The water molecule two hydrogen and one oxygen H20

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The water molecule two hydrogen and one oxygen
H20
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The shape of the water molecule results in it
forming the familiar six sided snowflake
structure. Also when water freezes the molecules
arrange themselves as shown above leaving empty
space within the six-sided ring structure. This
is why 1.0 gram of ice is larger in volume than
1.0 gram of liquid water.
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Water molecules continually evaporate from and
condense back onto the liquid water surface. If
the evaporation rate is larger than condensation
rate the humidity increases and if the
evaporation rate is smaller than the condensation
rate then the humidity decreases. The air is
said to be saturated when the evaporation and
condensation rates are equal.
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The air is said to be saturated when the
evaporation and condensation rates are equal. In
a closed system the saturation humidity increases
as the temperature increases. The reason for
this is that the evaporation rates increase and
hence the number of water vapor molecules the air
increases until the condensation rate matches
this higher evaporation rate.
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Water Cycle See hydrological Cycle Module
http//ga.water.usgs.gov/edu/watercycle.html
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Figure 2. The mean distribution of precipitable
water, or total atmospheric water vapor above the
Earth's surface, for 1992. This depiction
includes data from both satellite and
radiosonde observations. (Image courtesy of
Thomas Vonder Haar and David Randel, Colorado
State University, Fort Collins.) On average over
the whole Earth there is about 1 inch (25 mm) of
precipitable water in the atmosphere at any given
time. This is equivalent to about 1 weeks worth
of water use by humans.
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There is typically more water near the surface
than higher up. The stratosphere is quite dry.
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On average the near surface equatorial regions
(the tropics) have the largest humidity of
anywhere on Earth.
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Water vapor is extremely variable from place to
place and time to time. Water vapor amounts
recorded from Satellites can help forecasters
determine heavy precipitation events and also
help estimate nighttime lows from radiative
cooling.
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Since water molecules emit infrared radiation IR
detectors can be used to sense water vapor
amounts remotely.
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Since water vapor is a greenhouse gas it can trap
heat energy absorbed by the sun close to the
surface throughout the night. Les water vapor
implies cooler night-time temperatures. The
difference between day time and night time
temperatures is greatest in desert regions that
have little water vapor.
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In any given parcel of air there are nitrogen
oxygen, and water molecules (water vapor) among
many other gases in small quantities. The actual
humidity is a measure of how many water molecules
are actually in the air. The actual humidity can
be measured in a variety of units. Grams of water
per kilogram of air or grams of water per cubic
meter.
We will use vapor pressure as a measure of
humidity. As the molecules bounce around wildly
they exert an outward pressure. At sea level
this outward pressure balance the inward pressure
from the weight of all air above a location.
This is normal sea level pressure.
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We will use vapor pressure as a measure of
humidity. As the molecules bounce around wildly
they exert an outward pressure. At sea level,
this outward pressure balances the inward
pressure from the weight of all air above a
location. This is normal sea level pressure and
is about 1000 mb (milli-bars). Part of this
outward pressure comes from the water vapor
molecules.
When the vapor pressure is 20 mb approximately 2
of all air molecules are water vapor. When the
vapor pressure is 10 mb approximately 1 of all
air molecules are water vapor. When the vapor
pressure is 30 mb approximately 3 of all air
molecules are water vapor.
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Water boils when the internal vapor pressure
pushing out on a tiny bubble equals the
atmospheric pressure pushing inward on the
bubble. The graph above shows the saturation
vapor pressure on the y-axis versus temperature
on the x-axis. Water normally boils at 100 oC at
sea level because at that temperature the
saturation vapor pressure equals the sea level
pressure.
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At the top of Mt. Hood water boils at about 93 oC
Mt. Hood
Mt. Rainer
At the top of Mt. Rainer water boils at about 85
oC
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Amazingly Vancouver Wa and Tucson Az have the
same amount of water vapor in the air during
January.
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Dew point Temperature(directly linked to actual
humidity)
Temperature that results in saturation or dew
forming on surfaces. High dew point high actual
humidity Low dew point low actual humidity Dew
points in excess of 70 F will normally make one
uncomfortable.
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The dew point temperature is directly linked to
actual humidity
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Vancouver Wa and Tucson Az also have roughly the
same amount of water vapor in the air during July.
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Relative Humidity
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The relative humidity describes how much water is
in the atmosphere relative to the maximum
possible water vapor amount. In the above figure
the dark blue columns indicate the vapor pressure
(actual) and the total column height up to the
top of the light blue is the maximum possible
(saturation vapor pressure). Graphically we can
see that the far right column has the highest
relative humidity and the middle has the lowest.
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High relative humidity results in low evaporation
rates so our body does not cool itself very well
and we more easily suffer from heat stroke. The
Heat index is a quantitative measure of this
discomfort. The above example shows an air
temperature of 100 oF with relative humidity of
60 is equivalent to a dry 130 oF day. During
most years, heat stroke is the number 1 weather
related cause of death in the US.
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These examples of cooling from evaporation work
best in dry climates. Evaporative cooler,
porous clay pot, and canvas canteen
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If you soak the canvas wrapper the evaporative
cooling will keep you water cool. However just
wrapping your bottle in cloth with prevent
condensation from occurring on the bottle. Since
water vapor releases latent heat energy when it
condenses, the cloth wrapping isolates you water
bottle from warming up too fast from the
condenstaion.
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The RH will increase if more water vapor is added
to the air.
Fig. 4-11a, p. 93
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Since as the air temperature increases so does
the saturation vapor pressure. This causes the
relative humidity decrease as the air temperature
increases (denominator gets bigger)
Fig. 4-11b, p. 93
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RH is maximum when it is coolest. This is the
most likely time for fog, dew or frost to occur.
RH is minimum when it is warmest.
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Outside T -15 C Td -15 C RH 100
Inside T 20 C Td -15 C RH 8
Inside the cabin the air is uncomfortably dry.
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Air temp 35oC Dew point 10 oC RH 21
A dew point of 10 oC represents air with a
significant amount of water. However since it is
so warm here in this desert the relative humidity
is quite low. Evapo-transpiration occurs quite
rapidly from surface plants leaving the area
relatively dry. This same dew point with a mean
air temperature of 20 oC (68F) would correspond
to a comfortable 52 relative humidity and plant
life would likely flourish
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Air coming off the Pacific is relatively cool and
contains less water than the warm moist air
flowing into the Eastern US from the Atlantic and
Gulf of Mexico.
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Hygrometers measure humidity A Sling
Psychrometer is common
Well use the sling psychrometer in our week 4
lab.
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Computer data collection has almost completely
replace the old style hydrographs.