Classroom Pronunciation Reductions Grammar Conversation Reading Listening Vocabulary Activities Videos
Idioms Slang Acronyms Phonics Portmanteau Words Handwriting Alphabet Surveys Tests
Holidays Movies Everyday Environment Learning News Places Flashcards Study Literacy
World America History Drive Education Teaching Dictionary Resources About Contact
 
 
 
Cumuliform cloudscape over Swifts Creek, Australia.
Clouds

In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture (usually in the form of water vapor) from an adjacent source to raise the dew point to the ambient temperature. They are seen in the Earth's homosphere, which includes the troposphere, stratosphere, and mesosphere. Nephology is the science of clouds, which is undertaken in the cloud physics branch of meteorology. There are two methods of naming clouds in their respective layers of the homosphere, Latin and common.

Genus types in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names due to the universal adoption of Luke Howard's nomenclature that was formally proposed in 1802. It became the basis of a modern international system that divides clouds into five physical forms which can be further divided or classified into altitude levels to derive ten basic genera. The main representative cloud types for each of these forms are stratus, cirrus, stratocumulus, cumulus, and cumulonimbus. Low-level clouds do not have any altitude-related prefixes. However mid-level stratiform and stratocumuliform types are given the prefix alto- while high-level variants of these same two forms carry the prefix cirro-. Genus types with sufficient vertical extent to occupy more than one level do not carry any altitude related prefixes. They are classified formally as low- or mid-level depending on the altitude at which each initially forms, and are also more informally characterized as multi-level or vertical. Most of the ten genera derived by this method of classification can be subdivided into species and further subdivided into varieties. Very low stratiform clouds that extend down to the Earth's surface are given the common names fog and mist, but have no Latin names.
Tropospheric cloud classification by altitude of occurrence: Multi-level and vertical genus-types not limited to a single altitude level include nimbostratus, cumulonimbus, and some of the larger cumulus species.
In the stratosphere and mesosphere, clouds have common names for their main types. They may have the appearance of stratiform veils or sheets, cirriform wisps, or stratocumuliform bands or ripples. They are seen infrequently, mostly in the polar regions of Earth. Clouds have been observed in the atmospheres of other planets and moons in the Solar System and beyond. However, due to their different temperature characteristics, they are often composed of other substances such as methane, ammonia, and sulfuric acid, as well as water.

Tropospheric clouds can have a direct effect on climate change on Earth. They may reflect incoming rays from the sun which can contribute to a cooling effect where and when these clouds occur, or trap longer wave radiation that reflects back up from the Earth's surface which can cause a warming effect. The altitude, form, and thickness of the clouds are the main factors that affect the local heating or cooling of Earth and the atmosphere. Clouds that form above the troposphere are too scarce and too thin to have any influence on climate change. Clouds are the main uncertainty in climate sensitivity.

Warm air holds more water vapor than cool air. So if warm air with lots of water inside cools, it can form a cloud. These are ways air can cool enough to form clouds:
  • when air close to the ground is heated by the sun and rises to where the air is colder.
  • along weather fronts warmer air is cooled as it runs into colder air;
  • when air goes up the side of a mountain it cools as it goes higher;
  • when warm air goes over something colder such as cool water in a lake) or ground that is cooled at night it cools.
Cloud classification

Clouds are classified according to how they look and how high the base of the cloud is in the sky. This system was suggested in 1803. There are different sorts of clouds because the air where they form can be still or moving forward or up and down at different speeds. Very thick clouds with large enough water droplets can make rain or snow, and the biggest clouds can make thunder and lightning.

There are five basic families of clouds based on how they look:
  • Cirrus clouds are high and thin. The air is very cold at high levels, so these clouds are made of ice crystals instead of water droplets. Cirrus clouds are sometimes called mares' tails because they look like the tails of a horse.
  • Stratus clouds are like flat sheets. They may be low-level clouds (stratus), medium-level (altostratus), high-level (cirrostratus), or thick multi-level clouds that make rain or snow (nimbostratus).
  • Stratocumulus clouds are in the form of rolls or ripples. They may be low-level clouds (stratocumulus), medium-level (altocumulus), or high-level (cirrocumulus).
  • Cumulus clouds are puffly and small when they first form. They may grow into heap clouds that have moderate vertical extent (nothing added to the name), or become towering vertical clouds (towering cumulus).
  • Cumulonimbus clouds are very large cumulus-type clouds that usually develop cirrus tops and sometimes other features that give them their own unique look.
The following is a summary of the main cloud types arranged by how high they form:
High-Level clouds

High clouds form from 10,000 to 25,000 ft (3,000 to 8,000 m) in cold places, 16,500 to 40,000 ft (5,000 to 12,000 m) in mild regions and 20,000 to 60,000 ft (6,000 to 18,000 m) in the very hot tropics. They are too high and thin to produce rain or snow.

High-level clouds include:
  • Cirrus (Ci)
  • Cirrocumulus (Cc)
  • Cirrostratus (Cs)
Medium-level clouds

Middle clouds usually form at 6,500 ft (2,000 m) in colder areas. However, they may form as high as 25,000 ft (8,000 m) in the tropics where it's very warm all year. Middle clouds are usually made of water droplets but may also have some ice crystals. They occasionally produce rain or snow that usually evaporates before reaching the ground.

Medium-level clouds include:
  • Altocumulus (Ac)
  • Altostratus (As)
Low-level clouds

Low-level clouds are usually seen from near ground level to as high as 6,500 ft (2,000 m). Low clouds are usually made of water droplets and may occasionally produce very light rain, drizzle, or snow.

Low-level clouds include:
  • Stratocumulus (Sc)
  • Stratus (St)
When very low stratus cloud touches the ground, it is called fog.
Moderate-vertical clouds

These are clouds of medium thickness that can form anywhere from near ground level to as high as 10,000 ft (3,000 m). Medium-level cumulus does not have alto added to its name. The tops of these clouds are usually not much higher than 20,000 ft (6,000 m). Vertical clouds often create rain and snow. They are made mostly of water droplets, but when they push up through cold higher levels they may also have ice crystals.

Moderate-vertical clouds include:
  • Cumulus (Cu)
  • Nimbostratus (Ns)
Towering-vertical clouds

These clouds are very tall with tops usually higher than 20,000 ft (6,000 m). They can create heavy rain and snow showers. Cumulonimbus, the biggest clouds of all, can also produce thunderstorms. These clouds are mostly made of water droplets, but the tops of very large cumulonimbus clouds are often made mostly of ice crystals.

Towering-vertical clouds include:
  • Towering cumulus (Tcu)
  • Cumulonimbus (Cb)
Kiddle: Clouds
Wikipedia: Clouds
 
Cumulonimbus cloud seen from 38,000 feet.
Introduction to Clouds

We see clouds almost daily. But clouds are complicated and varied. In fact, the presence of a generic cloud means almost nothing without more details.

Clouds can grow very tall or appear flat as a pancake. They are typically white in color but can also be different shades of grey or brilliant yellow, orange or red. They can weigh tens of millions of tons yet float in the atmosphere.

Clouds can be harbingers of good weather or bad. Their absence can be a good thing after a flooding rain or a bad thing during a drought.

They provide relief from the heat of direct sunlight but also trap warmth, leading to higher temperatures.

Precipitation from clouds helps crops to grow, but can make driving more dangerous due to reduced visibility.

They come in infinite shapes and sizes yet we often recognize more familiar objects or animals.

Clouds can be carried along by winds of up to 150 mph (240 km/h) or can remain stationary while the wind passes through them.

They can form behind high flying aircraft or can dissipate as a plane flies through them. They are not confined to earth but are found on other planets as well.

What are clouds? They are the visible aggregate of minute particles of water and/or ice which form when water vapor condenses. Learn about clouds and how they form to become "Cloudwise".
The relative size of water molecules to condensation nuclei.
How Clouds Form

There are two ingredients needed for clouds to become visible; water, of course, and nuclei.

Nuclei

In one form or another water is always present in the atmosphere. However, water molecules in the atmosphere are too small to bond together for the formation of cloud droplets. They need a "flatter" surface, an object with a radius of at least one micrometer (one millionth of a meter) on which they can form a bond. Those objects are called nuclei.

Nuclei are minute solid and liquid particles found in abundance. They consist of such things as smoke particles from fires or volcanoes, ocean spray or tiny specks of wind-blown soil. These nuclei are hygroscopic meaning they attract water molecules.

Called "cloud condensation nuclei", these water-molecule-attracting particles are about 1/100th the size of a cloud droplet upon which water condenses.

Therefore, every cloud droplet has a speck of dirt, dust or salt crystal at its core. But, even with a condensation nuclei, the cloud droplet is essentially made up of pure water.
In an ideal atmosphere the saturation level of a parcel with a surface temperature of 85°F and a dew point of 65°F will cool to the saturation point at about 4,000 feet in elevation. Click image to change from English to metric unit.
Temperature's role

But having water attracting nuclei is not enough for a cloud to form as the air temperature needs to be below the saturation point. Called the dew point temperature, the point of saturation is where evaporation equals condensation.

Therefore, a cloud results when a block of air (called a parcel) containing water vapor has cooled below the point of saturation. Air can reach the point of saturation in a number of ways. The most common way is through lifting of air from the surface up into the atmosphere.

As a bubble of air, called a parcel, rises it moves into lower pressure since pressure decreases with height. The result is the parcel expands in size as it rises. This requires heat energy to be removed from the parcel. Called an adiabatic process, as air rises and expands it cools.

The rate at which the parcel cools with increasing elevation is called the "lapse rate". The lapse rate (the rate the temperature lapses or decreases) of unsaturated air (air with relative humidity <100%) is 5.5°F per 1000 feet (9.8°C per kilometer). Called the dry lapse rate, for each 1000 feet increase in elevation, the air temperature will decrease 5.5°F.

Once the parcel reaches saturation temperature (100% relative humidity) water vapor will condense onto the cloud condensation nuclei resulting in the formation of a cloud droplet.

But the atmosphere is in constant motion. As air rises drier air is added (entrained) into the rising parcel so both condensation and evaporation are continually occurring. So, cloud droplets are constantly forming and dissipating.

Therefore, clouds form and grow when there is more condensation on nuclei than evaporation from nuclei. Conversely, they dissipate if there is more evaporation than condensation. Thus, clouds appear and disappear as well as constantly change shape.
National Weather Service: Introduction to Clouds
National Weather Service: How Clouds Form
NWS Lesson: Smoking clouds
 
 
 
 
Search Fun Easy English
 
 
 
 
About    Contact    Copyright    Resources    Site Map