Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman have various types with different appearances, and some are simply a sight to behold. The question is, how are these types of clouds formed? Firstly, we have to know how clouds are formed. The water cycle explains that the water in water bodies such as lakes and oceans evaporates and turns into water vapour, which then rises up to the sky and condenses on the Earth's atmosphere due to lower temperatures. This is where clouds begin to form.

However, clouds need three things in order to form: Moisture, cooling air and condensation nuclei. Moisture is caused by water vapour, so there must be enough moisture in the air for condensation to occur. The temperature of the air should also be low enough so that the water vapour is able to condense. Condensation nuclei are the particles that are needed for the condensation process to occur, as the water vapour will need a solid medium for the water vapour to condense on. Some examples include dust, dirt and pollutants such as smoke particles.

Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman Perhaps the most interesting type of is the cloud, as it had just been discovered quite recently. It was added to the International in March 2017, and it is the first cloud formation added to the International Cloud Atlas since 1951.

The wavy appearance of asperitas clouds may be caused by atmospheric gravity waves propagating through the cloud. This happens when air in the atmosphere is displaced upward, causing gravity to pull it back down as gravity tries to restore equilibrium. It may be caused by meteorological disturbances such as thunderstorms, or cumulous convection within the troposphere (the lowest part of the atmosphere), which may lead to the vertical displacement of the air particles. As a result, it produces waves known as atmospheric gravity waves.

Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman Clouds can be classified into three categories of altitude: High-level, mid-level and low-level. High level clouds are clouds that are 16,500 - 45,000 feet above the ground. These include Cirrus, Cirrostratus and Cirrocumulus clouds. Mid-level clouds are 6,500 - 23,000 feet above the ground, and these include Altocumulus, Altostratus and Nimbostratus clouds. Lastly, we have low clouds which are less than 6,500 feet above the ground. Cumulus, Stratus, Cumulonimbus and Stratocumulus clouds fall under this category.

Cirrus: Characterised by its wispy and detached appearance. Made up of 100% ice crystals. They are formed when water vapour undergoes deposition into ice at higher altitudes where the atmospheric pressure is around 0.6 bar (or 60,000 Pa), and these conditions are usually present at the leading edge of a warm front (a density discontinuity at the front of a warm air mass)

Cirrostratus: Has characteristics of cirrus and stratus clouds. This is formed when the formation of cirrus clouds increases the temperature due to heat released as the water vapour freezes, usually by around 10 degrees celsius, which causes the filaments of the cirrus clouds to extend and form a sheet-like cloud which is the .

Cirrocumulus: Has characteristics of cirrus and cumulus clouds. They are made up of ice and ‘supercooled water’ which is water that remains a liquid even at temperatures below 0 degrees celsius due to its inability to undergo nucleation which is the process where water molecules begin to move closer to one another as it freezes. These clouds result when convection at high altitudes occurs. Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman Altocumulus: Cumulus clouds that have been carried into mid-level altitudes. (“Alto” in Latin means “tall” or “high”, but here it is used to describe clouds in mid-level altitudes) and are mainly made of water droplets, though ice crystals may form as they go up. They are normally found between warm and cold fronts.

Altostratus: Stratus clouds that have been carried into mid-level altitudes. They appear gray or blue-gray, and compose of ice crystals and water droplets. It is formed by the lifting of a large stable air mass that causes water vapour to condense. Altostratus clouds often form ahead of a warm front. These clouds can gradually thicken into a rain cloud (this is known as nimbostratus)

Nimbostratus: Stratus clouds that produce rain (The Latin word “Nimbus” indicates rain clouds) They are usually extremely dense, and occur along a warm front, where the rising warm air mass creates nimbostratus clouds. Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman Cumulus: Clouds that appear big and puffy. These clouds typically are in lower altitudes, but can go up into higher altitudes (altocumulus, cirrocumulus) and are the results of convection of bubbles of buoyant air. When the air rises, the temperature falls which causes the relative humidity to rise, releasing latent heat which warms the air further and causes further convection. During this phase, the water vapour condenses on various nuclei in the air, forming the puffy shape of the cloud.

Stratus: Flat, sheet-like clouds. Like the , it is typically formed in lower altitudes but can go up into higher altitudes (altostratus and cirrostratus clouds), and are formed when a sheet of water vapour condenses in the lower atmosphere. They can compose of water droplets, supercooled water droplets or ice crystals, depending on their temperature. Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman Cumulonimbus: Cumulus clouds that produce rain. This cloud can actually occupy low, mid and high levels at once. These clouds form when water droplets coalesce onto various nuclei in the air around it, causing surface tension and increasing the pressure on the droplets, and this pressure causes the droplets to evaporate. The water vapour produced then condenses on the larger droplets. When the larger droplets grow to around 20 to 30 micrometers, the droplets begin to fall as rain.

Stratocumulus: Has combined characteristics of stratus and cumulus clouds, forming a puffy sheet of clouds. These clouds are formed when the convective currents which normally carry the cumulus clouds are weaker. These currents create shallow cloud layers due to drier and stable air above, which create the as it prevents further vertical development. Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman In addition to the above, there are also some special types of clouds which are more uncommon. These are , Mammatus, Orographic and Lenticular clouds.

Contrails: Line-shaped clouds made by aircrafts. The exhaust produces condensation nuclei for the water vapour to condense onto, causing the lines of clouds to form in the shape of the plane’s trajectory.

Mammatus: Pouch-like clouds which hang below the base of a cloud. They are formed when the anvil cloud layer moves downwards into the air below. Evaporation of the cloud droplets cools down the air and makes it descend even more, and some of the cloud might get carried downwards with the air which causes the pouch-like shapes to form. Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman Orographic: Clouds that are shaped by the terrain below it, such as mountains and hills. They develop as a result of the lifting of air due to the earth’s topography. Water vapour in the air will condense into a cloud right above the mountain, making the shape of the cloud similar to the mountain below it.

Lenticular: Disc-shaped clouds which form when something disturbs the airflow such as mountains. This causes a circular movement of air, known as an eddy. When warm air gets trapped in the eddy, the vapour can condense to form a visible cloud, leading to the formation of the saucer-shaped clouds.

Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman Sources: 1. https://www.usgs.gov/special-topic/water-science-school/science/fundamental s-water-cycle?qt-science_center_objects=0#qt-science_center_objects 2. https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-are- clouds-58.html 3. https://scijinks.gov/clouds 4. https://www.weather.gov/jetstream/basicten 5. https://www.southernliving.com/culture/types-of-clouds 6. https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/cl ouds/high-clouds/cirrocumulus 7. https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/cl ouds/high-clouds/cirrus 8. https://www.youtube.com/watch?v=B9LyP0C2rsQ 9. https://www.metlink.org/other-weather/science-in-the-sky/cumulus-clouds 10. https://www.pinterest.com/pin/205195326741691058/ 11. https://www.forbes.com/sites/marshallshepherd/2016/06/18/beautiful-and-scar y-at-once-the-science-of-mammatus-clouds/#2f240ff32aa3 12. https://www.brockmann-consult.de/CloudStructures/orographic-clouds-descri ption.htm 13. https://www.candac.ca/candac/Outreach/CANDACcollaboration/index.php/en/t eacher-resources/activities-and-lesson-plans/100-cloud-in-a-jar 14. https://idahoaskascientist.com/2018/04/04/we-are-currently-studying-clouds-an d-rain-we-were-wondering-what-percentage-of-a-cloud-is-actually-water-and-w hat-percentage-of-a-rain-cloud-is-dust-and-or-other-things/ 15. https://en.wikipedia.org/wiki/Cirrus_cloud 16. https://www.skybrary.aero/index.php/Supercooled_Water_Droplets 17. https://en.wikipedia.org/wiki/Nimbostratus_cloud 18. https://en.wikipedia.org/wiki/Stratocumulus_cloud 19. https://en.wikipedia.org/wiki/Stratus_cloud 20. https://en.wikipedia.org/wiki/Altocumulus_cloud 21. https://en.wikipedia.org/wiki/Asperitas_(cloud) 22. https://eos.org/articles/science-explains-rough-and-chaotic-cloud-feature 23. https://eos.org/research-spotlights/global-atmospheric-model-simulates-fine-d etails-of-gravity-waves 24. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3923105/

Written by Timothy Design by Jennifer Lim and Rebecca Sopacua Edited by Michael Suherman