Introduction and terminologies
A cloud is a mass of liquid drops or frozen crystals of water or any other chemicals in the atmosphere over the surface of the earth (merriam-webster.com, erh.noaa.gov). Clouds are mainly formed as a result of vertical movement of air due to convection, forced gain in height over highlands or large scale vertical ascent due to merging of hot and cold air masses in the mid-latitudes. Clouds form and important part the water cycle, because they are the source of rain and snow, which transport water from the atmosphere to the surface of the earth. This maintains the water cycle on earth.
Cloud formation
Clouds form when water vapour in air condenses around aerosols (tiny solid particles from both manmade and natural sources, like dust, salt from sea water spray etc.) present in the atmosphere (Moran 148). These particles are also called the nuclei, around which the water vapour condenses. Generally these nuclei has a radii greater than 1.0 micron (1×10−6 m). There are two types of nuclei: cloud condensation (when water vapour condenses to form water droplets) nuclei and ice-forming nuclei (when tiny ice crystals accumulate and join to form bigger ice particles in the atmosphere). This condensation is caused by super saturation of air, adiabatic cooling of air and atmospheric instability (Ayoade 114).
Super saturation occurs when air is cooled to its dew point with its volume constant. Adiabatic cooling takes place when the volume of a rising air mass suddenly increase due to reduced pressure from the surrounding air in the higher atmosphere. Atmospheric instability arises due to temperature difference of a rising air mass with the surrounding air.
Cloud types
Clouds are classified into types based on two main criteria: shape size and appearance of a cloud, and the altitude of a cloud (Barry and Chorley 96). Based on the first critreria it’s divided into cirriform (fibrous appearance), stratiform (appear in layers) and cumuliform clouds (looks like heaps of cotton). Based on altitude above the surface of the earth, they are classified into High (6000 – 12000 m), medium (2000 – 6000 m) and low (ground level – 2000 m) clouds (see table 1 and figure 1).
Source: Ayoade, J.O. Introduction to Climatology for the Tropics. Wiley, 1983. Print.
Figure 1. Type of clouds based on altitude; Encyclopedia Britannica; britannica.com; Web; 30 Oct. 2013
Precipitation
Precipitation is the next stage of cloud formation, where water is deposited on the surface of the earth in liquid state (water: rain, dew etc.) or ice. In the global water cycle this is pathway followed by water from atmosphere to the surface of the earth. Formation of precipitation follows two main processes: the Collision-coalescence and Bergeron-Findeisen process (Barry and Chorley 100). They fall in the form of Drizzle, rain, snow, ice pellets and hail (Moran 163). Precipitation is of three main types: convection (due to condensation of water vapor in an air mass rising due to atmospheric instability), cyclonic (due to rise of an air mass after horizontal meeting of different air masses) and orographic (due to rise of an air mass due to movement over an elevated piece of land like mountains, hills etc.).
Storms
A thunderstorm is a very good example of the moisture changes and associated release of energy in an atmospheric system. They are characterized by extreme vertical movements of air, accompanied by thunder and lightning. They are caused by heated and unstable rising air mass, being triggered by atmospheric instability.
Hurricanes are string tropical storms that originate over the sea. When small warm pockets of sea are created they heat up the air above them, which rises. As the air rises it condenses to form huge clouds, and creates a pocket of low pressure. Due to the low pressure area more air from the surrounding area in drawn in, which spirals into the position of the storm. They follow an anti-clockwise motion in the Northern Hemisphere. These spiraling winds can reach a speed of 70 m/ hr. They form a huge column of cloud as high as 15 km, and can be 300 miles wide. When these reach land they can cause heavy rainfall and destruction due to high speed winds.
Works cited
Ayoade, J.O. Introduction to Climatology for the Tropics. Wiley, 1983. Print.
Barry, R., et al. Atmosphere, Weather and Climate. Taylor & Francis, 2012. Print.
Moran, J.M., M.D. Morgan, and P.M. Pauley. Meteorology: The Atmosphere and the Science of Weather. Prentice Hall, 1997. Print.
Merriam-Webster. "The Webster Dictionary". Web. 30 Oct. 2013.
National Oceanic and Atmospheric Administration. “Terms used by meteorologists, forecasters, weather observers, and in weather forecasts”. Web. 30 Oct. 2013.
