia Sea fog across the Golden Gate, San Francisco, CA © 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc. Evaporation Fog © 2007, John Wiley and Sons, Inc. Figure 7.26

© 2007, John Wiley and Sons, Inc. 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 © 2007, John Wiley and Sons, Inc.

Cloud Types and Identification 1 2 3 5 6 7 8 9 10 4 © 2007, John Wiley and Sons, Inc.

Cumulonimbus Development © 2007, John Wiley and Sons, Inc.