WATER’S ROLE IN THE ATMOSPHERE

HUMIDITYAmount of water vapor in the

airWater vapor adds pressure (called vapor pressure) to the air

Saturated air is air that is filled with water vapor to capacity

Capacity is temperature dependent – warm air has a much greater capacity

Measurements of humiditySpecific humidity

Quantity of water vapor in a given mass of air

Often measured in grams per kilogram

Relative humidityRatio of the air’s actual water vapor content to its potential water vapor capacity, at a given temperature

Expressed as a percent

Saturated airContent equals capacityHas a 100 percent relative humidity

Relative humidity can be changed in two waysAdd or subtract moisture to the air•Adding moisture raises the relative humidity

•Removing moisture lowers the relative humidity

Changing the air temperature•Lowering the temperature raises the relative humidity

•Raising the temperature lowers the relative humidity

Dew pointTemperature at which the air is saturated and the relative humidity is 100 percent

Cooling the air below the dew point causes condensation•E.g., cloud formation•Water vapor requires a surface condense on

Two types of hygrometers are used to measure humidityPsychrometer

•Compare the temperatures ofWet-bulb thermometer, andDry-bulb thermometer

• If the air is saturated (100 percent relative humidity) then both thermometers read the same temperature

•The greater the difference between the thermometer readings, the lower the relative humidity

Hair hygrometer – reads the humidity directly

ADIABATIC HEATING/COOLINGAdiabatic temperature changes occur when

Air is compressedMotion of air molecules increasesAir will warmDescending air is compressed due to increasing air pressure

Air expandsAir parcel does work on the surrounding airAir will coolRising air will expand due to decreasing air pressure

Adiabatic ratesDry adiabatic rate

Unsaturated airRising air expands and cools at 1°C per 100 meters

Descending air is compressed and warms at 1°C per 100 meters

Wet adiabatic rateCommences at condensation level

Air has reached the dew pointCondensation is occurring and latent heat will be liberated

Heat released by the condensing water reduces the rate of cooling

Rate varies from 0.5°C to 0.9°C per 100 meters

STABILITY OF AIRTwo types of air stability

STABLE AIR

Resists vertical displacement(sinking air, or air sitting at the surface)Cooler than surrounding airDenser than surrounding airWants to sink

No adiabatic coolingStability occurs when the environmental lapse rate is less than the wet adiabatic rate

Often results in widespread clouds with little vertical thickness

Precipitation, if any, is light to moderate

UNSTABLE AIR

Acts like a hot air balloon

Rising airWarmer than surrounding air

Less dense than surrounding air

Continues to rise until it reaches an altitude with the same temperature

Adiabatic cooling

Air is unstable when the environmental lapse rate is greater than the dry adiabatic rate

Clouds are often towering

Often result in heavy precipitation

Conditional instability occurs when the atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parcel of air

Determines to a large degreeClouds that developIntensity of the precipitation

PROCESSES THAT LIFT AIR

Orographic liftingElevated terrains act as barriersResult can be a rain shadow desert

Frontal wedgingCool air acts as a barrier to warm air

Fronts are part of the storm systems called middle-latitude cyclones

Convergence where the air is flowing together and rising

LIFTING AIR

CONDENSATION AND CLOUD FORMATION

CondensationWater vapor in the air changes to a liquid and forms dew, fog or clouds

Water vapor requires a surface to condense on

Possible condensation surfaces on the ground can be grass, a car window, etc.

Possible condensation surfaces in the atmosphere are tiny bits of particulate matterCalled condensation nucleiDust, smoke, etcOcean salt crystals which serve as hygroscopic (“water seeking) nuclei

Clouds

Made of millions and millions of Minute water droplets, or tiny crystals of ice

Classification based on Form (three basic forms)

Cirrus – high, white, thinCumulus

•Globular cloud masses•Often associated with fair weather

Stratus•Sheets or layers•Cover much of the sky

HeightHigh clouds

• Above 6000 meters

• TypesCirrusCirrostratusCirrocumulu

sMiddle clouds

• 2000 to 6000 meters

• rainy)

• Types (alto as part of the name_AltocumulusAltostratus

Low clouds• Below 2000

meters• Types

StratusStratocumulusNimbostratus

(nimbus means

Clouds of vertical development•From low to high altitudes•Called cumulonimbus•Often produce

Rain showersThunderstorms

HIGH CLOUDS

MIDDLE CLOUDS

LOW CLOUDS

FOG

Considered an atmospheric hazard

Cloud with its base at or near the ground

Most fogs form because ofRadiation cooling, orMovement of air over a cold surface

FOG

Types of fogFogs caused by cooling

Advection fog – warm, moist air moves over a cool surface

Radiation fogEarth’s surface cools rapidly

Forms during cool, clear, calm nights

ADVECTION FOG

RADIATION FOG

Upslope fogHumid air moves up a slopeAdiabatic cooling occurs

Evaporation fogsSteam fog

Cool air moves over warm water and moisture is added to the air

Water has a steaming appearanceFrontal fog, or precipitation fog

Forms during frontal wedging when warm air is lifted over colder air

Rain evaporates to form fog

UP-SLOPE FOG

EVAPORATION FOGS

Steam Fog

Frontal Fog

PRECIPITATION

Cloud dropletsLess than 10micrometers in diameter

Fall incredibly slow

Formation of precipitationBergeron process

Temperature in the clouds is below freezing

Ice crystals collect water vaporLarge snowflakes form and

Fall to the ground as snow, orMelt on their descent and form rain

Collision-coalescence processWarm cloudsLarge hygroscopic condensation nuclei

Large droplets formDroplets collide with other droplets during their descent

Forms of precipitationRain and drizzle

Rain – droplets have at least a 0.5 mm diameter

Drizzle – droplets have less than 0.5 mm diameter

Snow – ice crystals, or aggregates of ice crystals

Sleet and glazeSleet

Wintertime phenomenaSmall particles of iceOccurs when

•Warmer air overlies colder air•Rain freezes as it falls

Glaze, or freeing rain – impact with a solid surface causes freezing

HailHard rounded pellets

• Concentric shells• Most diameters range from 1-5 cm

Formation• Occurs in large cumulonimbus clouds with violent up-and-downdrafts

• Layers of freezing rain are caught up in up-and-downdrafts in the cloud

• Pellets fall to the ground when they become too heavy

RimeForms on cold surfaces•Freezing of super-cooled fog, or

•Cloud droplets

Rime

Measuring precipitationRain

Easiest form to measureMeasuring instruments

Standard rain gauge•Uses funnel to collect and conduct rain

•Cylindrical measuring tube measures rainfall in centimeters or inches

Recording gauge

Snow has two measurementsDepthWater equivalent

General ratio is 10 snow units to 1 water unit

Varies widely

AIR PRESSURE AND WINDAtmospheric Pressure

Force exerted by the weight of the air above

Weight of the air at sea level14.7 pounds per square inch1 kilogram per square centimeter

Decreases with increasing altitude

Units of measurementMillibar (mb) – standard sea level pressure is 1013.2mb

Inches of mercury – standard sea level pressure is 29.92 inches of mercury

Instruments for measuringBarometer

Mercury barometerInvented by Torricelli in 1643Uses a glass tube filled with mercury

Aneroid barometer“Without liquid”Uses an expanding chamberBarograph (continuously records air pressure)

WIND

Horizontal movement of air

Out of areas of high pressure

Into areas of low pressure

Controls of windPressure gradient force

Isobars – lines of equal air pressurePressure gradient – pressure changes over distance

Coriolis EffectApparent deflection in the wind direction due to Earth’s rotation

Deflection is To the right in the northern hemisphere

To the left in the southern hemisphere

Friction with earth’s surfaceOnly important near the surface

Acts to slow the air’s movements

Upper air windsGenerally blow parallel to isobars – called geostrophic winds

Jet stream“River” of airHigh altitudeHigh velocity (120-140 km/h)

CYCLONES AND ANTICYCLONES

CyclonesA center of low pressurePressure decreases toward the center

Winds associated withIn the Northern Hemisphere

Inward (convergence)Counterclockwise

In the Southern HemisphereInward (convergence)Clockwise

Associated with rising airOften bring clouds and precipitation

AnticycloneA center of high pressurePressure increases toward the center

Winds associated with In the Northern Hemisphere

Outward (divergence)Clockwise

In the Southern HemisphereOutward (divergence)Counterclockwise

Associated with subsiding airUsually bring “fair” weather

GENERAL ATMOSPHERIC CIRCULATION

Underlying cause is unequal surface heating

On the rotating earth, there are three pairs of atmospheric cells that redistribute the heat

Idealized global circulationEquatorial low pressure zone

Rising airAbundant precipitation

Subtropical high pressure zoneSubsiding, stable, dry airNear 30° latitudeLocation of great desertsAir traveling equatorward from the subtropical high produces the trade winds

Air traveling poleward from the subtropical high produces the westerly winds

Subpolar low pressure zoneWarm and cool winds interactPolar front – an area of storms

Polar high pressure zoneCold, subsiding airAir spreads equator-ward and produces polar easterly winds

Polar easterlies collide with the westerlies along the polar front

Influence of continents

Seasonal temperature differences

Influence is most obvious in the Northern Hemisphere

MonsoonSeasonal change in wind directionOccur over continents

During warm months•Air flows onto land•Warm, moist air from the ocean

Winter months•Air flows off the land•Dry, continental air

CIRCULATION IN THE MID-LATITUDES

ComplexOccurs in the zone of the westerliesAir flow is interrupted by cyclones

Cells move west to east in the Northern Hemisphere

Create anti-cyclonic and cyclonic flow

Paths of the cyclones and anticyclones are associated with the upper-level airflow

I CAN…

 I can explain the dynamics of the El Nino-Southern Oscillation and its effect on continental climates.  

I can explain differences between maritime and continental climates with regard to oceanic currents.  

I can describe the various conditions of formation associated with severe weather.  

I can describe the seasonal variations in severe weather.

EL NINOA counter current (ocean current that flows the opposite way) that flows southward along the coasts of Ecuador and Peru

THINK ABOUT THESE QUESTIONS…

Can you think of another ocean current that travels along the coast of a continent?

How does it affect the areas weather?

Can you explain what the weather would be like without an ocean current’s effect?

WarmUsually appears during the Christmas season

Blocks upwelling of colder, nutrient filled water, and anchovies starve from lack of food

Strongest El Nino on record occurred in 1997 and 1998 and causedHeavy rains in Ecuador and PeruFerocious storms in California

NOW, THINK ABOUT THESE QUESTIONS…How could the loss of the anchovies affect us?

What do you think a warm ocean current would do to the atmosphere of a location that normally had cold ocean water near it?

How do you know this?

Related to large-scale atmospheric circulationPressure changes between the eastern and western Pacific called the Southern Oscillation

Changes in trade winds creates a major change in the equatorial current system, with warm water flowing eastward

WHICH MAP SHOWS CONDITIONS THAT YOU WOULD LIKE TO EXPERIENCE?

Do you feel we are experiencing one of these phenomena this year?

Explain.

Effects are highly variable depending in part on the temperatures(how warm or cold they are) and size of the warm water pools

LOCAL WINDS

Produced from temperature differences

Small scale windsTypes

Sea and land breezesValley and mountain breezesChinook and Santa Ana winds

WIND MEASUREMENT

Two basic measurementsDirection Speed

DirectionWinds are labeled from where they originate (e.g., North wind – blows from the north toward the south)

Instrument for measuring wind direction is the wind vane

Direction indicated by eitherCompass pointsScale of 0° to 360°

Prevailing wind comes more often from one direction

Speed – often measured with a cup anemometer

Changes in wind directionAssociated with locations of

CyclonesAnticyclones

Often bring changes in Temperature Moisture conditions

GLOBAL DISTRIBUTION OF PRECIPITATION

Relatively complex patternRelated to global wind and

pressure patterns

High pressure regionsSubsiding airDivergent windsDry conditionse.g., Sahara and Kalahari deserts

Low pressure regionsAscending airConverging windsAmple precipitatione.g., Amazon and Congo basins

RELATED TO DISTRIBUTION OF LAND AND WATER

Large landmasses in the middle latitudes often have less precipitation than toward their centers

Mountain barriers also alter precipitation patternsWindward slopes receive abundant rainfall from Orographic lifting

Leeward slopes are usually deficient in moisture