A cloud is a visible mass of condensation droplets. Water moves into the atmosphere through the process of evaporation and transpiration. Water vapors join the dust particles in the atmosphere to form clouds. Finally, water returns to Earth as precipitation in the form of rain, snow, and hail. Clouds cannot be formed without water vapors. All clouds contain water vapors. Size of the clouds depends on several factors such as seasons, mountain ranges, bodies of water, volcanic eruptions and global warming.
Clouds generally form when the invisible water vapor in the air condenses into visible water droplets. When the air meets a cold surface, it is cooled to a saturation point and clouds form. It happens when air flows up the side of a mountain and cools as it rises higher into the atmosphere. Formation of clouds also takes place when warm air blows over a colder surface.
Clouds are generally heavy in weight. A cloud may look lighter to our eyes. However, the water in a cloud can contain a weight up to millions of tons. Volume of the cloud is high, but its net density is low enough to such an extent that the airs within the cloud are capable of keeping small droplets suspended. Water droplets always re-evaporate. A typical cloud droplet has a radius of 1 x 10-5 m and a terminal velocity of about 1-2 cm/s. This gives these droplets plenty of time to re-evaporate as they fall into the warmer air under the cloud. Most water droplets are formed around a tiny particle of smoke or dust (Marsh, & Henrick, 2000).
There are three types of clouds: Cirrus Clouds, Stratus Clouds and Cumulus clouds. Clouds are classified by the cloud base height.
Cirrus includes high forms of clouds. They are formed at an altitude above 18,000 feet in the cold region of the troposphere. In Polar Regions, they may form as low as 10,000 ft. They are generally white in color. Cirrus clouds have a fibrous (hair-like) appearance. Cirrus is composed of ice crystals originate from the freezing of supercooled water droplets. They are generally arranged in groups or lines. Cirrus sometimes gives the sky a milky appearance. Cirrus generally occurs in fair weather and point in the direction of air movement. Cirrus clouds are different from Cirrocumulus because of their fibrous or silky appearance. Cirrocumulus clouds are thin, white patches, sheets or a layer of clouds. Cirrocumulus is subdivided into small cloudlets. It may include silky, fibrous or smooth portions. Cirrostratus cloud produces a halo appearance. It presents a whitish cloud structure of fibrous appearance. It never blurs the outline of the sun or moon (Houze, 1993).
Altostratus clouds generally occur between 6,500 and 18,000 ft. They are named because they are middle height clouds. They are made of supercooled water droplets. An Altostratus cloud covers the whole sky. It normally has a gray or blue-gray appearance. It is lighter in color although it is darker than Cirrostratus. It is generally formed in the areas which witness rain and snow. Altostratus is caused by a condensation of a large air mass. These clouds are dangerous, because they can cause increase of ice deposit on aircrafts (Scorer, 1972).
The word stratus comes from the Latin word that means ‘to spread out’. Stratus clouds are low height clouds, which cover the entire sky like a blanket. They are formed at an altitude of below 6500-8000 feet (Rogers, & MK, 1989). Stratus clouds are horizontal and layered and give a dense gray appearance. They are arranged in groups, lines or rolls. Generally, we can see stratus clouds when a layer of warm air passes over a layer of cool air. When these two layers meet, the warm air is cooled. If the warm air is cooled below its dew point, the water vapor condenses to form a layer of clouds, called stratus clouds. We can find such clouds over the hills and mountains. These clouds are fogs that are formed above ground level. They can be formed either through the lifting of morning fog or when cold air moves at low altitudes over a region. Stratus clouds that are accompanied by precipitation are known as Nimbostratus. Altostratus and Cirrostratus are the examples of Stratus clouds that formed at higher altitudes (Scorer, 1972).
Cumulus clouds are vertical and thick. They are usually associated with fair weather. Their horizontal base and dome-shaped surface resemble a cauliflower. They are formed in the troposphere at a lower altitude of below 8,000 feet. Cumulus clouds occur at heights of 1,640 – 19,685 feet in elevation from the earth (Houze, 1993). They often occur scattered or in dense groups. The moisture in the air condenses when air currents rise to a certain height. Cumulus clouds can be seen mostly during summer. However, they may occur any time of year. Cumulonimbus, a form of Cumulus generally produces showers of rain, snow, hailstorms, or thunderstorms.
Color of the Clouds
Color of the cloud gives us an idea about the process going inside it. Clouds form when water vapor cools and condenses. These tiny particles of water droplets are relatively dense and sunlight cannot penetrate into the cloud. Thus it gets a white color. These small droplets may be transformed into big droplets and fall on the ground in the form of rain. When the droplets become larger, sunlight penetrates deep into the cloud. Some lights are reflected back, while other lights are absorbed by the cloud. This process of reflection and absorption changes the color of the cloud to gray and then black. Such clouds cause heavy rain.
Blue clouds occur due to the scattering of light within the cloud. The droplets found in the bluish cloud are well capable of producing rains. Another color observed by the scientists is greenish. Green cloud is produced when sunlight is scattered by ice. Green cloud is often dangerous and damaging. It causes heavy rain, hailstorms, strong winds and tornados. Yellow cloud is a rare form of the clouds. It occurs due to the presence of smoke. At the time of sunrise/sunset, color of the cloud turns orange, red or pink. It occurs due to the scattering of sunlight in the atmosphere. These colors are only the reflection of sunlight and do not cause any changes in weather (Rogers, & MK, 1989).
Climatic Influence of Clouds
Clouds influence the climatic conditions to a great extent. Now-a-days, the weather forecast is somewhat based on the formation of clouds over the atmosphere. It has been noticed that increase or decrease in cloud cover can affect the amount of radiation reflected from the earth and its atmosphere into space. Clouds have become an important factor in the study of global warming.
It is often noticed that climate results from an energy balance between absorbed sunlight and radiations emitted from earth. “When clouds reflect sunlight back toward space, it reduces the solar energy available on the earth. Clouds also reduce radiating heat losses to space (greenhouse effect). These processes depend on many parameters including cloud particle composition, cloud structure, cloud cover, and cloud location. Changes to any of these parameters can have significant implications on the climate” (Houze, 1993).
The greenhouse effect is weak for low altitude clouds and they cool earth’s surface. However, cold high altitude cirrus clouds can cool or warm the climate depending on the thickness. The greenhouse effect is very strong for high altitude clouds. Due to the complex structures, it is still difficult to understand the cirrus effect on climatic conditions. The main reason behind such difficulties is the inability to calculate the scattering sunlight in cirrus clouds. The scattering of sunlight contain numerous ice crystal shapes and sizes, and are irregular in structure.
The climatic importance of cirrus clouds has lead to many experiments over the last decade. “NASA has done a through research by analyzing remote sensing data from the earth, satellites and from NASA aircrafts. The formation of natural cirrus is simulated with and without aircraft exhaust soot particles to predict the impact of commercial air traffic on the frequency of cirrus occurrence and their impact on climate. It has concluded that cloud properties and climate effects are linked to each other. It is still carrying out experiments to understand the impact of clouds on the earth’s climate. NASA is experimenting new cloud model representations to improve its infrastructure to predict cloud effects on climate” (Marsh, & Henrick, 2000).
Impact of Clouds on Weather Change
Clouds have a considerable impact on the visibility. Precipitation further impacts visibility. It is to be noted that precipitation is associated with clouds. It is very necessary to read the weather from the state of clouds as sometimes they can be dangerous. Aircrafts and Helicopters are the worst-affected by bad weather. When they move under the clouds, visibility plays a crucial role. In the absence of clear visibility, accidents may occur. Also, aircrafts flying into cumulus clouds may experience violent disturbance in the atmosphere which are often hazardous.
Weather forecasting is a traditional belief which exists for centuries. From farmers to businessmen, everyone tried to develop the skill to read the clouds to predict the weather. And they have successfully done this. Now the modern science takes the help of remote sensing data, aerial surveys and satellite images for weather predictions. But the forecast of weather lies beneath the clouds although it is not an easy task to predict the weather accurately by reading the clouds. Red clouds form when sun shines at sunrise or sunset. However, there are differences in their timing of formation. When the morning skies are red, they give a warning message that weather won’t be good. If the skies become red in the evenings, it is considered that weather will be fair and fine.
The cauliflower shape of summer cumulus clouds is an indicator of moist surface air rising quickly into cooler, drier air. When it takes the shape of towers, these towers grow into thundershowers. The long and thin cirrus clouds of all high-altitude indicate that a moisture-bearing low pressure system is moving in from the west. This usually means an increase in wind speeds and increase in precipitation. It is helpful for the sailors to move in a direct direction.
Marsh, Nigel, & Henrik Svensmark. (2000). Cosmic rays, clouds, and climate. Space Science Review.
Scorer, R.S. (1972). Clouds of the World. Stackpole Books.
Houze, R. A. Jr. (1993). Cloud Dynamics. Academic Press, 573 pp.
Rogers, R. R., & M. K. Yau. (1989). A Short Course in Cloud Physics. 3d ed. Pergamon Press, 293 pp.