Understanding Aerosols, Cloud Formation, And Precipitation
Precipitation and evaporation are a process by which moisture is transferred from clouds to the earth’s atmosphere. Normally, warm air rushing westward towards the poles pushes warm water upwelling of deep cold water toward the equator, causing cool upwelling over the colder ocean water in the west. The air then continues descending steadily over warm surface water, while the precipitation on the ground responds through rainfall. The process continues indefinitely, although the precipitation will eventually move into the drainage systems and be taken to the runoff channels. Thus, we have cloud and rainfall cycles.
Precipitation and Evaporation – the Processes – Cloudy conditions are the result of warm clouds forming over warmer than average temperatures. Over a shorter period of time, cloud cover can persist for days or even weeks. As a rule, cloud cover is proportional to relative humidity. In other words, the larger the percentage of relative humidity, the drier it is because cloud droplets have lower temperatures than clouds.
Microphysical Processes and Mechanisms – Precipitation and Evaporation. The cloudiness is caused by precipitation falling to the ground. Droplets condense into droplets as a result of their attraction to cooler nearby air. As they condense, droplets become light and fall to the earth’s surface, where they further lose their moisture as they become heavier and fall again. The process repeats itself continually throughout the Earth’s atmospheric cycle.
As warm air rises to the top of the atmosphere to replace cold air, warm air also reaches the clouds. The warm air helps to reduce the amount of moisture in the atmosphere. When the warm air reaches the cloud, the water droplets condense and become ice crystals. When this process repeats itself, water droplets form into clouds and rain.
Cumulus Clouds – The Original Form. This is where all the rain is usually found on Earth because the original form of cirrus clouds (where the cloud is formed at the top of the atmosphere) is the only place where the clouds can exist. Because of this, cumulus clouds are often visible to the Earth as dark forms of rain. Cowl clouds are normally found in the winter when the warmest air is not available.
Formation of Clouds and Rain – The Climate. A warm climate encourages the development of more clouds, whether or not there is a real threat of rain. Cloud types depend on the climate, whether it is tropical or arid, so a heavy shower will not always mean a heavy cloud. For example, heavy showers may occur on a hot and humid summer day when there is no chance for cloud formation.
Global Warming. A rise in atmospheric pressure, which is caused by the rise in sea surface temperature can increase the chances of the formation of clouds and rain. As temperatures rise, clouds tend to develop at lower altitudes where cooler air can hold them and moisture-laden air can fall as precipitation. A positive feedback cycle is at work here, causing a decrease in rainfall as temperatures rise forcing evaporation to take place, thereby creating higher concentrations of humidity.
Understanding aerosol effects on cloud formation is important in understanding the recent changes in precipitation. Increasing global temperatures and reduced humidity along with an increase in cloud types occur due to global warming. A decrease in solar heating also has had a favorable effect, increasing the warm phase in the spring and late summer and decreasing the cool phase in the fall and winter. Understanding the physical process involved and the coupling between climate and microphysical processes is essential in understanding how to change precipitation.