Transcript Document

Announcements
• We will likely postpone some lectures and
the exams will likely be pushed back a day
• This shouldn’t affect what we can cover in
class—we have an extra day worked in
there
• More updates when Mr. Piety returns next
week
Outline for Lecture 7
Movement of Water through the Atmosphere
Water’s Changes of State
Water in the Atmosphere
Vapor Pressure and Saturation
Relative Humidity
2/20/03
Outline for Lecture 7 (cont.)
Dew Point Temperature
Adiabatic Temperature Changes
Humidity Measurement
New Wind Chill Chart
New wind chill vs. old
What does wind chill mean?
• It’s 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 don’t have any exposed surfaces?
• My point, exactly—bundle up.
• http://205.156.54.206/om/windchill/index.shtml
Hydrologic Cycle
atmosphere
Continuous cycle
oceans
land
Earth’s Water Balance
Volume of Water (km 3)
350,000
Ocean Processes are the dominant
source and sink for water into the
atmosphere.
300,000
250,000
200,000
150,000
100,000
50,000
0
Ocean Evap
Ocean Precip
Land Precip
Evap/trans
Runoff
process
Ocean Evap
Ocean Precip
Land Precip
Evap/trans
Runoff
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”.
Weather Machine
A calorie is a measure of heat energy (1 cal  1 g of H2O 1 °C)
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.
Water in the Atmosphere
Absolute Humidity and Mixing Ratio, what do they mean?
Absolute Humidity is the mass of water vapor in a given parcel of
air.
Absolute Humidity = mass of water vapor (grams)
volume of air (cubic meters)
Mixing Ratio (by mass) is the mass of water vapor in a unit of air
compared to the remaining mass of dry air.
Mixing Ratio
=
mass of water vapor (grams)
mass of dry air (kilograms)
Note that this is not the mixing ratio you may be used to!!!
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 air’s 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”
Vapor Pressure, RH, Dew Point
• Vapor pressure—how much water vapor is
actually there
• Dew Point Temperature—related to how
much water vapor is actually there
• Saturated vapor pressure—how much water
vapor could be there
• Relative humidity is the ratio of how much
is there to how much could be.
(a) 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
Saturation vapor pressure is
temperature dependent. The mass of
water vapor needed to reach saturation
nearly doubles every 10 °C
Saturation vapor pressure
The amount of water required to saturate 1 kg of dry air at various
temperatures.
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.
Relative Humidity
The ratio of the air’s 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.
How Relative Humidity Changes
Adding (removing) moisture increases (decreases) Relative Humidity.
RH Changes with Temperature
Heating (cooling) decreases (increases) Relative Humidity.
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.
That is not snow on the trees, the air temperature in this picture
is -22°F. The water deposited directly to ice from water vapor.
This phenomenon is called hoar frost or white frost.
Deposition
Frost
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 Earth’s surface cools, water vapor
condenses on the ground creating dew.
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.
A map of dew point can be useful because it
identifies wet and dry areas quickly.
Dew point map
Cool and Dry
Warm
and
moist
Measuring Humidity
Why do we measure relative humidity ?
If the relative humidity is known, other factors (mixing ratio,
absolute humidity) can be easily calculated.
Sling
Psychrometer
from USA today
Hair Hygrometer
Two methods to measure
Relative Humidity
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
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 it’s humid,
charges can drain away
along surfaces, so you
don’t get big sparks.
Adiabatic Temperature Changes
Frost and dew form (condense) on the Earth’s 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
Earth’s 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.
Adiabatic -occurring without the gain or loss of heat.
A good way to visualize the adiabatic process is a bicycle pump.
Adiabatic Warming
Adiabatic Cooling
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.
Adiabatic
V1 > V2
V1
V2
V1 < V2
V1 V2
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.
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 5°C/km
Dry adiabatic lapse rate 9.8°C/km
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
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
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 alltime 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.
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.
What the heck was that?!?
What the heck was that?!?
A “normal” nor’easter
What the heck was that?!?
What the heck was that?!?
What the heck was that?!?
This weekend
This weekend
This weekend
This weekend
This weekend
This weekend
• Terps take on the
Imperial Forces of
Evil (duke)
• Lots of rain
• Probably flooding
• …AND you have
homework.
• GAH!