CLOUD COVER AND SUNSHINE
Clouds are tiny droplets of water vapor, suspended and floating in the air. Their forms, shape, heights and movements indicate the sky and weather conditions of a particular place. On maps, places with an equal degree of cloudiness are joined by lines known as Isonephs.
Clouds are known to be caused due to rising air, which cools by expansion leading to the formation of water vapor droplets. Clouds can also be formed by temperature inversion. Cloud cover is known to:
- Aid formation of rain.
- Reflection of shortwave and radiation.
- Reduction of solar radiation.
MEASUREMENT OF CLOUD
The amount of cloud in the atmosphere is measured by Oktas. In other words, the amount of cloud in the air is expressed in Oktas or Eights of the skies cover.
CLASSIFICATION OF CLOUDS
There are four major types of clouds based on form, height and appearance. These are:
- High clouds.
- Medium clouds.
- Low clouds.
- Clouds with great vertical extent.
- High clouds: There are three sub-types of high clouds which have a height of 6000 – 12000m.
- Cirrus: It is made up of small ice crystals; white, wispy, fibrous or feathery in appearance. It is the cloud with the highest vertical height.
- Cirrocumulus: It is made up of ice crystals, looks like ripples in the mackerel sky forming thin cloud.
- Cirrostratus: This looks like a thin white milky cloud which is transparent and gives the sun and moon haloes.
- Middle clouds: There are two sub-types of this cloud with height 2100 – 6000m.
- Altocumulus: This is woolly, bumpy clouds arranged in layers which appears like waves in the blue sky.
- Altostratus: This cloud is composed of water droplets forming sheets of grey or watery looking clouds, partially or totally covering the sky.
- Low clouds: There are three sub-types of low clouds with height below 2100m.
- Stratocumulus: This cloud is formed in large globular masses bumpy-looking, soft and grey in appearance forming pronounced regular pattern. It usually indicates fair weather.
- Stratus: This cloud is fog-like low cloud, uniformly grey and thick stratus can reduce visibility of aircraft.
- Nimbostratus: This is a dark, dull cloud, clearly layered. It is also called rain cloud as it brings continuous rain, snow or sleet.
- Clouds with Great Vertical Extent: There are two sub-types of this and the height varies from 6000 – 9000m.
- Cumulus: It is a vertical cloud with rounded top and horizontal base, typical of humid tropical regions. It is associated with uprising convectional current.
- Cumulonimbus: This is a cloud of great vertical extent whose around tops are often spread out in form of anvil. It is frequently seen in tropical afternoons. It is also referred to as a ‘’thunder cloud’’ and brings convectional rain accompanied by lightning and thunder.
Sunshine is the amount of heat and the period the sun’s rays are received at a place. However, this amount of sunshine depends on the season, a factor determined by latitude and the position of the earth in its revolution around the sun.
On maps, places of equal sunshine are joined by lines called Isohels. Sunshine duration is measured by Sun-dial. Specifically, the number of hours and minutes of sunshine that a place receives each day is measured by sunshine recorder or light meter.