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Unformatted text preview: y become saturated (reach 100% RH). As it
continues to rise, condensation occurs and clouds form. The air continues to rise
and cool, but it now cools at a different rate, called the wet adiabatic lapse rate,
at 6°C per 1000 meters (actually it varies, but 6°C is a reasonable average and
we will use that value). The air is still rising and cooling as with the dry rate, but
now condensation is happening and latent heat is being converted to sensible
heat, partially offsetting the decrease. We’ll get back to these lapse rates shortly. Atmospheric Moisture:4 Clouds
We have been talking about condensation in the air and this forms clouds. What
has to happen is that relative humidity has to reach 100%, but that alone is not
enough. Water has to condense on something, so tiny particles have to be
present and the water condenses on these. These condensation nuclei are
dust, soot and salts floating around in the air. If no nuclei are present, the air can
become supersaturated, actually having more than 100% relative humidity. The
idea behind cloud seeding is that if you add condensation nuclei to
supersaturated air, then water drops will form and more rain will fall. It’s not clear
how successful this is in practice, but cloud seeding is being done here in West
Texas and elsewhere.
Clouds can be made of water drops or ice particles. Water drops can remain
liquid in clouds well below 0°C, but after about –12°C, they turn to ice. The
reason for this is that ice cannot form without ice nuclei, and these only form in
the atmosphere at –12°C and colder. Ice nuclei are tiny six-sided objects that a...
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- Fall '08