ES 1111

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ES 1111

• Moisture in the Atmosphere

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Moisture in the Atmosphere

• Water is part of a distinct system called the
  hydrological cycle
• Water is removed from the surface into the
  atmosphere by two processes
• Evaporation Water removed off a free water
  surface, like a lake, river, ocean, or even soil
• Transpiration Water released into the air by
  the stomata in leaves

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The Hydrological Cycle

• Figure 4.1, Page 56

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Evapotranspiration

• Both evaporation and transpiration result in the
  same thing water in the atmosphere
• Because they result in the same thing, we combine
  the two processes into one word
  evapotranspiration (ET)

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Rate of Evapotranspiration

• The rate of evapotranspiration is controlled by
• Energy availability
• Humidity gradient away from the surface
• Wind speed above the surface
• Water availability

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Water Availability

• An open water surface provides a continuous water
  source
• Transpiration can provide water up until a
  certain limit based upon the plants ability to
  pull water up through its roots and out its
  stomatae (rate of transpiration)

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Potential Evapotranspiration

• Rate that will occur from a well-watered,
  actively growing, short green crop covering the
  ground surface
• Essentially equal to the ET over a large open
  water surface
• Rate is controlled entirely by atmospheric
  conditions
• Measure of possible agricultural activity if the
  crop is well-watered
• Measured by an evaporation pan

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Evaporation Pan

• Figure 4.I.1, Page 58

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Actual Evapotranspiration

• Amount actually lost from the surface given the
  prevailing atmospheric and ground conditions
• Provides information of soil moisture conditions
  and the local water balance
• Measured by a lysimeter (difficult to maintain,
  not many in existence) that weighs the grass,
  soil, and water above

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Lysimeter

• Figure 4.I.2, Page 59

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Global Evaporation Map

• Difficult to construct due to sparse data
• Maximum rates are found over subtropical oceans
  (clearer skies in subtropics than at the Equator)
• Rates decrease as one goes poleward
• Land values less than ocean values

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Global Evaporation Map

• Figure 4.2, Page 61

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Saturation of the Air

• Saturation refers to the equilibrium condition
  where the rate of evaporation into the air equals
  the rate of condensation out of the air (in
  out)
• When the air is saturated, evaporation can still
  take place, as long as condensation of the same
  amount also takes place
• The amount of water vapor present in the
  atmosphere at saturation depends upon
• Temperature
• Ice versus water surface that water enters/leaves
• Pressure (can be ignored if dealing with same
  height)

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Type of Surface

• The amount of water vapor that can be present in
  the atmosphere depends on whether there is a
  plane of pure water as a surface, or a plane of
  pure ice
• Less water vapor can be present in the atmosphere
  at saturation over an ice surface than a water
  surface

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Measuring the Vapor Content

• There are a number of ways that we can measure
  and express the amount of water vapor content in
  the atmosphere
• Vapor Pressure
• Mixing Ratio
• Relative Humidity
• Dew Point
• Precipitable Water Vapor
• Others (absolute humidity, specific humidity)

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Vapor Pressure (e)

• Vapor pressure (e) is simply the amount of
  pressure exerted only by the water vapor in the
  air
• The pressures exerted by all the other gases are
  not considered
• The unit for vapor pressure will be in units of
  pressure (millibars and hectopascals are the same
  value with a different name)

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Mixing Ratio (w)

• The mixing ratio (w) is the mass of water vapor
  present in the atmosphere compared to the mass of
  dry air in a given volume of air
• Because water vapor is at most 4 of the
  atmosphere per volume, we use units of grams of
  water vapor per kilogram of dry air (to avoid
  decimals)

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Saturation Vapor Pressure (es) and Saturation
Mixing Ratio (ws)

• If we measure the vapor pressure when the air is
  saturated, we call that vapor pressure the
  saturation vapor pressure (es)
• If we measure the mixing ratio when the air is
  saturated, we call that mixing ratio the
  saturation mixing ratio (ws)
• Unless the air is saturated, the vapor pressure
  and mixing ratio will always be less than the
  saturation vapor pressure and the saturation
  mixing ratio
• The vapor pressure and mixing ratio will only be
  equal to the saturation vapor pressure and the
  saturation mixing ratio if the air is saturated

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Saturation Vapor Pressure vs. Temperature

• Figure 4.3, Page 62

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Relative Humidity (RH)

• The relative humidity (RH) is calculated using
  the actual water vapor content in the air (mixing
  ratio) and the amount of water vapor that could
  be present in the air if it were saturated
  (saturation mixing ratio)
• RH w/ws x 100
• The relative humidity is simply what percentage
  the atmosphere is towards being saturated
• Relative humidity is not a good measure of
  exactly how much water vapor is present (50
  relative humidity at a temperature of 80 degrees
  Fahrenheit will involve more water vapor than 50
  relative humidity at -40 degrees)
• Relative humidity can change even when the amount
  of water vapor has not changed (when the
  temperature changes and the saturation mixing
  ratio changes as a result)

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Dew Point (Td)

• The dew point temperature is the temperature at
  which the air will become saturated if the
  pressure and water vapor content remain the same
• The higher the dew point, the more water vapor
  that is present in the atmosphere
• The temperature is always greater than the dew
  point unless the air is saturated (when the
  temperature and dew point are equal)

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Precipitable Water Vapor (PWV)

• Precipitable water vapor (PWV) is the amount of
  water vapor present in a column above the surface
  of the Earth
• Measured in units of inches or millimeters
• It represents the maximum amount of water that
  could fall down to the surface as precipitation
  if all the water vapor converted into a liquid or
  a solid
• Can be measured easily by weather balloons or
  satellites

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Clouds

• When the air becomes saturated, water vapor may
  condense to form solid ice or liquid water
  droplets in the atmosphere, and this is what
  clouds are made of
• The type of cloud that is formed depends on what
  process led to the air becoming saturated

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Cloud Classifications

• Clouds can be basically classified based upon
  their visual appearance and their height above
  the ground (which influences whether they are
  made of ice or water)

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Classification Based on Appearance

• Clouds that are heaped up in appearance are
  called cumulus clouds
• Clouds that are flat and featureless in
  appearance are called stratus clouds
• Clouds that are very thin and whispy (resemble
  horses tails or flocks of hair) are called
  cirrus clouds

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Classification Based on Height

• Clouds that are low to the ground and are likely
  to be composed of liquid water only are not given
  any special prefix
• Clouds that are in the middle part of the
  troposphere and are likely to have a mixture of
  ice and water are given the prefix alto-
• Clouds that are in the upper part of the
  troposphere and are likely to only have ice are
  given the prefix cirro- (except cirrus clouds)

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Basic Cloud Classifications

• Appearance Heaped-Up Flat
• Height
• Low cumulus stratus
• Middle altocumulus altostratus
• High cirrocumulus cirrostratus

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Cloud Classifications

• A cloud that is causing precipitation to fall at
  the surface is given the prefix or suffix nimbo-
  or nimbus
• Stratus cloud nimbostratus
• Cumulus cloud cumulonimbus
• Cumulonimbus (a thunderstorm) is not classified
  as low, middle or high because of its extensive
  vertical development

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Cloud Formation

• Once again, clouds form whenever the air reaches
  saturation. This can happen by any of the
  following processes
• Cooling the air down and causing the temperature
  to equal to the dew point
• Adding water vapor to the air and causing the dew
  point to equal the temperature
• Mix two different bodies of air together to
  average out the moisture and temperature, thereby
  possibly resulting in saturation

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Cooling the Air

• Air can be cooled in any of the following ways
• Air coming into contact with a cold surface
• Vertical motion in the atmosphere

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Contact With a Cold Surface

• If the skies are clear and the wind calm, the
  surface may cool down rapidly due to the emission
  and loss of infrared radiation out to space. The
  surface cools down, and the air in contact with
  the surface also cools down. Once the dew point
  is reached, radiation fog forms at the surface
• If warm, moist air is blown by the wind
  (advected) over a cold surface and it cools down
  to its dew point, advection fog is the result

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Vertical Motions

• While radiation fog and advection fog may be
  important at some locations (San Francisco and
  Seattle), vertical motions in the atmosphere is
  the most common mechanism of cloud formation.
• Vertical air motions can be caused by
• Buoyant (unstable) ascent
• Forced ascent over sloping terrain
• Fronts and low pressure storm systems

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Parcels

• To understand how vertical air motion can result
  in saturation, it is best to introduce the
  concept of a parcel
• A parcel is simply a blob of air that we will
  move around and study what happens to it
• You may picture in your mind a balloon or a box
  of air as the parcel
• There are three rules to parcels
• There is no energy exchange between the parcel
  and the environment (the parcel is insulated)
• There is no mass exchange between the parcel and
  the environment (the parcel keeps the molecules
  it starts with)
• The parcel may change shape as needed (so the
  idea of a rigid box is not as good as a balloon)

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Surface Parcel

• We can start by forming a parcel right here in
  the room (picture filling a balloon with air that
  is from the classroom)
• The starting temperature of the air in the
  balloon will be identical to the room
  temperature. In addition, the pressure and
  humidity are also identical to that of the outside

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Lifting a Parcel Up

• What happens if we take our parcel and lift it
  vertically upward in the atmosphere?
• One variable that we know will change in the
  environment as we go up in the atmosphere is
  pressure. Recall that pressure always decreases
  rapidly with height
• If we lift our balloon up quickly, the pressure
  inside the balloon will be greater than the
  pressure outside of the balloon. What happens?
• The balloon will expand in size until the
  pressure inside the balloon equals the pressure
  outside

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The Meaning of Temperature

• The more scientifically precise definition of
  temperature is that it is the average kinetic
  energy of a substance
• Kinetic energy is the energy of motion. All the
  gas molecules are zipping around inside the
  parcel, and they have mass, so the gas molecules
  also have kinetic energy
• Some gas molecules are moving faster than others.
  When we take the temperature of the parcel, we
  are taking an average of how much kinetic energy
  there is of all the gas molecules

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Expansion Takes Work

• In physics, work is defined as a force being
  applied over a distance
• In order to do work, energy must be expended
• When our parcel expands after it is lifted, it is
  doing work by pushing out against the environment
  a certain distance
• Therefore, the parcel must use up energy in order
  to expand.
• The energy that is available to be used up is the
  kinetic energy of the molecules.
• With the kinetic energy being used up, the
  molecules must slow down
• With the molecules moving more slowly, the
  average kinetic energy of the molecules in the
  parcel decreases, and the temperature decreases

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Expansion Cools

• If you have ever let air out of a tire and felt
  it as it exited the tube, it should have felt
  cold
• Air leaving a tire feels cold because it is going
  from a higher pressure environment to a lower
  pressure environment, so it expands and cools

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Adiabatic Processes

• Notice that our parcel will cool even though heat
  is not leaving the parcel, it is cooling down due
  to internal processes
• Because it does not involve a heat exchange
  between the parcel and the environment, we use
  the word adiabatic to describe this process
  (without heat)

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Dry Adiabatic Lapse Rate

• If a parcel is dry (water vapor is not condensing
  or depositing out), a dry parcel will always cool
  at the same rate when it is lifted
• The dry adiabatic lapse rate is approximately
  equal to 10 C for every kilometer the parcel is
  lifted
• For example, a dry parcel at the surface with a
  temperature of 20 C will cool down to a
  temperature of -10 C if lifted up 3 kilometers

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Cooling Dry Adiabatically

• When a parcel is lifted vertically, the
  temperature will eventually cool down to the dew
  point (which also decreases slightly as the
  parcel is lifted)
• Once the parcel is saturated, water vapor will
  condense or deposit out, and recall that that
  those processes involve the release of latent
  heat into the parcel
• The level where the parcel becomes saturated is
  called the lifting condensation level (LCL)

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Saturated Adiabatic Lapse Rate

• Because latent heat is being released into the
  parcel during phase changes, the saturated parcel
  will not cool at the dry adiabatic lapse rate any
  longer
• The addition of latent heat counteracts the
  cooling that results from lifting, so a saturated
  parcel will cool more slowly than a dry parcel
• The saturated adiabatic lapse rate is not a
  constant value like the dry adiabatic lapse rate

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Lapse Rates and LCL

• Figure 4.6, Page 68

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Orographic Lift

• Orographic lift is when a parcel is forced to
  rise upward due to sloping terrain
• When wind blows towards a mountain, it can not
  blow through the mountain
• The wind is forced to rise up as it encounters
  this mountain
• This vertical motion results in cooling

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Frontal Uplift

• Air parcels can also rise upward in association
  with cold fronts and warm fronts
• Cold fronts have a steep vertical slope, so the
  lifting mechanism is more vigorous
• Warm fronts have a shallow vertical slope, but
  they still cause parcels to rise upward

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Frontal Uplift

• Figure 9.6(b), Page 157

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Buoyancy

• An air parcel may also rise vertically upward if
  it finds itself less dense than the surrounding
  environment
• If you take a beach ball and try to submerse it
  in a pool, the ball will shoot back up to the
  surface once you let go

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Ideal Gas Law

• The Ideal Gas Law (also called the equation of
  state) relates the pressure (P), density (?), and
  temperature (T) of a gas
• P ? R T, where R is a constant
• If pressure remains a constant and temperature
  increases, the density must decrease
• This explains how a hot air balloon works

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Buoyancy and Parcels

• In order to determine if a parcel is buoyant, we
  must compare the temperature of the parcel with
  the temperature of the environment
• Whenever the parcel is warmer than the
  environment, it will be less dense and continue
  to rise
• Whenever the parcel is colder than the
  environment, it will be more dense and sink down
• Whenever the parcel is the same temperature as
  the environment, the parcel will remain

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Stability Conditions

• Unstable When a parcel is displaced and it
  continues rising away from its original position
• Stable When a parcel is displaced and it
  returns to its original position
• Neutral When a parcel is displaced and it
  remains at its new position

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Stability and Buoyancy

• In order to determine if the atmospheric
  stability is stable, unstable, or neutral, we
  must compare the temperature of the parcel with
  that of the environment
• If Tp gt Te , then unstable
• If Tp lt Te , then stable
• If Tp Te , then neutral

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Clouds vs. Stability

• Cumulus clouds, because of their vertical
  development, are signs of instability
• Stratus clouds, because of their lack of vertical
  development, are signs of stability

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Diurnal Variations in Environmental Lapse Rates

• The temperature profile of the troposphere can
  change throughout the course of a day
• The temperature usually decreases with height at
  all levels in the late afternoon
• There usually is an inversion (temperature
  increasing with height) that starts to form in
  the evening and reaches maximum strength at
  sunrise
• The temperature near the surface changes the
  most, whereas the temperature farther up sees
  less change throughout the course of a day

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Diurnal Variations in Environmental Lapse Rates

• Figure 4.10, Page 71

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Other Means to Saturate

• Water vapor uptake can yield saturated parcels.
  Steam Fog over lakes is an example where water
  vapor uptake over the lake can result in
  saturation
• Mixing of two parcels together can also result in
  saturation. Seeing your breath on a winters day
  and jet contrails are examples of this