Atmospheric Moisture

As it continues to rise condensation occurs and

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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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