Last updated on July 13th, 2020 at 10:05 pm
“Just look up at them, refer to your past,
An observation gets you a valid forecast.”
In the previous article, we had stressed on the fact that the clouds make it difficult to account for all those local variations that make it challenging to have a particularly accurate forecast pretty distant into the future. But if one keeps an eye on the clouds that cover the sky, one can very accurately predict the local weather for at least the next 12 hours. And this is kind of interesting because the very cause for difficulty in predictions is helping to determine definitive forecast just by observation.
Though climate change issue is a global phenomenon and we are tempted to understand the global distribution and variation of cloud cover, we should keep in mind that clouds, as they depend on local convective wind currents which vary a lot, keep changing their shape and occurrence more frequently at local scales. We have different types of clouds as cloud formation takes place at different heights and temperatures keeping in mind that collision of air masses similar in temperature and moisture content is a prime necessity. Thus, instead of the global averages, the characters of clouds which vary with location, time of day and circulations are far more interesting and more relevant to the weather forecast.
Global Distribution and Character of Clouds
Clouds typically cover almost two-thirds of the planet. Clouds on average are about 270C colder than the surface. Oceans are significantly cloudier than continents. Slightly more than 70 percent of the sky over oceans is cloudy, but a little less than 60% of the total land area is usually covered with clouds. The reason being excessive water vapor available above oceans and larger time scale required to heat up the surface of oceans (large specific heat) to give rise to convective currents to driveway the clouds formed.
The tops of ocean clouds are generally slightly more than a kilometer lower than the tops of clouds over land, but ocean clouds reflect about 3% more sunlight on average than clouds over land. Above the oceans at low latitudes, clouds are more common in the morning than in the afternoon and the morning clouds are the most reflective of the day. Overland there are more clouds, with higher reflectivity, in the afternoon.
The cloudiest regions are tropics and the temperate mid-latitude storm zones; the subtropics and the polar regions have 10-20% less cloud cover. The non-uniform heating that earth experiences where the sun heats up the equator and the heating gradually decreases as we move towards the poles is the cause of these observations. Tropical cloud tops are substantially higher. High-latitude clouds are almost twice as reflective as most clouds at lower latitudes. In the tropics exceptionally large thunderheads often form, extending from the surface to an altitude of between twelve and fifteen kilometers owing to numerous and large-scale variations in local balance very often.
Types of clouds
Classifying types of clouds on the basis of altitude at which they occur in the atmosphere makes sense as the convective currents help bring two air masses of similar temperature and moisture content to help form clouds that will eventually determine their shape, characteristics, pattern and reason of occurrence.
Thus, we have these 4 basic types of clouds Cirrus (curls), Stratus (sheet), Cumulus (heaps) & Vertical displacement (towers) which are subdivided further as per their appearance in altitude:
- low-level clouds that lie below 2,000 m and mostly associated with drizzle, fog, and mist.
- middle clouds that form between 2,000 and 6,000 m and are associated with rain, irisation, and corona.
- high-level clouds that form above 6,000 m and are composed of ice crystals as at these altitudes the temperatures are freezing.
- Clouds of great vertical displacement which towers across the whole altitude and give rise to storms.
Illustration: Types of clouds and their occurrence, Credits: NWSAlbuquerque
How does each of them look & What to expect when you see them?
Cumulus is small, piled up with rounded, puffy tops with a brilliant white when sunlit, while their bottoms are flat and relatively dark. They are sharply outlined and stand out in low altitudes. Nicknamed as “fair weather” cumulus, it develops on clear, sunny days when the sun heats the ground directly below. It appears in the late morning, grows, then disappears towards evening.
Stratus hang low in the sky as a flat, featureless, uniform sheet of greyish cloud. Stratus are seen on dreary overcast days and are associated with light mist or drizzle.
Stratocumulus occurs in patches and has a dark-greyish appearance. They form when there’s weak convection in the atmosphere and have a wide variety of shapes.
Pic. 1. Stratocumulus completely fills the sky with Stratus interspersing on the right and Cumulus on the left-most side of the picture.
Altocumulus is recognized by white or grey patches that dot the sky in large rounded masses or are aligned in parallel bands. They are very often confused with Stratocumulus but we should remember that they belong to the middle atmosphere and the latter to the lower atmosphere. They are often spotted on warm and humid mornings, especially during summer. They can signal thunderstorms to come later in the day. These clouds are responsible for irisation.
Nimbostratus cover the sky in a dark grey layer. They can extend from the low and middle layers of the atmosphere and are thick enough to blot out the sun. They are the quintessential rain shower cloud. You’ll see it whenever steady rain is falling.
They appear as grey or bluish-grey sheets of cloud that partially or totally cover the sky at mid-levels. Not enough light shines through them, to cast shadows on the ground. They tend to form ahead of a warm front.
Pic. 2. Nimbostratus is clearly visible on the middle-left side accompanied by stratus and cumulus just above the sky-line. Cumulonimbus is clearly spans in the right above them and Altostratus is seen on the top-left of the picture.
Cirrus is thin, white, wispy, curly strands of clouds that streak across the sky. Because cirrus clouds occur at an altitude where low temperatures and low water vapor exist it is made up of tiny ice crystals rather than water droplets. Cirrus typically occurs in fair weather but can also accompany thunderstorms.
Cirrocumulus clouds are small, white patches of clouds often arranged in rows that live at high altitudes and are made of ice crystals. They are called “cloudlets”. The individual cloud mounds of cirrocumulus are much smaller than that of altocumulus and stratocumulus. Cirrocumulus clouds are rare and relatively short-lived.
Cirrostratus clouds are transparent, whitish clouds that cover the entire sky. A dead giveaway to distinguishing cirrostratus is to look for a halo around the sun or moon. Cirrostratus indicates a large amount of moisture is present in the upper atmosphere. They’re also generally associated with approaching warm fronts.
Pic. 3. Cirrocumulus is visible in the middle-right part of the picture. The top portion is covered with Cirrostratus interspersed with some cirrocumulus.
Cumulonimbus clouds span the low, middle, and high layers. They grow from cumulus and rise into towers with bulging upper portions that look like cauliflower. They are called thunderheads, because torrential rain, vivid lightning, and thunder come from it. The tops of such clouds may reach up to 60,000 feet or more into the sky. Ice crystals become sheared off and are carried away by strong winds aloft forming a flattened shield of cirrus that spread out in the shape of an anvil. So, if you see one you can be sure there’s a nearby threat of severe weather.
Pic. 4. The picture shows the initial phase of the formation of a Cumulonimbus from Cumulus. The presence of Nimbostratus on the left provides evidence of rain in a few hours. At high altitudes, we can see some cirrus. Clearly, a thunderstorm is coming.
We also have some more type of clouds whose appearance is rare but have very interesting features:
- Condensation trails (Aviaticus) – Contrails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruise altitudes several miles above the Earth’s surface. Contrails are composed primarily of water, in the form of ice crystals.
- Noctilucent or night-shining clouds – Noctilucent clouds are tenuous cloud-like phenomena in the upper atmosphere of Earth. They are the highest clouds in Earth’s atmosphere, located in the mesosphere at altitudes of around 76 to 85 km. They consist of ice crystals and are only visible during astronomical twilight. They are often confused with northern lights.
The observations make sense and are valid. Pictures 1,4 & 2 were taken on 15th September at 5 pm, 3 pm & 7 pm respectively and each of them suggested a potential thunderstorm which is consistent with our real-life observations. Just by seeing the various types of clouds that span the sky, if I was able to predict the weather of Mumbai then any interested person also can do the same.
Thus, knowing about what type of clouds are present in the sky comes pretty handy to forecast accurately the local weather for a minimum of 12 hours.
As an exercise for applying what you have learned now let me leave you guys with one more picture for self-forecast. Leave us with your answers in the comments section. We will provide the answer very soon.
All pictures are my own. Courtesy: Prabhu Prasad Swain
“There are no rules of architecture for a castle of dreams in clouds.”
“This is the last article named “Clarity in Judgement” in the series of two articles named “Clouds in Judgement“. Hope you guys had a good experience and learned something interesting from this effort. Don’t forget to give comments and feedback.”
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