HUMIDITY AND CLOUDS
Water vapor is one of the elements which constitute the atmosphere.
Humidity refers to the amount of water vapor present in the atmosphere at a particular time and place.
The term ‘evaporation’ is used to explain the process of conversion of water into water vapor. Sometimes the ice instead of getting changes into the state of water directly converted into vapor. This process is called ‘sublimation’.
At the present level of global temperature, only one percent of the total amount of water on the earth’s surface can exist in the form of water vapor.
The amount of water vapor constantly varies from 0 to 4 percent of the total volume of atmospheric gases.
The total amount of water vapor of the atmosphere is almost equal to the total amount of rainfall for 10 days.
Humidity and Temperature:
There is direct relationship between the temperature of air and the amount of water vapor it can withhold. With increasing of temperature the capacity of the air to withhold moisture also increases and vice-versa.
In case the temperature of the air decrease, the Humidity of the air is precipitated in the form of dew, mist etc. This process is called ‘Condensation’.
The amount of atmospheric Humidity can be expressed in three types namely
1. Absolute Humidity
The actual amount of water vapor present in the air is Absolute humidity.
It is expressed in terms of grains or grams.
The actual amount of humidity present in cubic feet of air is expressed in grains.
Similarly, the total water vapor present in a mass of one cubic meter is expressed in terms of grams.
Absolute Humidity is highest near the equator and decreases polewards. Similarly, it is highest near the surface and decreases as one go higher up. At the same time, Absolute humidity is more near the coasts and decreases towards the interior lands.
2. Relative Humidity
It is the ratio between actual water vapor present and the maximum capacity of the air to hold moisture at a particular level of temperature.
It is expressed in terms of percentage.
The relative humidity is constantly changing with the variation of temperature. In fact relative humidity determines the amount of evaporation. If the relative humidity is low, it results in more evaporation and vice-versa.
3. Specific Humidity
It is the amount of water vapor present in one kilogram of air.
It is measured in terms of grams.
It is highest of about 18 grams in the humid tropics and lowest of 0.2 grams near the poles.
specific humidity thus varies from one place to another depending on temperature.
The line joining the places of an equal amount of water vapor is called ‘Isohygric’.
The amount of atmospheric humidity is measured by an instrument called a ‘Hygrometer’.
Clouds are droplets of water or tiny ice crystals which collect around the dust particles present in the atmosphere.
The water droplets and tiny ice crystals that remain suspended in the air can be disturbed by the slightest movement of the air.
They are masses that consist of high density and volume and hence it is visible to naked eyes.
All forms of precipitation occur from the clouds.
They play different roles in the climate system like being the bright objects in the visible part of the solar spectrum, they efficiently reflect light to space and thereby helps in the cooling of the planet.
Clouds are formed when the air becomes saturated or filled, with water vapor. The warm air holds more water vapor than cold air.
Clouds differs in height, shape, color transparency etc.
These are found from a few meters of height from the ground level extended up to 13 Km.
Most of clouds forms are confined to the troposphere.
Types of Clouds:
Luke Howard of Britain in his article ‘On Modification of Clouds’ in 1803 has considered four types of clouds. These are
1. CIRRUS CLOUDS
Cirrus clouds are formed at high altitudes (8,000 – 12,000m). They are thin and detached clouds having a feathery appearance. They are always white in color.
2. cumulus clouds
Cumulus clouds look like cotton wool. They are generally formed at a height of 4,000 – 7,000 m. They exist in patches and can be seen scattered here and there. They have a flat base.
3. STRATUS CLOUDS
As their name implies, these are layered clouds covering large portions of the sky. These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.
4. NIMBUS CLOUDS
Nimbus clouds are black or dark gray. They form at middle levels or very near to the surface of the earth. These are extremely dense and opaque to the rays of the sun. Sometimes, the clouds are so low that they seem to touch the ground. Nimbus clouds are shapeless masses of thick vapour.
The World Meteorological Organization presented a detailed International Atlas of Clouds mentioning three main groups and ten main types of clouds.
5. HIGH CLOUDS
They can reach above 6000 meters or 20,000 feet.
They are also known as Cirrus Clouds.
They are usually thin and are made up of ice.
They often indicate fair weather and hence do not produce rain.
Types of High Clouds
1. Cirrus (Ci)
They are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet (6,000 meters), they are composed of ice crystals that originate from the freezing of supercooled water droplets.
2. Cirrostratus (Cs)
They are high, very thin, comprise a uniform layer, and are composed of ice crystals. It is difficult to detect and is capable of forming halos when the cloud takes the form of thin cirrostratus nebulosus.
3. Cirrocumulus (Cc)
They are small rounded puffs-shaped clouds, that usually appear in long rows high in the sky and are usually white, but sometimes appear grey.
6. MIDDLE CLOUDS
They form between 6,500 feet and cirrus level or from 2000 to 6000 meters.
They are also known as “Alto” clouds.
They frequently indicate an approaching storm.
They may sometimes produce Vigra.
Sometimes the raindrops evaporate before reaching ground. This phenomenon is called Vigra.
Types of Middle Clouds:
These clouds are in the form of continuous sheets or veils, grey or blue-gray in color. They are composed of ice crystals and water droplets. In its thinner areas, the sun can still be visible as a round, dim disk. These clouds may often form ahead of storms with continuous rain or snow.
They are greyish sheet cloud, characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus.
7. LOW CLOUDS
They lie below 6,500 feet, which means from the surface to 2,000 meters.
Low clouds are also known as Stratus Clouds.
They may appear dense, dark, and rainy (or snowy) and can also be cottony white clumps interspersed with blue sky.
Types of Low Clouds:
1. Strato Cumulus
Usually arranged in a large dark, rounded or globular mass, usually in groups, lines, or waves.
Usually looks like a huge grey blanket that hangs low in the sky that resembles fog, comprises a uniform layer and appears dull, if these clouds are warm it means rain and if it is cold it snows.
They are known as ‘Rain Clouds’ and they are dark, thick and accompanied by light to moderately falling precipitation.
8. GREAT VERTICAL EXTENT CLOUDS
They are most dramatic types of clouds.
Great Vertical Extent Clouds are also known as the Storm Clouds.
They rise to dramatic heights, and sometimes well above the level of transcontinental jetliner flights