917 Lecture, Moisture, Clouds
917 Lecture, Moisture, Clouds
Family Homecoming Special Event
"Can Climate Engineering Serve as a Complementary Step to Aggressive
Dr. Michael MacCracken, The Climate Institute, Washington, DC
Friday, Sept. 25 at 4:00 pm in Olin 1, with cookies
Dalton’s Law of Partial Pressures
The total pressure of the air is the sum of the partial
pressures of its component gases.
Ptotal = PN2 + PO2 + e + Ptrace
Total = Nitrogen + Oxygen + Water Vapor + Trace Gases
Moisture is critically important to all sorts of weather
If our atmosphere was as dry as Mars, we would have winds and
sandstorms, and that’s about it.
On Earth, we have dew, fog, mist, clouds, frost, rain, snow, sleet,
hail, ice, colored sunsets, rivers, lakes, oceans, and many other
phenomena -- all because of moisture.
Moisture controls the heat budget of the planet
Water vapor is a major greenhouse gas.
Clouds, snow and ice reflect sunlight back to space.
Sensible heat is carried by winds and ocean currents
Oceans store heat and moderate coastal climates.
Snow is an effective insulator.
Vegetation alters both the heat and moisture budgets.
Latent (or hidden) heat is exchanged when water changes from one phase to
Latent heat powers weather systems
Sling psychrometer -- ventilated wet bulb and dry
Hair hygrometer -- hair lengthens with humidity
(about 2.5%). Twisted hair generates torsion.
Dew Cell -- measures humidity by an electrical
conductor coated with LiCl, a chemical whose
conductivity varies with moisture.
one free surface
Sublimation, melting, and evaporation all require heat from the
Deposition, freezing, and condensation all release heat to the environment.
Temperature and phase changes of 1 gram of ice as energy is added.
Vapor pressure, e is that portion of the total air pressure exerted by the water vapor.
From Dalton’s Law of Partial Pressures, e = P (H2O).
Saturation vapor pressure, es is the maximum possible e at a given T. It assumes
that the water vapor and a plane water surface are in equilibrium.
Relative Humidity RH = e/es x 100%
Air does not “hold” water vapor; the dry gases in the air coexist with water vapor
near the ground.
It is temperature (mainly) that affects the rate of water vaporization.
Imagine water molecules leaving and entering Johnson Pond. When they are
inequilibrium (for a given T) then you measure e.
As temperatures drop overnight… e lowers and RH rises
If a fog covers the pond, then you can assume e ~ es
Dew Point, Td is the temperature to which air must be cooled at
constant pressure to reach saturation. Dew point indicates the
temperature at which dew, frost, fog may occur.
Often the dew point temperature in the evening is a good predictor
of the overnight minimum temperature? Why?
High Td means high absolute humidity.
If T and Td are almost the same then RH is high.
If T = Td, then RH = 100%
Anthes Fig. 1.9
pressure and density as
a function of T
Insert is for liquid and ice
below 0 oC
The boiling point is defined as the temperature at which the
saturation vapor pressure of a liquid (es) is equal to the
surrounding atmospheric pressure.
For water, es = 1 atm. at 100 C.
At pressures greater than 1 atm. (e.g. in a pressure cooker), the
boiling temperature is greater than 100 C.
At pressures lower than 1 atm. (e.g. on a mountain top), the boiling
temperature is less than 100 C.
when es = 1 atm.
Clouds are aesthetically pleasing. They generate
rainbows, cloudbows, halos, and gorgeous sunsets.
Cloudscapes are half of all “landscapes.” Clouds
indicate important meteorological processes such
as convection, fronts, and mountain waves. Clouds
are the source of rain, snow, thunder, lightning, hail,
and many other weather phenomena. The Earth
from space appears to be about 50% covered by
white clouds of all shapes.
Eternal Clouds, let us appear; let us arise from the roaring
depths of Ocean, our father; let us fly towards the lofty
mountains, spread our damp wings over their forestladen summits. . . let us shake off the rainy fogs, which
hide our immortal beauty and sweep the earth from afar
with our gaze
-- Aristophanes, The Clouds
I am the daughter of Earth and Water,
And the nursling of the Sky;
I pass through the pores of the ocean and shores;
I change, but I cannot die.
-- Percy Bysshe Shelley, The Cloud
Four Basic Cloud Types
High, cloud bases above 5.5 km (18,000 ft)
High with lumps “mackerel sky”
Middle, cloud bases 2-5.5 km (7,000 - 18,000 ft)
Mid level with lumps
Low, cloud bases 0-2 km (surface to 7,000 ft)
Vertical Development, cloud bases 1 km (3500 ft) thick 10 km (30,000 ft)
Showery and stormy