PRESSURE
TEMPERATURE
MOISTURE
We remember that
pressure is the weight of a
column of air above a
surface (and only the air, even if an
airplane, rooftop, bird, etc. happens to be in
that particular column – but why does
pressure not include those items? b/c they are
effectively weightless, either supported by a
wall (for a rooftop) or buoyant on their own
(for a plane/bird/insect etc.)
In meteo., we have a very convenient way to measure
atmospheric pressure: Millibars (abbreviated “mb”)
Standard sea-level pressure is 1013.2 mb.
500 mb is considered the “halfway” point of the
atmosphere (half of the atmosphere lies beneath
500mb, the other ½ lies above it)
100 mb is often around the level of the tropopause (so
90% of air lies in the troposphere– no wonder most
weather occurs there!)
Because air pressure decreases naturally as we rise in the
atmosphere, or up a mountain, we must make corrections to the air
pressure owing to elevation above sea level.
These corrections are easily made by adding the air pressure
that would be exerted by the air column at that elevation.
Here we have a great
example of the “700 mb
surface” – and also the
strong relationship between
pressure and temperature
-- Notice (lower-panel) that the
700 mb “surface” (shaded) slopes
downward from the equator to the
poles. According to this graphic, at
the equator, you’d have to rise
3,200 meters (2 miles) to reach
700 mb; at the pole, you only need
to rise 2,800m to reach 700 mb.
This difference in height is
directly related to the temperature
of the air: warm air is less-dense
& thus occupies greater volume.
In panel A, temp is uniform, &
700 mb surface is flat.
Example of a constant-pressure surface weather map: 500 hPa. Can identify regions of
high height and low height. Data for this map come from radiosonde (balloon)
releases at 00Z and 12Z daily. Visit
http://weather.unisys.com/gfsx/init/gfsx_500p_init_eur.html
How to interpret this
chart: a warm column
of air will normally
expand. Thus the 500
millibar (mb, aka hPa)
level will be found
higher above the earth’s
surface. For instance:
the 500 hPa heights
over Austria are ~ 5800
m, but over the north
Atantic are only ~ 5300
m. This 500 m
difference is one of the
factors that generate
wind (& hence weather)
On this chart: colors represent 500 hPa height, contoured every 60 m; dark
lines represent sea level pressure, contoured every 4 mb
Equation of state & relationships
P=ρRT
Pressure = density x gas constant x temperature
• Thus, if we hold P constant (like on a “constant-pressure” weather
map, 500 mb etc.), and T increases, what must density do?
• Relationship becomes more complex when all 3 variables change at
the same time – and when we add water vapor (which affects density)
to the air
• Adding water vapor makes air less dense. Why?
– Avagadro’s rule: for constant P, volume, & T, n (# of molecules)
must remain constant. Therefore, if we displace O2 (atomic weight
32) and N2 (atomic weight 28) with H2O (atomic weight 18), we
have lowered the density of that parcel of air
Water and
the Water
Cycle
The U.S. Geological Survey estimates that the Earth has about 326 million
cubic miles of water. This includes all of the water in the oceans,
underground and frozen in glaciers.
The Hydrologic Cycle involves the continuous circulation of
water in the Earth-atmosphere system. Of the many processes
involved in the hydrologic cycle, the most important are
• evaporation,
• transpiration,
• condensation,
• precipitation, and
• runoff.
Evaporation
 the process by which water changes from a liquid to a gas or
vapor
 Evaporation is the primary pathway that water moves from the
liquid state back into the water cycle as atmospheric water vapor.
 Studies have shown that the oceans, seas, lakes, and rivers
provide nearly 90 percent of the moisture in the atmosphere via
evaporation, with the remaining 10 percent being contributed by
plant transpiration.
Evaporation is a cooling
process
Transpiration is the evaporation of water from plants
through stomata. Stomata are small openings found on the
underside of leaves that are connected to vascular plant
tissues. In most plants, transpiration is a passive process
largely controlled by the humidity of the atmosphere and the
moisture content of the soil. Of the transpired water passing
through a plant only 1% is used in the growth process of the
plant. The remaining 99% is passed into the atmosphere.
How much water do plants transpire?
During a growing season, a leaf will transpire many times
more water than its own weight.
 an acre of corn gives off about 3,000-4,000 gallons
(11,400-15,100 liters) of water each day,
 a large oak tree can transpire 40,000 gallons (151,000
liters) per year.
Condensation is the process by which water vapor in the
air is changed into liquid water. In the atmosphere,
condensation may appear as clouds, fog, mist, dew or frost,
depending upon the physical conditions of the atmosphere.
Condensation is crucial to the water cycle because clouds may
produce precipitation, which is the primary route for water to
return to the Earth's surface.
Condensation is the opposite of evaporation. Condensation
releases heat to the atmosphere
 At any given temperature, there will eventually be an equilibrium
between the number of molecules evaporating, and the number of
molecules condensing.
 When the number of molecules evaporating balances the number
of molecules condensing, we say that the air above the liquid water
is saturated.
Latent heat is critical to
energetics of the atmosphere
Key to understanding latent heat
is to separate the “air parcel”
from the “environment”
Latent heat is exchanged from
the parcel to the environment
Clausius-Clapeyron derivation
Start with the differential relationship
between vapor pressure and temperature:
where es is saturation water vapor pressure over a plane water
surface, T is a temperature, Lv is latent heat of evaporation, and
Rv is water vapor gas constant.
Integrate the equation, note that at 273 K, es is 611.2 Pa and that
Lv is slightly a function of temperature. We get:
Tells us the saturation vapor
pressure of water increases
with increasing temperature
Warmer air has
greater saturation
vapor pressure
and thus more
energy (Gibbs
free energy) is
available for
phase change
to/from gas
Amount of moisture in the
atmosphere is a function of
temperature
Thus July vapor pressures are
typically greater than January
vapor pressures
We can describe the actual amount of water vapor in the
atmosphere at any given time in a number of different ways:
Absolute humidity = density of water vapor (gm of water
vapor/m3 of air)
Specific humidity = portion of atmospheric mass accounted for
by water vapor = mass of water vapor/(mass of dry air + mass
of H2O vapor)
Mixing ratio = mass of water vapor/mass of dry air
Vapor pressure = portion of total atmospheric pressure (P) due
to water vapor (i.e. the partial pressure of water vapor) = e
Relative humidity = actual water vapor in air/maximum water
vapor possible
Relative humidity depends on two factors:
 the actual amount of moisture in the atmosphere
 the temperature
 remember that temperature determines how much water
vapor can be in the air at saturation
We can express/calculate Relative Humidity in a variety of ways:
RH=[(vapor pressure)/(saturation vapor pressure)] X 100%
RH=[(mixing ratio)/(saturation mixing ratio)] X 100%
Remember, RH =
vapor pressure /
saturation vapor
pressure
VP remains
relatively constant
during the day
(varies with
evaporation &
precipitation)
SVP, though, varies
directly with temp
Thus as temp
warms during the
day, RH decreases
Because RH varies greatly each day, it is not a good
measure of moisture in the atmosphere
So you can have a change in relative humidity in
one of two ways:
1) Change the amount of water vapor available; if there is
liquid water present, for instance, a lake, you can have an
increase in relative humidity by evaporation from the surface
of the lake. You’re adding water vapor, so the humidity
increases.
Steam fog over a lake.
2) The other way is to change the temperature of the air,
while holding the water vapor constant. Even though there is
no water source, and no water vapor is added, a lowering of
air temperature results in a rise of relative humidity.
The amount of water vapor that could be present at
saturation is less at the lower temperature, so the existing
amount of water vapor represents a higher percentage of the
saturation level of the air.
Similarly, a rise in temperature results in a decrease in
relative humidity, even though no water vapor has been
taken away.
Key point to remember: Given that the amount of
water vapor is held constant, then if you
-- reduce the temperature, the relative humidity
goes up
-- increase the temperature, the relative humidity
goes down.
If the amount of water
vapor in the air remains
constant, the relative
humidity will change only
as a result of the change
in temperature; therefore,
the relative humidity will
be higher in the morning
(coolest part of the day)
than in the afternoon
(warming part of the
day).
Only about 3,100 cubic miles of
this water is in the air, mostly as
water vapor, but also as clouds
or precipitation, at any one
time, the Geological Survey
estimates.
While this is a small share of
Earth's water, our planet would be
very different without it. If Earth's
air didn't contain as much
humidity as it does, our weather
would be like that of Mars: And,
without all of this water in all of its
forms, life on Earth would
probably not exist.
The storehouses for the vast majority of all water on Earth
are the oceans:
 of the 332,500,000 cubic miles (mi3) (1,386,000,000 km3)
of the world's water supply, about 321,000,000 mi3
(1,338,000,000 km3) is stored in oceans (~ 96.5%)
 it is also estimated that the oceans supply about 90% of
the evaporated water that goes into the water cycle.
The vast majority, almost 90 percent, of Earth's ice mass is in
Antarctica, while the Greenland ice cap contains 10 percent of
the total global ice mass.
Photos from: www.uscg.mil/.../polarsea/ Antarctica_Photos.htm
Estimate of global water distribution
Water source
Water volume, in cubic
miles
Water volume, in cubic
kilometers
Percent of
freshwater
Percent
of
total
water
Oceans, Seas, & Bays
321,000,000
1,338,000,000
--
96.5
Ice caps, Glaciers, &
Permanent Snow
5,773,000
24,064,000
68.7
1.74
Ground water
5,614,000
23,400,000
--
1.7
Fresh
2,526,000
10,530,000
30.1
0.76
Saline
3,088,000
12,870,000
--
0.94
Soil Moisture
3,959
16,500
0.05
0.001
Ground Ice & Permafrost
71,970
300,000
0.86
0.022
Lakes
42,320
176,400
--
0.013
Fresh
21,830
91,000
0.26
0.007
Saline
20,490
85,400
--
0.006
Atmosphere
3,095
12,900
0.04
0.001
Swamp Water
2,752
11,470
0.03
0.0008
Rivers
509
2,120
0.006
0.0002
Biological Water
269
1,120
0.003
0.0001
Source: Gleick, P. H., 1996: Water resources. In Encyclopedia of Climate and Weather, ed. by S. H.
Schneider, Oxford University Press, New York, vol. 2, pp.817-823.
• Water freezes at 32o F (0oC) and boils at 212oF (100oC) at
sea level, but 186.4°F at 14,000 feet
• Water has a high specific heat index. This means that water
can absorb a lot of heat before it begins to get hot
• The high specific heat index of water helps regulate the rate
at which air changes temperature, which is why the
temperature change between seasons is gradual rather than
sudden, especially near the oceans
According to columnist Cecil Adams, "a modest-size cloud,
one kilometer in diameter and 100 meters thick, has a mass
equivalent to one B-747 jumbo jet." But, with all that mass
being spread over such a large volume of space, the density,
or weight (mass) for any chosen volume, is very small.
Even though a cloud
weighs tons, it doesn't fall
on you because the rising
air responsible for its
formation keeps the cloud
floating in the air.
http://www.straightdope.com/classics/a980313a.html,
Although the atmosphere may not be a great storehouse of water, it is the
superhighway used to move water around the globe. Evaporation and
transpiration change liquid water into vapor, which ascends into the atmosphere
due to rising air currents. Cooler temperatures aloft allow the vapor to condense
into clouds and strong winds move the clouds around the world until the water
falls as precipitation to replenish the earthbound parts of the water cycle.
There is always water in the atmosphere (i.e., RH never equals 0%). If all
of the water in the atmosphere rained down at once, it would only cover the
ground to a depth of 5 to 50 mm.
By examining precipitable water content, you get an idea of not only the
quantity of moisture in the depth of the troposphere, but also the temperature
for the layer
Precipitation
Precipitation is the result
when the tiny condensation
particles grow too large for
the rising air to support, and
thus fall to the earth.
Precipitation is water
released from clouds in the
form of rain, freezing rain,
sleet, snow, or hail.
It is the primary connection in
the water cycle that provides
for the delivery of
atmospheric water to the
Earth.
Most precipitation falls as rain.
Ice caps influence the weather
In some regions, precipitation collects throughout the year as ice or snow.
Just because water in an ice cap or glacier is not moving does not mean that it
does not have a direct effect on other aspects of the water cycle and the weather.
Ice is very white, and since white reflects sunlight (and thus, heat), large ice fields
can determine weather patterns. Air temperatures can be higher a mile above ice
caps than at the surface, and wind patterns, which affect weather systems, can be
dramatic around ice-covered landscapes.
Some glacier and ice cap facts
 Glacial ice covers 10-11 percent of all land.
 According to the National Snow and Ice Data Center (NSIDC), if all glaciers
melted today the seas would rise about 230 feet (70 meters).
 During the last ice age (when glaciers covered more land area than today) the
sea level was about 400 feet (122 meters) lower than it is today. At that time,
glaciers covered almost one-third of the land.
 During the last warm spell, 125,000 years ago, the seas were about 18 feet
(5.5 meters) higher than they are today. About three million years ago the seas
could have been up to 165 feet (50.3 meters) higher.
 Largest surface area of any glacier in the contiguous United States: Emmons
Glacier, Washington (4.3 square miles or 11 square kilometers)
Extent of global ice
during the last ice age
(~20,000 years ago)