hydrology presentation on rainfall

Properties of WaterProperties of Water• Physical States

– only natural substance that occurs naturally in three states on the earth’s surface

• Heat Capacity– Highest of all common solids and liquids

• Surface Tension– Highest of all common liquids

• Latent Heat of Fusion– Highest of all common substances

• Compressibility– Virtually incompressible as a liquid

• Density– Density of seawater is controlled by temperature, salinity and pressure– Liquid has maximum density at +4oC; solid phase has lower density!

Properties of Water (cont’)Properties of Water (cont’)

• Radiative Properties– transparent to visible wavelengths– virtually opaque to many infrared wavelengths– large range of albedo possible

• water 10 % (daily average) • Ice 30 to 40%• Snow 20 to 95%• Cloud 30 to 90%

Molecular Structure of WaterMolecular Structure of Water

Water's unique molecular structure and hydrogen bonds enable all 3 phases to exist in earth's atmosphere.Sublimation & deposition describe the non-incremental changes between solid and vapor phases.

water moleculeice

Sublimation

Deposition

Energy associated with phase changeEnergy associated with phase change

Why does it take so much Why does it take so much energy to evaporate water?energy to evaporate water?

• In the liquid state, adjacent water molecules attract one another– “-” charge on O attracted to “+” charge on H– we call this hydrogen bonding

• This same hydrogen bond accounts for surface tension on a free water surface– column of water “sticks together”

Sublimation – Sublimation – evaporate ice directly to water vaporevaporate ice directly to water vapor

Take one gram of ice at zero degrees centigrade

Energy required to change the phase of one gram of ice to vapor:

Add 80 calories to melt the iceAdd 100 calories to raise the temperature to 100 degrees CAdd 540 calories to evaporate the liquid

Total Energy ADDED for sublimation of 1 gram of ice:

80 + 100 + 540 = 720 calories

Deposition –Deposition –

convert vapor directly to iceconvert vapor directly to ice

Take one gram of water vapor at 100 degrees Centigrade

Release 540 calories to condenseRelease 100 calories to cool temperature of liquid to oCRelease 80 calories to freeze water

Total energy RELEASED for deposition of 1 gram of ice

540 + 100 + 80 = 720 calories

Water vapor pressureWater vapor pressure

• Molecules in an air parcel all contribute to pressure

• Each subset of molecules (e.g., N2, O2, H2O) exerts a partial pressure

• The VAPOR PRESSURE, e, is the pressure exerted by water vapor molecules in the air– similar to atmospheric pressure, but due only to the

water vapor molecules– often expressed in mbar (2-30 mbar common at

surface)

Water vapor saturationWater vapor saturation

• Water molecules move between the liquid and gas phases

• When the rate of water molecules entering the liquid equals the rate leaving the liquid, we have equilibrium– The air is said to be

saturated with water vapor at this point

– Equilibrium does not mean no exchange occurs

Relationship between eRelationship between eSS and T and T

• The saturation vapor pressure of water increases with temperature– At higher T, faster water

molecules in liquid escape more frequently causing equilibrium water vapor concentration to rise

– We sometimes say “warmer air can hold more water”

• There is also a vapor pressure of water over an ice surface– The saturation vapor

pressure above solid ice is less than above liquid water

Saturation vapor pressure depends upon temperature

eeSS vs T schematic vs T schematic

How do we express the amount of How do we express the amount of water vapor in an air parcel?water vapor in an air parcel?

• Absolute humidity– mass of water vapor/volume of air (g/m3)– changes when air parcel volume changes

• Specific humidity– mass of water vapor/mass of air (g/kg)

• Mixing ratio– mass of water vapor/mass of dry air (g/kg)

• Specific humidity and mixing ratio remain constant as long as water vapor is not added/removed to/from air parcel

• Dew point temperature

Expressing the water vapor Expressing the water vapor pressurepressure

• Relative Humidity (RH) is ratio of actual vapor pressure to saturation vapor pressure– 100 * e/eS

– Range: 0-100% (+)– Air with RH > 100% is supersaturated

• RH can be changed by– Changes in water vapor content, e– Changes in temperature, which alter eS

Dewpoint TemperaturesDewpoint Temperatures

• Dewpoint is a measure of the water vapor content of the air• It is not a measure of temperature!

Which environment has higher Which environment has higher water vapor content?water vapor content?

Why is the southwest coast Why is the southwest coast of the US hot and dry while of the US hot and dry while

the Gulf coast is hot and the Gulf coast is hot and moist?moist?

• Both are adjacent to large bodies of water

• Both experience onshore wind flow on a regular basis

• Why does one have a desert like climate and the other ample moisture and rainfall?

The cold water temperatures typically found off the west coast of continentsare a result of oceanic upwelling which ocean currents typically cause inthese locations

Humidity reflects water tempsHumidity reflects water temps

Water vapor is distributed Water vapor is distributed throughout the atmospherethroughout the atmosphere

• Generally largest amounts are found close to the surface, decreasing aloft– Closest to the source - evaporation from

ground, plants, lakes and ocean– Warmer air can hold more water vapor

than colder air

Take-Away PointsTake-Away Points

1. Weather is driven by unequal solar heating and cooling

2. Air motions are affected by the Coriolis Effect and “centrifugal” force

3. High and Low Pressure Systems4. Air flows parallel to pressure contours

(Geostrophic winds)5. Air masses meet along sharp boundaries

or fronts6. Weather is inherently chaotic and that

limits our ability to forecast it

The SeasonsThe Seasons

Uneven solar heating on EarthUneven solar heating on Earth

• Solar energy in high latitudes:– Has a larger “footprint”– Is reflected to a greater

extent– Passes through more

atmosphere– Is less than that

received in low latitudes

Earth’s seasonsEarth’s seasons

• Earth’s axis is tilted 23½º from vertical

• Northern and Southern Hemispheres are alternately tilted toward and away from the Sun

• Causes longer days and more intense solar radiation during summer

Oceanic heat flowOceanic heat flow

• A net heat gain is experienced in low latitudes

• A net heat loss is experienced in high latitudes

• Heat gain and loss are balanced by oceanic and atmospheric circulation

Physical properties of the Physical properties of the atmosphere: Composition (dry atmosphere: Composition (dry

air)air)

Gas PercentNitrogen (N2) 78.1%

Oxygen (O2) 20.9%

Argon (Ar) 0.9%Carbon dioxide (CO2)

0.036%

All others Trace

Physical properties of the Physical properties of the atmosphere: Temperatureatmosphere: Temperature

• Troposphere is:– Lowermost part of

the atmosphere– Where most

weather occurs• Temperature of

troposphere cools with increasing altitude

Physical properties of the Physical properties of the atmosphere: Densityatmosphere: Density

• Warm, low density air rises

• Cool, high density air sinks

• Creates circular- moving loop of air (convection cell)

Physical properties of the Physical properties of the atmosphere: Water vaporatmosphere: Water vapor

• Cool air cannot hold much water vapor, so is typically dry

• Warm air can hold more water vapor, so is typically moist

• Water vapor decreases the density of air

Physical properties of the Physical properties of the atmosphere: Pressureatmosphere: Pressure

• A column of cool, dense air causes high pressure at the surface, which will lead to sinking air

• A column of warm, less dense air causes low pressure at the surface, which will lead to rising air

Physical properties of the Physical properties of the atmosphere: Movementatmosphere: Movement

• Air always moves from high-pressure regions toward low-pressure regions

• Moving air is called wind

The Coriolis effectThe Coriolis effect

• The Coriolis effect– Is a result of Earth’s rotation– Causes moving objects to follow curved

paths:• In Northern Hemisphere, curvature is to right• In Southern Hemisphere, curvature is to left

– Changes with latitude:• No Coriolis effect at Equator• Maximum Coriolis effect at poles

A merry-go-round as an A merry-go-round as an example of the Coriolis effectexample of the Coriolis effect

• To an observer above the merry-go-round, objects travel straight

• To an observer on the merry-go-round, objects follow curved paths

The Coriolis effect on EarthThe Coriolis effect on Earth

• As Earth rotates, different latitudes travel at different speeds

• The change in speed with latitude causes the Coriolis effect

Figure 6-9a

Missile paths demonstrate the Missile paths demonstrate the Coriolis effect Coriolis effect

• Two missiles are fired toward a target in the Northern Hemisphere

• Both missiles curve to the right

Figure 6-9b

Wind belts of the worldWind belts of the world

Figure 6-10

Characteristics of wind belts Characteristics of wind belts and boundariesand boundaries

Region/Latitude Wind belt or boundary name

Characteristic

Equatorial (0-5º) Doldrums Low press. boundary

5-30º Trade winds Persistent easterlies

30º Horse latitudes High press. boundary

30-60º Prevailing westerlies Mid-latitude winds

60º Polar front Low press. boundary

60-90º Polar easterlies Cool easterly winds

Polar (90º) Polar high pressure High press. boundary

Coriolis effect influences air Coriolis effect influences air movementmovement

• Northern Hemisphere winds curve to the right as they move from high to low pressure

• Causes wind to circulate:– Clockwise around high-

pressure regions– Counterclockwise around

low-pressure regions

Air masses that affect U.S. Air masses that affect U.S. weatherweather

Origin and paths of tropical Origin and paths of tropical cyclonescyclones

• Tropical cyclones are intense low pressure storms created by:– Warm water– Moist air– Coriolis effect

• Includes:– Hurricanes– Cyclones– Typhoons

Hurricane occurrenceHurricane occurrence

• Hurricanes have wind speeds of at least 120 kilometers (74 miles) per hour

• Worldwide, about 100 storms grow to hurricane status each year

• In the Northern Hemisphere, hurricane season is generally between June 1 and November 30

Hurricane structureHurricane structure

• Hurricanes have:– Circular cloud

bands that produce torrential rain

– The ability to move into the mid-latitudes

– A central eye

Hurricanes produce storm Hurricanes produce storm surgesurge

• Storm surge:– Is a rise in sea level

created by hurricane coming ashore

– Can be up to 12 meters (40 feet) high

– Causes most destruction and fatalities associated with hurricanes

Climate regions of the oceanClimate regions of the ocean

How a greenhouse worksHow a greenhouse works

• Sunlight passes through the clear covering of a greenhouse

• It converts to longer wavelength heat energy

• Heat cannot pass through the covering and is trapped inside

The heating of Earth’s The heating of Earth’s atmosphereatmosphere

Anthropogenic gases that Anthropogenic gases that contribute to the greenhouse contribute to the greenhouse

effecteffectGreenhouse Gas Relative

contribution

Carbon dioxide (CO2)

60%

Methane (CH4) 15%

Nitrous oxide (N2O) 5%

Tropospheric ozone (O3)

8%

CFC-11 4%

CFC-12 8%

Carbon dioxide is increasing in Carbon dioxide is increasing in the atmospherethe atmosphere

• As a result of human activities, carbon dioxide in the atmosphere has increased by 30% since 200 years ago

Earth’s average temperature is Earth’s average temperature is risingrising

• Earth’s average surface temperature has risen at least 0.6°C (1.1°F) in the last 130 years

• May be related to increase in atmospheric carbon dioxide

Predicted changes with Predicted changes with increased greenhouse warmingincreased greenhouse warming• Higher than normal sea surface

temperatures that could affect world climate• More severe droughts or increased

precipitation• Water contamination and outbreaks of

water-borne diseases• Longer and more intense heat waves• Shifts in the distribution of plants and

animals• Potential melting or enlargement of polar

ice caps

Atmospheric CirculationAtmospheric Circulation


Asymmetric EarthAsymmetric Earth


Atmospheric CirculationAtmospheric Circulation


Zonal and Meridional FlowZonal and Meridional Flow


Semi-Permanent Features, Semi-Permanent Features, JanuaryJanuary


Semi-Permanent Features, JulySemi-Permanent Features, July


Rotation EffectsRotation Effects


The Coriolis EffectThe Coriolis Effect

• Due to moving on a rotating earth

• Things on equator are moving faster than points near poles

• Affects:– Winds– Ocean Currents– Tides


The Coriolis EffectThe Coriolis Effect

• Things moving toward the equator are deflected west

• Things moving poleward are deflected east

• Deflected to Right in Northern Hemisphere

• Deflected to Left in Southern Hemisphere


The The Coriolis Coriolis EffectEffect


““Centrifugal” Force Does Not Centrifugal” Force Does Not ExistExist

• When anything turns, the only forces that act are in the direction of the turn

• These forces are called centripetal (center-seeking) force

• “Centrifugal” force is an illusion• “Centrifugal” force is due to inertia

and centripetal force opposing each other


High Pressure SystemsHigh Pressure Systems

3. High and Low Pressure Systems

High Pressure SystemsHigh Pressure Systems

• Air flows out from center• Spin clockwise in Northern

Hemisphere• No air mixing• Stable, fair weather• Sinking Air, few clouds• Long duration can result in

inversions, pollution• Winter: often extreme cold

– Cold Air is Dense– Clear Skies and Radiational Cooling


Low Pressure SystemsLow Pressure Systems


Why Counterclockwise?Why Counterclockwise?


Low Pressure SystemsLow Pressure Systems

• Air flows in toward center• Spin counter-clockwise in Northern

Hemisphere• Mixes air of different properties• Associated with fronts• Stormy, sometimes violent

weather• Passage often results in sharp

change in weather conditions3. High and Low Pressure Systems

Geostrophic WindsGeostrophic Winds• As air flows in or out of pressure cells,

Coriolis Effect deflects it• At surface, friction limits the deflection.

– Winds blow about 45 degree angles to isobars

• Aloft, friction not a factor– Deflection continues until limited by pressure

gradient (winds can’t go against pressure)– Winds blow parallel to contours– This is called geostrophic flow

Geostrophic FlowGeostrophic Flow

Geostrophic FlowGeostrophic Flow

4. Air flows parallel to pressure contours (Geostrophic winds)

1905 Weather Map of US1905 Weather Map of US

First U.S. Weather Map With First U.S. Weather Map With FrontsFronts

Fronts and Low Pressure Fronts and Low Pressure SystemsSystems

5. Air masses meet along sharp boundaries or fronts

FrontsFronts


Warm FrontsWarm Fronts


Warm FrontsWarm Fronts

• Gradual Onset• Warm Air over Cool

Air• Little Turbulence• Weather Rarely

Violent


Cold FrontsCold Fronts


Cold FrontsCold Fronts

• Abrupt Onset• Cold Air Lifting Warm Air• Considerable Turbulence• Weather Sometimes Violent• Thunderstorms Common• Can Spawn Tornadoes


Old Low Pressure SystemsOld Low Pressure Systems


Occluded FrontsOccluded Fronts


Occluded FrontsOccluded Fronts

• Two fronts merge• Any two types of front can occlude• Most common: Cold Front

overtakes Warm Front• Starts off like a warm front,

finishes like a cold front


Weather PredictionWeather Prediction


ChaosChaos

x

Chaos TheoryChaos Theory

Does Not Mean:• Cloned Dinosaurs will run amok• Systems do not follow physical laws• Systems behave with wild

unpredictability• Systems do not have limits• Phenomena cannot be predicted

6. Weather is inherently chaotic and that limits our ability to forecast it

Chaos TheoryChaos Theory

Does Mean:• Small differences compound over

time• There are limits to how accurately

phenomena can be predicted• Examples:

– Weather– The Planets– Traffic

6. Weather is inherently chaotic and that limits our ability to forecast it

Who Cares About Water Anyways?Who Cares About Water Anyways?

• Phase changes of water important for energy transport in atmosphere

• Clouds!• Precipitation!

OverviewOverview

• Cloud: Collection of liquid water drops or ice crystals

• Clouds form as– 1) Water vapor condenses onto small particles

known as condensation nuclei to form liquid water drops, or

– 2) Water vapor deposits onto small particles known as ice nuclei that allow for ice crystal formation

• In a cloud, water can be present in all three phases at the same time

Hydrologic CycleHydrologic Cycle

• Global precipitation = Global evaporation

TermsTerms

• Evaporation: liquid water molecules break bonds with other molecules to escape to gaseous phase

• Condensation: Water vapor returns to liquid state

• Sublimation: Ice changes directly to water vapor

• Deposition: Water vapor changes directly to ice

• Transpiration: Plants releasing water vapor into air

Temperature and EvaporationTemperature and Evaporation

• Evaporation occurs when liquid water molecules gain enough kinetic energy to break bonds– The higher the temperature of water, the

higher the kinetic energy of its molecules, thus the higher the evaporation rate

• Evaporation is a cooling process

Balance of Evaporation & CondensationBalance of Evaporation & Condensation

• Net Condensation: Condensation exceeds evaporation

• Net Evaporation: Evaporation exceeds condensation

• Vapor Pressure: Water vapor’s contribution to total pressure

Equilibrium Vapor Pressure & TemperatureEquilibrium Vapor Pressure & Temperature

Birth of CloudsBirth of Clouds

• Relative Humidity: (vapor pressure / equilibrium vapor pressure) * 100 – At saturation (rate of condensation = rate of

evaporation), RH = 100%– Clouds form when RH exceeds 100% by a few

tenths of a percent– Water vapor condenses onto CCN…some

hygroscopic, meaning they attract water vapor– In summer, when RH exceeds 80%, net

condensation occurs on some particles (pollution), leading to haze…associated with poor air quality

Mechanisms to induce cloud formationMechanisms to induce cloud formation

• For clouds to form, there must be net condensation

• We can get this by cooling the air– As temperature lowers, molecular

speeds decrease, and water vapor gathers near CCN

– The amount of cooling needed is inversely proportional to the amount of water vapor in the air

Fog Formation by Cooling AirFog Formation by Cooling Air

Cooling via liftingCooling via lifting

• Air pressure (density) decreases with height

• Rising air parcels expand, cooling as they do work on environment

• If vapor pressure = equilibrium vapor pressure => condensation

Clouds due to LiftingClouds due to Lifting

Orographic Lifting: Lifting by TerrainOrographic Lifting: Lifting by Terrain

• Windward side of mountain, facing prevailing wind, is extremely wet

• Leeward side, sheltered from wind, very dry…known as rain shadow

Clouds due to TerrainClouds due to Terrain

Orographic Lifting: CaliforniaOrographic Lifting: California

Mixing Warm & Cold Air MassesMixing Warm & Cold Air Masses

Assessing Air’s Moisture ContentAssessing Air’s Moisture Content

• Problems with RH because denominator depends on temperature– Cold, dry air masses can have a high RH, even if

they hold little water vapor– Relative humidity varies with time of day

Dew Point: Absolute measure of water vaporDew Point: Absolute measure of water vapor

• Dew Point: Temperature air must be cooled (at constant pressure) to reach saturation– Less than or equal to temperature – Higher the dew point, more water vapor in air– Frost point if air temperature below 32ºF– Measured with a hygrometer or sling

psychrometer– Changes by evaporating water into air, mixing

drier air from above, wind blowing in moist or dry air from another region (air dries behind cold front, moistens before cold front)

Applying Dew Point to Weather ForecastingApplying Dew Point to Weather Forecasting

• 1) Cloud Base Height– Temperature of rising air decreases

faster than dew point…has a decent chance of eventually reaching dew point

• 2) First-Guess Low Temperature• 3) Severe Weather

– High dew points indicate enhanced risk

The Earth’s Hydrologic CycleThe Earth’s Hydrologic Cycle

hydrology presentation on rainfall

Documents

gram of water vapor

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