Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt

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Understanding Weather and Climate 3rd
EditionEdward Aguado and James E. Burt

• Anthony J. Vega

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Part 2. Water in the Atmosphere

• Chapter 5.
• Atmospheric Moisture

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Introduction

• Over 70 of the planet is covered by water
• Water is unique in that it can simultaneously
  exist in all three states (solid, liquid, gas) at
  the same temperature
• Water is able to shift between states very easily
• The hydrologic cycle refers to the regular cycle
  of water through the earth-atmosphere system
• Liquification of water occurs frequently at
  normal Earth temperatures
• Occurs when air is saturated with respect to
  water vapor
• The addition of water vapor, or the lowering of
  temperature, in saturated air will lead to
  condensation

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• Evaporation
• Occurs if energy is available to a water surface
• Water vapor increases in air as surface water
  evaporates
• Upon saturation, condensation will begin and
  water will return to the surface
• Saturation marks an equilibrium between
  evaporation and condensation
• Saturation may occur in the presence or not in
  the presence of dry air, so that the statement
  that air holds water is erroneous

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• Changes of state may also occur with regard to
  water vapor changing directly to ice
• Deposition
• Or, the inverse situation
• Sublimation
• Indices of Water Vapor Content
• Humidity indicates the amount of water vapor in
  air
• Humidity expressed through a variety of ways
• Each has advantages and disadvantages
• All indices refer solely to water vapor and
  exclude liquid and frozen states
• Vapor Pressure
• Simply the amount of pressure exerted on the
  atmosphere by water vapor
• Dependent upon both temperature and density of
  the vapor with density most important

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The movement of water vapor molecules exerts
vapor pressure on surfaces

• The maximum water vapor pressure which can occur
  is termed saturation vapor pressure
• Saturation vapor pressure is solely temperature
  dependent
• It exponentially increases with temperature such
  that high temperatures may have extremely high
  saturation vapor pressures compared to lower
  temperatures

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Exponential increase in saturation vapor pressure
with increase in temperature

• Absolute Humidity
• Indicates the density of water vapor expressed in
  g/m3
• Changes as air volume changes
• Specific Humidity
• Represents a given mass of water vapor per mass
  of air in g/kg
• Term does not vary with air volume fluxes
• Does not change with temperature changes
• Saturated air has the highest specific humidity
  for a given temperature and pressure saturation
  specific humidity

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• Mixing Ratio
• Very similar to specific humidity in that it
  expresses the mass of water vapor relative to air
  mass
• However, mixing ratio expresses the amount of
  water vapor relative only to a mass of dry air
• Maximum mixing ratio saturation mixing ratio
• Relative Humidity
• Most commonly used expression of water vapor
  content
• Indicates the amount of water vapor in the air
  relative to the possible maximum
• Given as a percentage
• Does not indicate the amount of air which is
  water vapor but instead describes the amount
  present relative to a saturation point
• The saturation point, thus the relative humidity
  term, is relative to air temperature and total
  water vapor present

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• More water vapor can exist in warm air than cold,
  the term is sometimes misleading
• An example involves the diurnal distribution of
  RH in which the highest RH occurs in the morning
  during the coolest time of the day
• The lowest RH values will be recorded in late
  afternoon, the time of greatest air temperature
• This makes high temperature/high relative
  humidities (90oF, 90 RH, or so) impossible
• Because of temperature dependency the term cannot
  be used to compare moisture content at different
  locations having different temperatures

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The relationship between RH and temperature
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• Dew Point
• The dew point temperature is the temperature at
  which saturation occurs in air
• Reached either by increasing water vapor content
  or by chilling air (while holding moisture
  content constant)
• Good indicator of moisture content in air
• Relatively high dew points indicate abundant
  atmospheric moisture
• Dew points can be only equal or less than air
  temperatures
• If saturation is reached and air temperatures
  cool further, water vapor is removed from the air
  through condensation
• When air reaches saturation at temperatures below
  freezing the term frost point is used

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Dew point/temperature relationships in a)
unsaturated air b) and c) saturated air
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• Methods of Achieving Saturation
• Air may become saturated through the addition of
  water vapor to air at a constant temperature
• Example light fogs formed beneath clouds as
  vapor is added through falling raindrops
• Or by mixing cold air with warm, moist air
• Example Contrails and steam fogs which develop
  as cold air passes over warm water bodies
• Or by cooling air to the dew point
• The most common way
• Effects of Curvature and Solution
• Condensed water suspended in the atmosphere is
  typically curved
• Impurities also exist
• Both factor into phase shifts

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• Effect of Curvature
• Small drops exhibit greater curvature than larger
  ones
• Curvature influences saturation vapor pressure
  with highly curved drops requiring RHs in excess
  of 100 to remain liquid
• For very small drops, supersaturation may
  approach 300
• Hygroscopic aerosols acting as condensation
  nuclei help keep RHs below these extremes
• Condensation onto such particles, called
  heterogeneous nucleation, causes dissolution of
  the aerosol

Larger drops have less curvature than smaller
ones
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Small droplets require higher RHs to remain
liquid

• Effect of Solution
• Evaporation from solutions is less than from pure
  water
• This directly opposes curvature influences such
  that condensation typically occurs at RHs near
  100
• Hygroscopic nuclei abound in the atmosphere from
  many natural (salt, dust, ash, etc.) sources and
  anthropogenic (combustion derivative) sources
• Very small condensation nuclei lead to very tiny
  water drops haze

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• Ice Nuclei
• Atmospheric water does not freeze at 0oC (32oF)
• Leads to the presence of supercooled water
• Ice crystal formation requires ice nuclei
• A rare temperature dependent substance similar in
  shape to ice
• Examples clay, ice fragments, bacteria,
  volcanics, etc.)
• Ice nuclei become active at temperatures below
  -4oC
• Between -10o and -30oC (14-22oF), saturation may
  lead to ice crystals, supercooled drops, or both
• Below -30oC, clouds are composed solely of ice
  crystals
• At or below -40oC (-40oF) spontaneous nucleation,
  the direct deposition of ice with no nuclei
  present, occurs

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• Measuring Humidity
• The easiest way to measure humidity is through
  use of a sling psychrometer
• A pair of thermometers one of which has a wetted
  cotton wick attached to the bulb
• The two thermometers measure the wet and dry bulb
  temperature
• Swinging the psychrometer causes air to circulate
  about the bulbs
• When air is unsaturated, evaporation occurs from
  the wet bulb which cools the bulb
• Once evaporation occurs, the wet bulb temperature
  stabilizes allowing for comparison with the dry
  bulb temperature
• The wet bulb depression is found with a greater
  depression indicative of a dry atmosphere
• Charts gauge the amount of atmospheric humidity
• Aspirated and hair hygrometers are alternatives

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• High Humidities and Human Discomfort
• Temperature extremes account for more fatalities
  than severe storms, of all types, combined
• High temperature extremes are compounded by
  humidity (and other factors such as wind and
  intensity of sunlight)
• The effect of humidity and high temperatures can
  be expressed in a heat index
• Humans are cooled by the release of perspiration
  which cools the body by evaporating into air
• When the atmosphere has a high moisture content,
  the rate of evaporation is effectively reduced
• This leads to a reduction in the cooling power of
  perspiration
• This increases the apparent temperature of the
  air leading to heat related health risks
• Muscle cramps, heat exhaustion, heat stroke
  (potentially fatal)

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• Cooling Air to the Dew or Frost Point
• Most condensation processes occur as air is
  chilled to the dew point
• Air temperature changes either from direct energy
  exchanges (diabatic processes) or from those
  involving no net energy exchange (adiabatic
  processes)
• Diabatic Processes
• Involve the direct addition or removal of heat
  energy
• Example Air passing over a cool surface loses
  energy through conduction
• Energy is always transferred from areas of high
  temperature toward those of lower temperatures
• The Second Law of Thermodynamics
• Diabatic processes are typically associated with
  fog development

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• Adiabatic Processes
• Cloud formation typically involves temperature
  changes with no net exchange of energy
• Such processes occur according to the First Law
  of Thermodynamics
• Rising air expands through an increasingly less
  dense atmosphere causing a decrease in internal
  energy and a corresponding temperature decrease
• Parcels expand and cool at the dry adiabatic
  lapse rate
• 1oC/100 m (5.5oF/1000 ft)
• Sinking parcels experience exactly proportional
  compression warming
• Parcels may eventually reach the lifting
  condensation level, the height at which
  saturation occurs
• Parcels then cool at the saturated adiabatic
  lapse rate
• 0.5oC/100 m (3.3oF/1000 ft)

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Dry adiabatic cooling
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• The Environmental Lapse Rate
• The environmental (ambient) lapse rate (ELR)
  refers to an overall decrease in air temperature
  with height
• This rate, which changes diurnally from place to
  place, stems from the fact that air located
  farther from surface heating is typically cooler
  than that nearer the surface

A comparison of adiabatic and environmental
cooling rates
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• Forms of Condensation
• Many forms of either liquid or solid condensation
  can occur depending on particular process
  characteristics
• Dew
• Liquid condensation on surface objects
• Diabatic cooling of surface air typically takes
  place through terrestrial radiation loss on calm,
  cool, clear nights
• Surface air becomes saturated and condensation
  forms on objects acting as condensation nuclei
• Frost
• Similar to dew except that it forms when surface
  temperatures are below freezing
• Deposition occurs instead of condensation
• May be referred to as white or hoar frost

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• Frozen Dew
• Occurs when normal dew formation processes occur
  followed by a drop in temperature to below
  freezing
• Ensures a tight bond between ice and the surface
• Causes black ice on roadways
• Fog
• Simply a surface cloud when air either cools to
  the dew point, has moisture added, or when cooler
  air is mixed with warmer moister air
• Radiation Fog
• Occurs when near surface air chills diabatically
  to saturation through terrestrial radiation loss
  on clear cool nights
• Require a slight breeze to vertically mix air
  through a shallow column

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Dew and Frost
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• If winds exceed about 5km/hr (3 mph) warmer air
  from aloft will mix with the near surface air and
  evaporate the fog
• After sunrise, the fog evaporates from below due
  to surface heating

Radiation fog in the Central Valley of California
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• Advection Fog
• Occurs when warm moist air moves across a cooler
  surface
• Air is chilled diabatically to saturation
• Common on the U.S. west coast as warm, moist air
  from the central Pacific advects over the cold
  California ocean current
• Frequently develop near boundaries of opposing
  ocean temperatures
• Example Off the northeast coast of the U.S.
• Upslope Fog
• The only fog developed through adiabatic cooling
• Occur when air is advected over land surfaces
  which increase in elevation
• A common occurrence in the Great Plains of the
  U.S. where warm, moist air advects from the Miss.
  River Valley towards the Rocky Mountains

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Different types of fog found throughout the U.S.
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• Formation and Dissipation of Cloud Droplets
• Clouds are mainly associated with adiabatic
  cooling of rising air
• Dew points decrease as air rises at the shallow
  dew point lapse rate
• 0.2oC/100 m (1.1oF/1000 ft)
• Approximately 50 m above the lifting condensation
  level, all condensation nuclei have condensed
  water attached
• Leads to additional growth of those drops over
  the creation of new drops
• Process soon stops leaving drops to slowly
  evaporate or sublimate

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End of Chapter 5 Understanding Weather and
Climate 3rd EditionEdward Aguado and James E.
Burt