w ater ’ s r ole in the a tmosphere. h umidity amount of water vapor in the air water vapor adds...
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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