AS 120 Principles of Aeronautical Science

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Atmospheric Moisture
CH 5. pp. 122-152
Hydrologic Cycle
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Water is an Odd Molecule

• Boiling point is much higher than one would
  expect from molecular weight.

Substance Molecular weight Boiling Temp
Oxygen 32 -183 oC
Nitrogen 28 -196 oC
Water 18 100 oC
Carbon Dioxide 44 -79 oC
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Water is a Polar Molecule
Net positive charge on one side and a negative
side on the other.
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Polar Molecule

• Electrical negative will attract positive charge.
• The electrical attraction of the polar molecules
  is quite strong.
• The attraction makes water molecules join
  together, raising the temperature of the boiling
  and freezing point.
• Polar attraction also makes water a good liquid
  solvent. Other molecules bind to water in
  solution.
• Water one of the very few substances that is
  densest in its liquid state
• Implications in the atmosphere?

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Ice Crystal

• 105 degree separation of hydrogen causes
  formation of a six sided crystal.

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Water Vapor

• As the number of water vapor molecules in the air
  increase in number, they will have more chance of
  being close to another molecule and being
  attracted to each other.
• After there are a certain number of molecules in
  the air, any more molecules will cause the
  molecules to clump together forming a liquid.
• This point is the saturation point.

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Saturation Point

• Saturation point dependent upon
• Temperature (the higher the temperature the more
  the molecules can escape from each others
  influence)
• Pressure (the higher the pressure, the higher the
  number of molecules present)

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Atmospheric Moisture
Methods of achieving saturation

1. Adding water vapor to the air
2. Mixing cold air with warm, moist air
3. Lowering the temperature to the dew point

Lets talk about 3
Air Temperature can change by Diabatic
Adiabatic Processes
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Atmospheric Moisture
Diabatic Processes
Processes that involve the removal/input of heat
Increase Heat Increase Temp. Increase volume
Adiabatic Processes
Processes that do not involve the removal/input
of heat
Expansion of air Increase volume Decreases
Temp.
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Evaporation/Condensation

• Imagine a container of water covered with a lid
• Some molecules go from liquid to gas
  (evaporation), and some go from gas to liquid
  (condensation).
• The air above the liqud surface is at saturation
  when the number of molecules escaping equals the
  number of molecules reentering the water.

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Saturation

• Saturation occurs when evaporationcondensation
  (the air cant hold any more water vapor)

Increasing the temperature of the air increases
the amount of water vapor it can hold (it takes
more water vapor to reach saturation point)
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Moisture Measurements

• There are a number of measurements we can use to
  specify the amount of moisture (also referred to
  as humidity) in the air
• absolute humidity
• specific humidity
• vapor pressure
• saturation vapor pressure
• relative humidity
• mixing ratio
• saturation mixing ratio
• wet-bulb temperature
• dew-point temperature

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Absolute Humidity

• Absolute Humidity mass of water vapor/volume of
  air
• So, absolute humidity is like a water vapor
  density, commonly express in grams/m3
• Abs humidity is not a useful measurement for
  humidity--because it changes with volume and
  temperature changes that occur in the atmosphere.
  Why?

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Expansion/Compression

• What happens to the volume of a rising or sinking
  air parcel?
• Consider a parcel of air at 1000mb
• The parcel exerts 1000 mb of outward pressure to
  counteract the atmospheric pressure acting on the
  parcel
• If no energy is added or taken away from the
  parcel, then the force of the molecules bumping
  into the side of the parcel will be constant

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Expansion

• Now, we raise the parcel to 500 mb
• The outside pressure on the parcel decreases, so
  the volume of the parcel increases as the parcel
  expands

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Specific Humidity

• Specific humidity mass of water vapor/total
  mass of air
• For example, in a parcel, the mass of water vapor
  is 1g
• The total mass of the parcel (N2, O2, Ar, H2O,
  other trace gasses) is 1 kg
• Mass H20/Total mass of the parcel of air
  (including the water vapor)
• Specific Humidity is 1g/kg
• Specific humidity is not effected by expansion
  and compression changes in the air parcel
• Much more useful in meteorology

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Latitudinal Distribution of Specific Humidity
Figure 5.9

• High at the equator
• Low at the poles

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Absolute Specific Humidity
Figure 5.7
For a given mass of water vapor in an air parcel,
the absolute humidity changes as the parcel
volume changes (lifts or descends).
Figure 5.8
Specific humidity is concerned with the mass of
vapor to mass of air, and is not affected by
changes in parcel volume.
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Mixing Ratio

• Mixing ratio mass of water vapor/mass of dry
  air
• For example In a parcel, the mass of water
  vapor is 1g and the mass of the dry air in the
  parcel is 1.0 kg
• The mixing ratio is 1g/kg

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Vapor Pressure

• Remember from Chap 1 that pressure is the force
  of collisions of molecules against a surface
• The total pressure is the sum of the pressures of
  the different molecules

If the total pressure in this parcel was 1000
mb-- Nitrogen would contribute 780 mb, Oxygen 210
mb, and Water vapor 10 mb.
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Actual Vapor Pressure(Daltons Law)

• The total pressure of the air parcel is due to
  the sum of partial pressures of each of the
  gasses comprising the parcel
• The pressure due to water vapor is called the
  actual vapor pressure

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Distribution of Actual Vapor Pressure
January
July
Higher in moist regions Higher in warmer areas
and warmer season Very low in cold air
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Saturation Vapor Pressure

• Actual vapor pressure tells us the total water
  vapor content of the air..
• Saturation vapor pressure indicates how much
  water vapor pressure is present when the air is
  saturated
• Dependent on air temperature
• It takes less moisture to saturate a cold parcel
• Warm air can hold much more water vapor than cold
  air

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Relative Humidity

• How much water vapor is there divided by how much
  it can hold (X 100)RH
• Basically Content/capacity
• Relative Humidity can be calculated by
• Actual vapor pressure/saturation vapor pressure
• Actual mixing ratio/saturation mixing ratio
• We can change RH in two ways
• Change the amount of vapor in the air OR
• Change the air temperature
• Relative Humidity is affected by both temperature
  and pressure
• Relative humidity is high at equator and poles

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• In most places, vapor content changes only
  slightly over a day, yet RH varies widely each
  day
• This is due to daily change in air temperature.
  Lowest temperatures of early morning result in
  highest RH values, warm afternoon result in
  lowest RH values

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Relative and Specific Humidity

• Relative humidity (RH) as an indicator of
  saturation reveals that desert air is far from
  saturated, and that cold polar air nears
  saturation.
• Graphs of RH contrast with specific humidity in
  the deserts and poles.

Specific Humidity
Relative Humidity
Figure 5.14
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Dew Point Temp (TDP)

• The temperature that the air must be cooled to
  (assuming no changes in water vapor or pressure)
  in order for saturation to occur
• If the dew point is lt 32oF, its called Frost
  Point
• Important measurement to predict formation of
  dew, frost, fog and minimum air temperaturesas
  well as a good indicator of severe weather,
  cumulus cloud ceiling heights
• High TDP is a good indicator of the airs actual
  vapor content.

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Seasonal Dew Point Maps
Figure 5.12A
Figure 5.12B

• January, July Average Dew Point Temperatures
• High Dew Pointsplentiful moisture
• Low Dew Pointsvery dry air

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Dew Point Reality

• Can you feel dew point?
• On a typical summer day the following is true
• Dew Point(F) Perception
• 75..........................Extremely
  uncomfortable
• 70-74........................Very humid, quite
  uncomfortable
• 65-69........................A bit uncomfortable
  for most people
• 60-64........................Ok for most, but
  everyone begins to
• feel the
  humidity
• 55-59........................Comfortable
• 50-54........................Very comfortable
• lt49........................Feels like the
  west, very pleasant, feels a bit dry
  to some

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Wet Bulb Temperature

• The lowest temperature that can be reached by
  evaporating water into the air
• A good measure of how cool the skin can become by
  sweating
• Note the wet bulb temperature will always be
  less than or equal to the temperature
• If you are a runner
• T 90F, RH 90...High wet-bulb temperature.
• T 90F, RH 10...Low wet-bulb temperature.
• You feel more comfortable when wet-bulb
  temperature is low
• Wet-bulb temperature is related to the heat index

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Measuring Moisture
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Moisture MeasurementsHair Hygrometer

• Human hair lengthens with increasing RH. (why
  you have a bad hair day)
• Levers amplify the change in length.
• Measures Relative Humidity directly.
• Relatively cheap instrument.

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Moisture MeasurementsSling Psychrometer

• Two thermometers one wet, one dry. Move through
  the air (either fan or swing around).
• Wet thermometer measures wet bulb temperature

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Moisture measurementsDew Point

• Cool mirror until dew (or frost) forms on mirror.
  
• Used by NWS at all automated surface observing
  stations.

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Moisture MeasurementsElectrical Hygrometer

• Uses a chemical film that absorbs moisture which
  changes the electrical resistance.
• Used in radiosonde measurements.