Ch 6 – Atmospheric Moisture

Ch 6 – Atmospheric Ch 6 – Atmospheric MoistureMoisture

Ch 6 – Atmospheric MoistureCh 6 – Atmospheric Moisture

• IntroductionIntroduction– In Chapter 1, we learned that water In Chapter 1, we learned that water

vapor is a variable gas, occupying only a vapor is a variable gas, occupying only a small percentage of the volume of the small percentage of the volume of the gases in the atmosphere.gases in the atmosphere.

– Although water vapor is around us in Although water vapor is around us in only small quantities, it has major only small quantities, it has major consequences, not the least of which consequences, not the least of which include icing, thunderstorms, freezing include icing, thunderstorms, freezing rain, downbursts, whiteouts, frost, and rain, downbursts, whiteouts, frost, and lightning (Lester, 2006).lightning (Lester, 2006).


• IntroductionIntroduction– In this chapter, we look at the basics of In this chapter, we look at the basics of

atmospheric moisture, a term which is used atmospheric moisture, a term which is used here to imply the presence of H2O in any one here to imply the presence of H2O in any one or all of its states: water vapor, water, or ice.or all of its states: water vapor, water, or ice.

– We examine the transformation between We examine the transformation between states and the importance of air temperature states and the importance of air temperature in the transformation process.in the transformation process.

– When you complete this chapter, you will When you complete this chapter, you will understand the causes and effects of state understand the causes and effects of state changes, how clouds form and dissipate, and changes, how clouds form and dissipate, and how precipitation is produced (Lester, 2006).how precipitation is produced (Lester, 2006).


• IntroductionIntroduction– You will also know how clouds and You will also know how clouds and

precipitation are classified and observed precipitation are classified and observed (Lester, 2006).(Lester, 2006).


• Section A – Moisture CharacteristicsSection A – Moisture Characteristics– State ChangesState Changes– Vapor PressureVapor Pressure– Relative HumidityRelative Humidity– Dewpoint TemperatureDewpoint Temperature


• Section B – CloudsSection B – Clouds– Cloud FormationCloud Formation

• Water VaporWater Vapor• Condensation NucleiCondensation Nuclei• CoolingCooling• Latent Heat and StabilityLatent Heat and Stability

– Cloud and Visibility ObservationsCloud and Visibility Observations• Standard Cloud ObservationsStandard Cloud Observations• VisibilityVisibility• Cloud TypeCloud Type• Other Useful Cloud ObservationsOther Useful Cloud Observations


• Section C – PrecipitationSection C – Precipitation– Precipitation CausesPrecipitation Causes– Precipitation CharacteristicsPrecipitation Characteristics

•TypesTypes

• Intensity and AmountIntensity and Amount

– METARMETAR– Hydrologic CycleHydrologic Cycle


• Section A: Moisture CharacteristicsSection A: Moisture Characteristics– State Changes State Changes

•Water vapor – Water vapor – is a colorless, odorless, tastless is a colorless, odorless, tastless gas in which the molecules are free to move gas in which the molecules are free to move about, as in any gasabout, as in any gas

•Water – Water – in the liquid state (water), molecules in the liquid state (water), molecules are restricted in their movements in comparison are restricted in their movements in comparison to water vapor at the same temperatureto water vapor at the same temperature

•Ice – Ice – as a solid ice, the molecular structure is as a solid ice, the molecular structure is even more rigid and the freedom of movement even more rigid and the freedom of movement is greatly restrictedis greatly restricted


• Change of state – Change of state – refers to the transition from refers to the transition from one form of H2O to anotherone form of H2O to another


• ***The processes by which water vapor is ***The processes by which water vapor is added to unsaturated air are evaporation added to unsaturated air are evaporation and sublimationand sublimation


• Change of state – Change of state – refers to the transition from one form of refers to the transition from one form of H2O to anotherH2O to another– Melting – Melting – ice to waterice to water– Evaporation – Evaporation – water to vaporwater to vapor– Sublimation – Sublimation – ice directly to vapor without water as an ice directly to vapor without water as an

intermediate state; figure 6-1intermediate state; figure 6-1– Condensation – Condensation – vapor to watervapor to water– Freezing – Freezing – water to icewater to ice– Deposition – Deposition – vapor directly to ice without water as an vapor directly to ice without water as an

intermediate stateintermediate state– Latent heat – Latent heat – the amount of heat energy that is the amount of heat energy that is

absorbed or released when H2O changes from one state absorbed or released when H2O changes from one state to anotherto another

– Sensible heat – Sensible heat – heat that can be felt and measured heat that can be felt and measured when the molecules pass to lower energy stateswhen the molecules pass to lower energy states


• Vapor PressureVapor Pressure– Partial pressure – Partial pressure – in the mixture of in the mixture of

atmospheric gases, each individual gas exerts atmospheric gases, each individual gas exerts a partial pressurea partial pressureVapor pressure (VP) – Vapor pressure (VP) – the the partial pressure exerted by water vapor (H2O partial pressure exerted by water vapor (H2O in gaseous form)in gaseous form)

– Saturation – Saturation – occurs when the same amount of occurs when the same amount of molecules are leaving a water surface as are molecules are leaving a water surface as are returningreturning

– Saturation vapor pressure (SVP) – Saturation vapor pressure (SVP) – figure 6-figure 6-2; the vapor pressure exerted by the molecules 2; the vapor pressure exerted by the molecules of water vapor in this equilibrium conditionof water vapor in this equilibrium condition


• ***The amount of water vapor needed to ***The amount of water vapor needed to saturate the air largely depends on air saturate the air largely depends on air temperaturetemperature


• Boiling – Boiling – occurs when SVP equals the total air occurs when SVP equals the total air pressurepressure


• Relative Humidity – Relative Humidity – it is often useful to determine how it is often useful to determine how close the atmosphere is to saturationclose the atmosphere is to saturation– this information can help you anticipate the formation of this information can help you anticipate the formation of

clouds or fogclouds or fog– this is done by measuring the amount of water vapor in this is done by measuring the amount of water vapor in

the atmosphere in terms of actual VP and then estimating the atmosphere in terms of actual VP and then estimating SVP from a temperature measurement; figure 6-3SVP from a temperature measurement; figure 6-3

– the degree of saturation is then computed by taking the the degree of saturation is then computed by taking the ratio of VP and SVP and multiplying it by 100ratio of VP and SVP and multiplying it by 100

– the result is called relative humiditythe result is called relative humidity– it expresses the amount of water vapor actually in the air it expresses the amount of water vapor actually in the air

as a percentage of the amount required for saturationas a percentage of the amount required for saturation•RH (%) = (VP/SVP) x 100RH (%) = (VP/SVP) x 100


• Dewpoint TemperatureDewpoint Temperature– Dewpoint – Dewpoint – the temperature at which the temperature at which

condensation first occurs when air is cooled at condensation first occurs when air is cooled at a constant pressure without adding or a constant pressure without adding or removing water vaporremoving water vapor

•dew point temperature is always less than dew point temperature is always less than the air temperature, with one exceptionthe air temperature, with one exception

– when the air is saturated (RH = 100%), when the air is saturated (RH = 100%), the temperature and dew point are equalthe temperature and dew point are equal


• ***Dew point refers to the temperature to ***Dew point refers to the temperature to which air must be cooled to become which air must be cooled to become saturatedsaturated


• Temperature-Dew point Spread – Temperature-Dew point Spread – a very a very useful quantity that relates RH and dew pointuseful quantity that relates RH and dew point– also called the dew point depressionalso called the dew point depression– it is the difference between the air temperature it is the difference between the air temperature

and dew pointand dew point•when the temperature-dew point spread is when the temperature-dew point spread is

small, the RH is highsmall, the RH is high•when the spread is very large, the RH is low; when the spread is very large, the RH is low;

figure 6-4figure 6-4


• Dew – Dew – a condensation product that forms when a condensation product that forms when the ground or other object (such as the wings of a the ground or other object (such as the wings of a parked airplane) loses heat energy through night parked airplane) loses heat energy through night time (nocturnal) radiationtime (nocturnal) radiation


• White dew – White dew – if the temperature falls below 32 if the temperature falls below 32 degrees F (0 degrees C) after dew is present, it degrees F (0 degrees C) after dew is present, it will freezewill freeze


• ***Frost forms when the temperature of the ***Frost forms when the temperature of the collecting surface is at or below the dew collecting surface is at or below the dew point of the adjacent air and the dew point point of the adjacent air and the dew point is below freezingis below freezing– Frost is considered hazardous to flight Frost is considered hazardous to flight

because it spoils the smooth flow of air because it spoils the smooth flow of air over the wings, thereby decreasing lifting over the wings, thereby decreasing lifting capabilitycapability


• Section B: Clouds - Section B: Clouds - suspension of water suspension of water droplets and / or ice crystals in the atmospheredroplets and / or ice crystals in the atmosphere– Cloud Formation - Cloud Formation - three requirements for three requirements for

cloud formation are water vapor, condensation cloud formation are water vapor, condensation nuclei and coolingnuclei and cooling

– Water Vapor - Water Vapor - cloud development requires cloud development requires adequate water vaporadequate water vapor•cloud formation favored in air with high RH cloud formation favored in air with high RH

(small temperature dew point spread)(small temperature dew point spread)


– Condensation Nuclei - Condensation Nuclei - Condensation Nuclei Condensation Nuclei are microscopic particles such as dust and salt are microscopic particles such as dust and salt that provide surfaces on which water vapor that provide surfaces on which water vapor undergoes condensation to form water droplets undergoes condensation to form water droplets or deposition to form ice crystalsor deposition to form ice crystals

•fog is usually more prevalent in industrial fog is usually more prevalent in industrial areas because of an abundance of areas because of an abundance of condensation nucleicondensation nuclei


• ***Fog is usually more prevalent in ***Fog is usually more prevalent in industrial areas because of an abundance of industrial areas because of an abundance of condensation nuclei from combustioncondensation nuclei from combustion


• Cooling -Cooling -contact cooling, advection fog often forms in coastal contact cooling, advection fog often forms in coastal areasareas– conditions favorable for the formation of radiation fog over a conditions favorable for the formation of radiation fog over a

land surface are clear skies, little or no wind, and a small land surface are clear skies, little or no wind, and a small temperature-dew point spreadtemperature-dew point spread

– if the fog is very shallow (less than 20 feet deep) it is called if the fog is very shallow (less than 20 feet deep) it is called ground fogground fog

– valley fog forms when radiational cooling causes cool dense valley fog forms when radiational cooling causes cool dense air to pool in a valleyair to pool in a valley

– river valleys with ample supplied of moisture are favorable river valleys with ample supplied of moisture are favorable locations for this type of foglocations for this type of fog

– rain shadow; upslope fograin shadow; upslope fog– the types of fog that depend on wind in order to exist are the types of fog that depend on wind in order to exist are

advection fog and upslope fogadvection fog and upslope fog– steam fog is common over unfrozen water bodies in the cold steam fog is common over unfrozen water bodies in the cold

months of the yearmonths of the year


• ***Advection fog often forms in coastal ***Advection fog often forms in coastal areasareas

• ***Conditions favorable for the formation of ***Conditions favorable for the formation of radiation fog over a land surface are clear radiation fog over a land surface are clear skies, little or no wind and a small skies, little or no wind and a small temperature-dew point spreadtemperature-dew point spread

• ***The types of fog that depend upon wind ***The types of fog that depend upon wind in order to exist are advection fog and in order to exist are advection fog and upslope fogupslope fog


• Latent Heat and Stability -Latent Heat and Stability -saturated adiabatic saturated adiabatic processprocess– saturated adiabatic lapse rate (SALR)saturated adiabatic lapse rate (SALR)

•varies between 3C per 1,000 ft for very cold varies between 3C per 1,000 ft for very cold temperatures and 1C per 1,000 ft for very temperatures and 1C per 1,000 ft for very hot temperatureshot temperatures

•condensation levelcondensation level

•conditional instability – air parcel must conditional instability – air parcel must become saturated in order to be unstablebecome saturated in order to be unstable


• Cloud and Visibility Observations -Cloud and Visibility Observations -learning to learning to observe clouds and visibility is essential for observe clouds and visibility is essential for proper interpretation of METAR and for flight proper interpretation of METAR and for flight safetysafety– Standard Cloud Observations - Standard Cloud Observations - sky sky

condition condition

•cloud height, cloud amount, cloud typecloud height, cloud amount, cloud type


– Visibility Visibility

•tower visibilitytower visibility

•prevailing visibilityprevailing visibility

•runway visibility (RVV)runway visibility (RVV)

•runway visual range (RVR)runway visual range (RVR)


– VFR conditionsVFR conditions

•VFR: ceiling > 3,000 ft AGL and visibility > 5 VFR: ceiling > 3,000 ft AGL and visibility > 5 smsm

•MVFR: ceiling 1,000 to 3,000 ft AGL and / or MVFR: ceiling 1,000 to 3,000 ft AGL and / or visibility 3 to 5 smvisibility 3 to 5 sm

• IFR conditionsIFR conditions– IFR: ceiling 500 to < 1,000 ft AGL and/or IFR: ceiling 500 to < 1,000 ft AGL and/or

visibility 1 to < 3 smvisibility 1 to < 3 sm– LIFR: ceiling <500 feet AGL and / or LIFR: ceiling <500 feet AGL and / or

visibility < 1 smvisibility < 1 sm


• ***A ceiling is defined as the height above ***A ceiling is defined as the height above the earth’s surface of the lowest layer the earth’s surface of the lowest layer reported as broken or overcast, or as the reported as broken or overcast, or as the vertical visibility into an indefinite ceilingvertical visibility into an indefinite ceiling


• Cloud Type - Cloud Type - ten basic cloud types < 6500 feet AGL ten basic cloud types < 6500 feet AGL – low (ST) Stratuslow (ST) Stratus– (SC) Stratocumulus(SC) Stratocumulus– (NS) Nimbostratus(NS) Nimbostratus

•6,500 to 20,000 ft AGL6,500 to 20,000 ft AGL– Middle (AC) AltocumulusMiddle (AC) Altocumulus– (AS) Altostratus, High > 20,000 ft AGL (AS) Altostratus, High > 20,000 ft AGL – (CC) Cirrocumulus(CC) Cirrocumulus– (CS) Cirrostratus(CS) Cirrostratus– (CI) Cirrus(CI) Cirrus– Clouds with vertical development (CU) CumulusClouds with vertical development (CU) Cumulus– (CB) Cumulonimbus (CB) Cumulonimbus


• the four families of clouds are high, middle, low the four families of clouds are high, middle, low and those with extensive vertical developmentand those with extensive vertical development– a high cloud is composed mostly of ice crystalsa high cloud is composed mostly of ice crystals

• the stability of the air before lifting occurs the stability of the air before lifting occurs determines the structure or type of clouds which determines the structure or type of clouds which form as a result of air being forced to ascendform as a result of air being forced to ascend

• heights of cumulus cloud bases can be heights of cumulus cloud bases can be determined by H = (T – DP) / 4.4determined by H = (T – DP) / 4.4– Cloud Height – cloud layer, cloud heightCloud Height – cloud layer, cloud height


• FogFog– vertical visibility (VV) or indefinite ceiling is the vertical visibility (VV) or indefinite ceiling is the

vertical distance that an observer or remote vertical distance that an observer or remote sensing device on the ground can see into a sensing device on the ground can see into a low cloud or other obscuring phenomenonlow cloud or other obscuring phenomenon

– cloud height is the height of the base of the cloud height is the height of the base of the cloud above the groundcloud above the ground

– when sky is completely obscured the reported when sky is completely obscured the reported cloud height is actually the vertical visibilitycloud height is actually the vertical visibility


• Cloud Amount – amount of sky covered by each Cloud Amount – amount of sky covered by each cloud layercloud layer– celestial domecelestial dome– clear (SKC) – no clouds presentclear (SKC) – no clouds present– clear (CLR) – no clouds below 12,000 ft AGLclear (CLR) – no clouds below 12,000 ft AGL– few (FEW) – 1/8 to 2/8 sky coverfew (FEW) – 1/8 to 2/8 sky cover– scattered (SCT) – 3/8 to 4/8 coveragescattered (SCT) – 3/8 to 4/8 coverage– broken (BKN) – 5/8 to 7/8 coveragebroken (BKN) – 5/8 to 7/8 coverage– overcast (OVC) – 8/8 coverageovercast (OVC) – 8/8 coverage


• Obscurations and ceilings – obscuration, ceiling, Obscurations and ceilings – obscuration, ceiling, indefinite ceiling, vertical visibilityindefinite ceiling, vertical visibility– a ceiling is defined as the height above the a ceiling is defined as the height above the

earth’s surface of the lowest layer reported as earth’s surface of the lowest layer reported as broken or overcast, or as the vertical visibility broken or overcast, or as the vertical visibility into an indefinite ceilinginto an indefinite ceiling

•PIREPS pilot’s reports PIREPS pilot’s reports


• ***The four families of clouds are high, ***The four families of clouds are high, middle, low and those with extensive middle, low and those with extensive vertical developmentvertical development

• ***A high cloud is composed mostly of ice ***A high cloud is composed mostly of ice crystalscrystals

• ***The stability of the air before lifting ***The stability of the air before lifting occurs determines the structure or type of occurs determines the structure or type of clouds which form as a result of air being clouds which form as a result of air being forced to ascendforced to ascend


• Other Useful Cloud Observations - Other Useful Cloud Observations - satellite satellite images from the Geostationary Orbiting images from the Geostationary Orbiting Environmental Satellite (GOES)Environmental Satellite (GOES)


• Section C: Precipitation Section C: Precipitation – Precipitation Causes Precipitation Causes

•condensation / depositioncondensation / deposition

•collision / coalescence – large droplets fall faster collision / coalescence – large droplets fall faster than smaller particles capturing them as they than smaller particles capturing them as they descenddescend

• ice crystal processice crystal process

•super cooled water droplets are primary cause of super cooled water droplets are primary cause of aircraft icingaircraft icing

• ice crystals grow at the expense of super cooled ice crystals grow at the expense of super cooled water droplets in the ice crystal processwater droplets in the ice crystal process


• Precipitation CharacteristicsPrecipitation Characteristics– Types Types

•DrizzleDrizzle•RainRain• rain showersrain showers• freezing drizzlefreezing drizzle• freezing rainfreezing rain•black iceblack ice• the presence of ice pellets at the surface is evidence that the presence of ice pellets at the surface is evidence that

there may be freezing rain at a higher altitudethere may be freezing rain at a higher altitude– ice pelletsice pellets

•SnowSnow• snow showerssnow showers•VirgaVirga• fall streaksfall streaks


• ***The presence of ice pellets at the surface ***The presence of ice pellets at the surface is evidence that there may be freezing rain is evidence that there may be freezing rain at a higher altitudeat a higher altitude


• Intensity and Amount Intensity and Amount – drizzle or snowfall intensity and associated drizzle or snowfall intensity and associated

visibilityvisibility

• light > ½light > ½

•moderate > ¼ but < or = ½moderate > ¼ but < or = ½

•Heavy < or = ¼Heavy < or = ¼

•snow depthsnow depth– heavy snow warningheavy snow warning

blizzardblizzard


• METAR METAR – METAR format; Figure 6-27 page 6-27METAR format; Figure 6-27 page 6-27– Visibility – reported in statute milesVisibility – reported in statute miles– Present Weather Present Weather – intensity symbols intensity symbols

• light (-)light (-)•moderate (no sign)moderate (no sign)•or heavy (+)or heavy (+)•ProximityProximity•descriptor; see Figure 6-28 page 6-28; list of descriptor; see Figure 6-28 page 6-28; list of

contractions for various weather phenomena; see contractions for various weather phenomena; see Figure 6-29 page 6-29Figure 6-29 page 6-29


• Hydrologic Cycle Hydrologic Cycle – on and below the surface of the earth, on and below the surface of the earth,

moisture is stored as ice and snow, and as moisture is stored as ice and snow, and as water in lakes, rivers, ground water and the water in lakes, rivers, ground water and the oceansoceans

– water vapor removed from the earth’s surface water vapor removed from the earth’s surface may be transported far from its source by the may be transported far from its source by the windwind

– clouds form and produce precipitation that clouds form and produce precipitation that returns the moisture to the earth’s surface.returns the moisture to the earth’s surface.

SummarySummary

• You have learned some important You have learned some important details about H2O and its three details about H2O and its three states in the atmosphere.states in the atmosphere.

• Changes of state and the associated Changes of state and the associated latent heat exchanges have latent heat exchanges have important effects on cloud formation important effects on cloud formation and dissipation and on atmospheric and dissipation and on atmospheric stability (Lester, 2006).stability (Lester, 2006).

SummarySummary

• You should now understand that there You should now understand that there are important differences between the are important differences between the way clouds are formed and the way clouds are formed and the processes by which precipitation is processes by which precipitation is produced.produced.

• On the very practical side, you should On the very practical side, you should know the basic elements of cloud and know the basic elements of cloud and precipitation observing and reporting precipitation observing and reporting (Lester, 2006).(Lester, 2006).

SummarySummary

• Your ability to recognize the 10 basic cloud Your ability to recognize the 10 basic cloud types and the types and characteristics of types and the types and characteristics of precipitation give you valuable observational precipitation give you valuable observational tools which will help you evaluate the state of tools which will help you evaluate the state of the atmosphere and its likely effect on flight.the atmosphere and its likely effect on flight.

• In the next chapter, you will apply your In the next chapter, you will apply your knowledge of all of the basic physical knowledge of all of the basic physical processes gained thus far to understand how processes gained thus far to understand how atmospheric storms and other circulations atmospheric storms and other circulations arise, and to determine their structures and arise, and to determine their structures and future behavior (Lester, 2006).future behavior (Lester, 2006).

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Ch 6 – Atmospheric Moisture