Geography Exam #2

Importance of Water Vapor

• Energy cycle

  • Water is a greenhouse gas

  • Transports latent heat

• Rain and Snow

• Weather Hazards

• Adiabatic Processes: Affects air stability

Vapor Pressure

Pressure exerted by water vapor

Clausius-Clapeyron Relationship

Warmer air masses can hold more water vapor

Saturation vapor pressure

maximum capacity of air to hold water vapor

• Increases exponentially with temperature

• Reason that summer days are often humid

Relative Humidity

water-vapor pressure relative to saturation vapor pressure (%)

Specific Humidity

water-vapor mass relative to total air mass (g H2O / kg air)

Dew Point

Temperature at which air of given vapor pressure becomes saturated (= air reaches 100% relative humidity)

Cloud Formation

• Aggregations of small water droplets

• They are liquid water, not vapor

• Form when air becomes saturated with vapor

• Require cloud-condensation nuclei

• If air mass exceeds its saturation pressure, water will condense from vapor to liquid → clouds form

• Two ways to form clouds

  • Add moisture

  • Reduce temperature → Air passes dew point

Cloud Classification

• Stratus (layered)

• Cumulus (puffy)

• Cirrus (wispy)

• Each subdivided according to elevation and size

• All formed by condensation of water vapor to droplets

• Clouds with “nimbus” bring rain

• Cumulonimbus are the only ones that produce hail, thunder, and lightning

Atmospheric Stability

• Stability- tendency of air parcel to remain in place (stable) or rise upwards (unstable)

• Two opposing forces: gravity and buoyancy

• Convective circulation is an example of instability (air rising and falling)

Temperature and water vapor

• Warming an air mass tends to destabilize air masses (like hot air balloon)

• Adding water vapor also tends to destabilize an air mass (Adiabatic process)

Adiabatic processes

• processes that occur with no net heat exchange between a parcel of air and environment

• Rising parcel → pressure decreases and volume increases (Ideal gas law)

• Expansion causes temperature to drop, because expansion of air decreases internal heat energy (gas molecules moving away from each other, lose energy during collisions)

Why moist air cools more slowly

• Rising airmass cools (adiabatic pressure)

• Cooler airmasses have a lower saturation capacity (Clausius-Clapeyron Relationship)

• Water condenses when air reaches saturation (dew point)

• Condensing water loses latent heat energy → converts to sensible heat energy → slows rate of cooling

Moist vs. Dry Adiabatic rates

• An airmass that is warmer than surrounding air will tend to be buoyant, continue to rise

• Because moist air masses cool more slowly, they will rise further

• Airmass switches from dry to moist adiabatic rate when it cools and reaches dew point

Environmental Lapse Rate

• Lapse rate at particular time and location

• Governs the temperature profile

• Determines the stability of the atmosphere

• Average in Troposphere- 6.4 C / km or 3.5 F / 1000 ft

Thermal Equator

Line connecting zones of maximum surface temp

4 forces that drive atmospheric motion

• Gravity and Buoyancy

• Pressure Gradient Force

• Coriolis Force

• Friction

Intertropical Convergence Zone (ITCZ)

• Convergence of Hadley Cells, band of persistent clouds right around the equator

• Moves seasonally, north in summer, south in winter

Hadley Cells

• Heating at equator

  • Air warms, expands (due to solar heating)

  • Water evaporates (due to solar heating)

• Clouds form as air rises, hits a ceiling (tropopause)

• High-altitude air moves away, cools, descends

• Descending air warms, absorbs moisture

• Lower pressure near surface, higher pressure near the tropopause in these areas

Global Patterns of Surface Air Pressure

• Subtropical highs- Descending air from Hadley cell

• Polar Lows- Boundary between warm westerlies rising over polar air, stronger in winter

• Position changes seasonally

Pressure Belts

• Polar High

• Subpolar Low

• Subtropical High

• ITCZ

• Subtropical High

• Subpolar Low

• Polar High

Surface Wind Patterns

• Polar Easterlies

• Midlatitude Westerlies

• Tropical Easterlies

• Tropical Easterlies

• Midlatitude Westerlies

• Polar Easterlies

Areas of weak / variable winds

• Doldrums (equator)

• Horse Latitudes (subtropics)

Cross-Section of atmosphere patterns

Jet Stream

• Bands of high-speed upper-atmosphere air flow, found where pressure gradients are steepest

• Example of Geostrophic Flow

• Not fixed in one place, moves around with time

• Two per Hemisphere

  • Polar Jet Stream (typically stronger)

  • Subtropical Jet Stream

Rossby Waves

• Giant meanders in the Jet Stream

• Caused by upper-atmosphere variations in pressure- highs (ridges north) and lows (troughs south)

• Responsible for much of our weekly weather variations

• Effects at Earth’s Surface

  • Upper-air slowing → Upper-air Convergence → Air pushed to surface → Surface High pressure

  • Upper-air speeds up → Upper-air Divergence → Air pulled away from surface → Surface Lo Pressure

Polar Vortex

• Very fast air that moves around the poles in the stratosphere in the winter

• Large waves in the polar jet can draw cold air southward into the mid-latitudes (cold air outbreak)

Sea Breezes

• Caused by daily differential heating of land/ocean

• Day- Land heats faster → hot air rises → low surface pressures → air replaces from ocean. Surface winds move onshore

• Night- Reverses, surface winds move offshore

Monsoons

• Same process as sea breezes, instead caused by seasonal differential heating of land and ocean

• Summer- Surface winds move onshore → lots of rain

• Winter- Surface winds move offshore

Air Masses

• Regional masses of air with mostly homogenous properties (temperature, humidity, stability)

• Air masses are modified as they move

Lake Effect Snow

• Example of Air Masses being modified

• Air mass picks up moisture and energy from lake, releases snow downwind

North American Air Masses

• Continental Arctic (cA)

• Continental Polar (cP)

• Maritime Polar (mP)

• Maritime Tropical (mT)

• Continental Tropical (cT)

Purpose of Atmospheric Lifting

Produces clouds and (perhaps) precipitation

Orographic Atmospheric Lifting

• Air masses move laterally, pushed up and over mountain

• Windward- In the direction of wind flow, distinguished by heavy rainfall

• Leeward- Away from the wind flow, distinguished by dry weather

• Rain shadow- Dry weather on the leeward side

• Chinooks- Warm and dry winds that flow downslope

Convectional Atmospheric Lifting

• Surface warms, heats air masses, which rise

• Convective Storms- Strong surface heating + Unstable atmosphere = Storm

  • Can be a single cell or cluster, can produce thunder / lightning

  • Vertical movement caused by surface heating

  • Stages:

    1. Moist air rises, cool, condenses, releases latent heat

    2. Heavy rain produce downdrifts

    3. Downdrifts weaken and then suppress updraft

Convergent Atmospheric Lifting

• Air masses converging on low-pressure zone push air upwards. Important in tropics where temp differences are small.

• Related to the ITCZ

Cold Fronts

• Advancing cold air quickly lifts warm air

• Steep front → strong precipitation, storms, severe weather

• Move relatively fast

Warm Fronts

• Advancing warm air slides over cold air

• Gradual lifting → drizzle and overcast skies

• Move relatively slow

Cyclones

• Weather patterns around a low pressure zone

• Centered on low pressure zone

• Low pressure causes surface convergence

• Draws in warm air from south and cold air from north

• Cold and warm fronts rotate around central low

Cyclonic Storm Tracks

• Carried west to east by westerlies

• Moisture sources- Pacific, Gulf of Mexico

• Track shifts seasonally

Hurricanes

• Intense form of tropical storm

• High risk to life and property, due to high wind speeds and storm surges

Formation of Tropical Cyclones

• Also start as a line of mild low-pressure, called easterly wave

• Air converges on low pressure and rises, creating clouds and thunderstorms

• Rising air strengthens the low-pressure systems and air starts to rotate

Formation of Hurricanes

• Warm ocean surface generates convection and vertical movement

• Moist air condenses and releases latent heat- rain starts

• Creates warmer air that is more buoyant and air continues to rise

• Rising air causes low-pressure zone at the surface. Winds converge and spiral inwards

• Continued convection from the warm ocean surface intensifies storm

• Fastest wind and greatest damage often to right of eye and travel path

• Spawn in warm subtropical waters, due to warm water with lots of energy and Coriolis rotation

Storm Surge

• Water rises due to drop in air pressure and push of water to coast

Importance of the Ocean

• Water- Global hydrosphere- 97.2% of water

• Carbon- Global carbon cycle- 90% of carbon is in oceans, in intermediate and deep waters

• Buffer- Absorbing ~25% of all CO2 we emit from fossil fuels

• Energy- Transports about 25% of heat energy to high latitudes

Ocean Vertical Structure

Deep ocean is colder (thermocline), saltier (Halocline), denser (Pycnocline), darker

Ocean’s two layers

• Shallow mixed ocean (top 100-150m)

  • Circulation driven by wind

  • Well-mixed

  • Sunlit → Photosynthesis

• Deep ocean (below 100m)

  • Circulation driven by density gradients

  • Slow circulation (1000 yrs)

Ocean temperature at different latitudes

• Ocean sea surface temps show a clear latitudinal gradient- warmest at equator, coldest at poles.

  • Therefore, thermal stratification is strongest in low latitudes, weakest at poles

Ocean Surface Circulation

• Wind-driven

• In Tropics- winds cause water divergence

• In Subtropics- winds cause water convergence

• Dominated by subtropical gyres

Ekman Spiral

• Wind drags ocean surface

• Surface drags subsurface

• Water is deflected by Coriolis

Gyres

• large, circular ocean currents centered on subtropical convergence

• Five subtropical gyres dominate ocean circulation

Gulf Stream

• Western arm of North Atlantic Gyre

• Carries heat from equator to Arctic and warms Europe and coastal North America

Major warm ocean currents

• East Australian Current

• Kuroshio Current

• Gulf Stream

Upwelling

• Process in which cold, deep water is brough to the surface

• Coastal upwelling

  • Wind + Coriolis force pushes surface water away from coastline

  • Allows deep, nutrient-rich water to upwell

Deep Ocean Circulation

• Driven by density gradients

• Linked together into one “global conveyor belt”

  • Called the “Thermohaline Circulation”

    • Meaning it is driven by temperature and salinity

• Slow- takes about 1000 years for deep ocean to complete 1 cycle

• In most areas, stable stratification prevents overturning

Ways to make ocean surface water dense enough to sink

• Make it colder

• Make it saltier

  • Ice formation and brine exclusion (poles)

  • Evaporate lots of water (Mediterranean)

• Most deep water formation occurs at poles

Ocean Acidification

• Oceans absorb 1/4 to 1/3 of CO2 that we release

  • Good- “ocean sink” slows down rater of climate change

  • Bad- Added CO2 increases the acidity of oceans

• Ocean acidification

  • CO2 and water react to form Carbonic Acid (H2CO3)

  • Carbonic Acid dissolves to release hydrogen ions

  • Recall that acidity measures the concentration of H+ in liquid

  • Ocean Acidity has increased over the last 20 years

    • Ocean pH has increased by 0.1

    • 26% increase in concentration of hydrogen ions

  • A problem because H+ reacts with and removes Co3-2 which many species use to build their shells

Uses of Surface Water

• Agriculture

• Drinking water

• Power plant cooling

• Mining and Manufacturing

• Recreation

Hazards of Surface Water

• Flooding

• Droughts

• Mudslides and Landslides

Hydrologic Cycle

• Precipitation transfers water from atmosphere to ocean and land (supply)

• Evaporation transfers water from oceans and land to atmosphere (demand)

• Transpiration transfers water from plants to atmosphere (demand)

• Runoff transfers water from land to oceans. Rechange soil and groundwater includes transfer of water within soil column (surplus)

Surface Water Balance - Precipitation

• Highest in tropics due to ITCZ + Hadley Cells

• Also high in subtropical regions where monsoons are strong

• (In US) high in the Pacific Northwest and Eastern US, caused by moisture advection from oceans, mountain rain shadows

• Atmospheric Rivers- Narrow plumes of water vapor carried by the westerlies

  • Ex: Pineapple Express in California

  • One atmospheric river can carry more water than the Amazon river

Surface Water Balance - Evapotranspiration

• Evaporation and Transpiration, often combine into Evapotranspiration

• Evapotranspiration Global Patterns

  • Highest in areas with heat and plenty of water, subtropical oceans

  • Lowest in areas with low water availability, under the descending limb of Hadley Cell, or cold areas, the poles.

PET- Potential Evapotranspiration

• “Potential” means maximum possible, given unlimited water supply

• Highest when high temps, high sunlight, and windy.

AET- Actual Evapotranspiration

• Actual ET, limited by supply of water

• Highest in warm areas with high water availability

• AET is always less than or equal to PET

Water Balance

• Deficits and Surpluses result from imbalances between supply and demand

• Deficits cause water loss from land surface, reductions in surface, soil, and groundwater reserves

• Surpluses can go to stream runoff, rechange soil moisture, recharge groundwater.

  • Runoff is high when precipitation is high (tropics, coasts), and ET is low (arctic).

• Supply > Demand = Surplus

• Demand > Supply - Deficit

Percolation

The gravitational pull of water downwards through pore spaces in soil and rock

Porosity

The amount of pore space in rock or sediments

Permeability

The degree of connectedness among pores

• Ex: Swiss cheese, High Porosity, Low Permeability

Types of water in soil

• Hygroscopic Water- Bound too tightly to soil particles for plants to access

• Capillary water- Most accessible to plants

• Gravitational water- Escapes down too quickly for plants to access

Grain sizes in soil

• Coarse sand- Pore spaces big, high gravitational water loss

• Fine grains (clay)- pore spaces small, mostly hydroscopic water

• Silt- Grain sizes between sand and clay

• Loams- Mixture of clay, silt and sand (ideal)

Groundwater Recharge

Water that percolates down through soil enters the groundwater

Vadose Zone vs. Saturated Zone

V- Mixture of air and water in the ground

S- All pores filled by water in the ground

Water Table

Top of the saturated zone of the ground

Basic Behavior of Ground Water

• Water flows by gravity, pulled downwards

• Flow is slow due to high friction, can take decades

• Water table surface mostly follows contours of land surface

• Wells create cones of depression, lowered water table

Aquifers vs. Aquicludes

• Aquifers- Permeable rocks and sediments, reservoirs of groundwater

  • Unconfined Aquifers- Permeable sediments and rocks above

  • Confined Aquifers- Capped by impermeable layer of rock

• Aquicludes- Impermeable rocks and sediments

Groundwater Issues

• Groundwater Mining- When rate of water pumping is higher than rate of recharge

• Groundwater Contamination- When toxic materials are introduced into groundwater

  • Landfills, Agricultural runoff, Septic tanks, Industrial waste, Oil and gas drilling.

• Fracking- Fracturing rocks underground to increase permeability, return natural gas

  • A slurry of water, chemicals, and sands pumped underground to fracture rocks

  • Will these chemicals and natural gas contaminate groundwater?

Aridity vs. Drought

• Aridity- Long-term scarcity of water (Saharan desert)

• Drought- Short-term lack of water (Wisconsin sometimes experiences)

Effects of climate change on water

• Warming increases moisture transport, making wet seasons and events wetter

• Warming increases evaporative demand and drought severity

• Wet get wetter, Dry get dryer