Announcements

1
Announcements

• We will likely postpone some lectures and the
  exams will likely be pushed back a day
• This shouldnt affect what we can cover in
  classwe have an extra day worked in there
• More updates when Mr. Piety returns next week

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Outline for Lecture 7

• Movement of Water through the Atmosphere
• Waters Changes of State
• Water in the Atmosphere
• Vapor Pressure and Saturation
• Relative Humidity

2/20/03
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Outline for Lecture 7 (cont.)

• Dew Point Temperature
• Adiabatic Temperature Changes
• Humidity Measurement

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New Wind Chill Chart
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New wind chill vs. old
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What does wind chill mean?

• Its related to how fast heat will be removed
  from an exposed surface
• So 30ºF with 20 mph winds will feel like 17ºF
  with little or no wind
• What if you dont have any exposed surfaces?
• My point, exactlybundle up.
• http//205.156.54.206/om/windchill/index.shtml

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Hydrologic Cycle
Continuous cycle
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Earths Water Balance
Ocean Processes are the dominant source and sink
for water into the atmosphere.
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Weather Machine
Energy is absorbed or released from the
atmosphere during phase changes. This heat is
called latent (hidden) because the heat used
does not produce a temperature change. The
energy that is liberated during condensation
helps to drive the weather machine.
A calorie is a measure of heat energy (1 cal ? 1
g of H2O 1 C)
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Phase Changes
Evaporation Conversion of Liquid to Gas
(Vapor). Transpiration absorption of water by
plants through their roots release to the
atmosphere. Condensation Conversion of Gas to
Liquid. Melting Conversion of Solid to
Liquid. Freezing Conversion of Liquid to
Solid. Sublimation Conversion of Solid to
Gas. Deposition Conversion of Gas to Solid.
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Water in the Atmosphere
Absolute Humidity and Mixing Ratio, what do they
mean?
Note that this is not the mixing ratio you may be
used to!!!
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Vapor Pressure, RH, Dew Point
Neither the Absolute Humidity nor the Mixing
Ratio are easy to determine directly.
Therefore, other measurements, such as Vapor
Pressure, Relative Humidity, and Dew Point are
used to describe the moisture content of the
atmosphere.
Vapor Pressure - the part of the total
atmospheric pressure exerted by water vapor.
Relative Humidity (RH) - is the ratio of the
airs actual water vapor content compared with
the amount of water vapor required for
saturation at that temperature (and pressure).
Dew Point Temperature - is the temperature to
which a parcel of air would need to be cooled to
reach saturation. AKA dew point
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Vapor Pressure, RH, Dew Point

• Vapor pressurehow much water vapor is actually
  there
• Dew Point Temperaturerelated to how much water
  vapor is actually there
• Saturated vapor pressurehow much water vapor
  could be there
• Relative humidity is the ratio of how much is
  there to how much could be.

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• Dry air introduced
• No vapor pressure
• (b) Water evaporates,
• pressure increases in the
• vessel
• (c) Equilibrium between
• evaporation and condensation
• is reached. The air is now
• saturated.
• (d) When the vessel is heated
• the vapor pressure increases
• (more water evaporates
• than condenses). This process
• continues until a new equilibrium
• point (vapor pressure) is reached.

Vapor Pressure
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Saturation vapor pressure
Saturation vapor pressure is temperature
dependent. The mass of water vapor needed to
reach saturation nearly doubles every 10 C
The amount of water required to saturate 1 kg of
dry air at various temperatures.
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The atmosphere behaves much like a closed vessel.
Gravity prevents water vapor, and other gases,
from escaping into space.
atmosphere
Again, like the closed container, water is
constantly being exchanged between the gas and
liquid phases.
liquid surface
Q. What determines whether the rate of
evaporation exceeds the rate of condensation
(net evaporation) or vice versa?
A. 1. The temperature (molecular motion) of the
water or ice surface. 2. The amount of water
vapor surrounding the water or ice.
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Relative Humidity
The ratio of the airs actual water vapor content
to the amount of water vapor required for
saturation at that temperature and pressure.
Relative humidity indicates how near the air is
to saturation rather than the actual quantity of
water vapor in the air.
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How Relative Humidity Changes
Adding (removing) moisture increases (decreases)
Relative Humidity.
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RH Changes with Temperature
Heating (cooling) decreases (increases) Relative
Humidity.
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Natural Changes in RH

• Daily (diurnal) changes in temperature
• (hot in the day and cold in the night).
• Temperature changes that result as air moves
  horizontally
• from place to another.
• Temperature changes caused as air moves
  vertically in the
• atmosphere

Typical daily variation in Temperature and
Relative Humidity for a spring day In Wash, DC.
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Frost
That is not snow on the trees, the air
temperature in this picture is -22F. The water
deposited directly to ice from water vapor. This
phenomenon is called hoar frost or white frost.
Deposition
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Dew Point Temperature
Dew Point temperature - the temperature to which
a parcel of air would need to be cooled to reach
saturation.
The term dew point temperature reflects the
concept that in the evening, when the Earths
surface cools, water vapor condenses on the
ground creating dew.
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Comments on Moisture in the Atmosphere
Relative Humidity is a measure of how close the
air is to being saturated, whereas Dew Point
Temperature is a measure of the actual moisture
content of the atmosphere at that
point. Understanding the above (and looking at
table table 4-1) we can see why, for instance,
air over Fort Meyers, Florida with a dew point
of 25 C contains twice as much water vapor as
air over St Louis with a dew point of 15 C and
four times the air over Tucson, Arizona with a
dew point of 4 C.
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Dew point map
A map of dew point can be useful because
it identifies wet and dry areas quickly.
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Measuring Humidity
Why do we measure relative humidity ?
If the relative humidity is known, other factors
(mixing ratio, absolute humidity) can be easily
calculated.
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Sling Psychrometer
Hair Hygrometer
from USA today
Two methods to measure Relative Humidity
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Humidity and Static Electricity

• A winter phenomenon
• In winter, the air is very very dry
• Nighttime temperature is about 20 ºF
• Dewpoint can be no higher than that
• Heat the air up to 70 ºF

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Humidity and Static Electricity

• Humidity is then only 15!!
• How about a nice day in spring?
• T 70 ºF, dew point is 50 ºF, RH 50
• When its humid, charges can drain away along
  surfaces, so you dont get big sparks.

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Adiabatic Temperature Changes
Frost and dew form (condense) on the Earths
surface in the early morning (typically the
coolest time of the day) because heat is readily
exchanged from the ground to the air above
it. -The ground cools until water vapor condenses
upon striking it. On the other hand, cloud
formation often occurs far from the Earths
surface and during the hottest time of day. We
conclude that a different process must be taking
place to cool the air to saturation .
Adiabatic temperature changes are often
responsible for cloud formation.
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Adiabatic
Adiabatic -occurring without the gain or loss of
heat.
A good way to visualize the adiabatic process is
a bicycle pump.
Adiabatic Cooling
Adiabatic Warming
The energy used to compress the air molecules in
to a smaller volume increases the motion of the
gas which causes a rise in temperature.
As the air expands into the increased volume
the molecules move less rapidly and the air
temperature decreases.
V1 gt V2
V1 lt V2
V1
V2
V1
V2
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Adiabatic Cooling Condensation
It is helpful to think of adiabatic processes as
occurring within an air parcel. Think of an air
parcel as being a few hundred cubic meters in
volume.
It is also convenient to ignore mixing of
surrounding air with the air parcel.
This process is known as entrainment.
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Rate of Adiabatic Cooling
The rate at which adiabatic cooling occurs with
altitude is dependent on the moisture content in
the atmosphere.
Wet adiabatic lapse rate 5C/km
Dry adiabatic lapse rate 9.8C/km
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What the heck was that?!?

• TOP 10 LIST FOR BALTIMORE, MD SNOWFALLS
• 1ST FEBRUARY 16-18 2003......28.2 INCHES
• 2ND JANUARY 27-29 1922......26.5 INCHES
• 3RD FEBRUARY 11-12 1983......22.8 INCHES
• 4TH JANUARY 7-8 1996......22.5 INCHES
• 5TH MARCH 29-30 1942......22.0 INCHES
• 6TH FEBRUARY 11-14 1899......21.4 INCHES
• 7TH FEBRUARY 18-19 1979......20.0 INCHES
• 8TH MARCH 15-18 1892......16.0 INCHES
• 9TH FEBRUARY 15-16 1958......15.5 INCHES
• 10TH JANUARY 25 2000......14.9 INCHES

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What the heck was that?!?

• TOP 10 LIST FOR WASHINGTON, D.C. SNOWFALLS
• 1ST JANUARY 27-28 1922......28.0 INCHES
• 2ND FEBRUARY 11-13 1899......20.5 INCHES
• 3RD FEBRUARY 18-19 1979......18.7 INCHES
• 4TH JANUARY 6-8 1996......17.1 INCHES
• 5TH FEBRUARY 16-18 2003......16.7 INCHES
• 6TH FEBRUARY 11-12 1983......16.6 INCHES
• 7TH FEBRUARY 15-16 1958......14.4 INCHES
• 8TH FEBRUARY 7 1936......14.4 INCHES
• 9TH FEBRUARY 16-18 1900......14.3 INCHES
• 10TH JANUARY 29-30 1966......13.8 INCHES

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What the heck was that?!

• Central Park -- New York City recorded 19.8
  inches, making this the 4th ranked
• Boston, Mass. (Logan Airport) -- Both a new
  all-time record and a new 24-hour record set with
  27.5 inches of snow in this storm
• Potomac highlands and Shenandoah/Blue Ridge were
  in the 3 to 4-1/2 foot range. Garrett County, MD
  190 inches or 16 feet of snow this winter so
  far. Normal 100 inches.

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What the heck was THAT?!?

• Heavy rain forecast for this weekend
• currently calling for over 2 of rain.
• This falling on already saturated ground, and
  melting the snow should lead to some flooding of
  the Potomac.

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What the heck was that?!?
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What the heck was that?!?
A normal noreaster
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What the heck was that?!?
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What the heck was that?!?
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What the heck was that?!?
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This weekend
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This weekend
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This weekend
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This weekend
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This weekend
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This weekend

• Terps take on the Imperial Forces of Evil (duke)
• Lots of rain
• Probably flooding
• AND you have homework.
• GAH!