Introduction to Experiment 4: Clouds Week of 2 May, 2016

Clouds and Climate Clouds play an important role in climate. They reflect sunlight back to space (cooling impact on climate). And they absorb infrared radiation from below and radiate it downwards and upwards (warming impact on climate). Low clouds are thought to cool climate and higher ice clouds are thought to warm climate. On balance, cooling is thought to win out. Uncertainties about how clouds will change in a warming climate result in uncertain predictions of future climates.

Clouds Formation and Modification Clouds result when air is cooled enough to reach the dew point. Dew point: The temperature at which water condenses from vapor to liquid is the dew point temperature. It depends upon the amount of water vapor in the air. The when the air is saturated (100% Relative Humidity), the dew point temperature equals the air temperature. If the air is less humid than that, the dew point temperature is lower than the air temperature.

Clouds Formation and Modification Clouds result when air is cooled enough to reach the dew point. As air is cooled to its dew point, dew will occur. That is condensation will take place. When that condensation occurs on a surface, you will see water droplets on that surface (dew). When the condensation occurs on particles in the air, you will see fog or clouds. ndbnano.com water.usgs.gov

Clouds Formation and Modification Clouds result when air is cooled enough to reach the dew point. At temperatures below the dew point, water vapor condenses on particles in the air (cloud condensation nuclei-ccn). These particles can be natural or human-caused. The resulting droplets are called cloud particles or cloud droplets and are much larger than the ccn on which they were formed. Particles in the air (aerosol) are ubiquitous. Near the Earth’s surface there are often thousands of particles per cubic centimeter (cc). After a diesel engine passes, concentrations can rise to hundreds of thousands of particles per cc. In the stratosphere, the number can fall to 10 cc-1 (per cc). Changing the number and nature of these particles can change the nature of clouds and precipitation.

In this experiment, you will: Observe what happens to temperature when air is compressed and when it expands. This will help you understand what happens when an air parcel near the surface is heated and rises. It then cools. Observe what happens to the number and size of cloud droplets as the number of cloud condensation nuclei is increased and decreased. This will help you understand ways in which the albedo can be changed by changing the number of particles in the atmosphere.

Cloud Formation Clouds result when air is cooled enough to reach the dew point. Usually air is cooled by moving it upward. As the air moves upward, it expands and cools. It expands because pressure is less at higher altitudes. We have experienced cooling because as we go up mountains, we experience cooler air. Air can also cool by radiation or contact with a cold surface. Ground fogs sometimes form when cloud-free air allows the infrared emitted by the ground to escape to space and cool the low lying air. When air is cooled to the dew point, water vapor often condenses on particles to form fogs. When air moves down it is compressed and warms. Clouds do not form in this situation.

Impact of CCN on Cloud Formation When air is cooled enough to reach the dew point, water vapor condenses on particles in the air and form droplets. If there are more particles in the air that can serve as CCN (right composition and size), then there will be more cloud droplets. The available water will be spread out over more drops and there will be smaller droplets. These clouds tend to be brighter and to reflect more light back to space. Aerosol particles can have more impacts on clouds and climate. (http://earthobservatory.nasa.gov/Features/Aerosols/page4.php)

Physical processes in our clouds-in-a-bottle experiment. The water and water-soaked wick in the bottom of the bottle keep the air in the bottle humid. When the air is compressed, it warms and more water evaporates into the warmer air. (The air warms more than is shown by our thermocouple since it is slow to warm and is susceptible to radiation errors.) When the air expands, it cools (It cools more than is shown by our thermocouple). The water vapor becomes supersaturated. Water vapor condenses on particles in the air forming droplets we can see. If there are more particles, we get more droplets. In time the clouds in the bottle go away. To explain this properly, we would have to do a better job measuring the temperature of the air. So for the moment, we will ignore this.

Physical processes in our clouds-in-a-bottle experiment. When we allow the flame to burn in the bottle, we are introducing millions of invisible particles into the bottle. They are too small to see. When we pass air through a filter, we remove the particles. So when we inject filtered air into the bottle, we have put in particle-free air and some particle-laden air is forced out of the bottle. So putting filtered air into the bottle reduces the number of aerosol particles in the bottle.

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