The sky is always blue at the corner of West 72nd Street and Central Park West in New York City. That is because of “Sky”, a series of mosaic murals installed on the walls of that subway station by artist Yoko Ono.
Collectively spanning 973 square feet, the six murals create views of bright blue skies dotted with white, puffy, fair-weather clouds. Messages of hope, such as “love”, “dreams”, and “yes”, are also scattered across the subterranean skyscapes.
The murals, which are based on photographs of the sky, were designed to look slightly different from different angles. As such, they appear to mimic the movement of real clouds.
This collection of mosaics is part of the public art program, MTA Arts and Design. It was added to the station in 2018 as part of a renovation project.
Yoko Ono’s “Sky” at the W 72 subway station. Photo Credit: Melissa Fleming
For most people in the US, the month of June is associated with warm temperatures and abundant sunshine. For parts of coastal California, however, it is a month known for cloudy and relatively cool conditions. This regional phenomenon called “June Gloom” is the result of the interaction of several natural elements, including geography, ocean currents, and weather patterns.
With the California Current running south along the coast from the Gulf of Alaska, the water in the area is cold. Ocean temperatures in the region usually hover in the upper 50s to low 60s during the summer, cooling the air that flows over it.
Another significant factor is the temperature inversion aloft created by the North Pacific High, a semi-permanent area of high pressure. This is part of a larger planetary circulation of air known as a Hadley cell, a current of high altitude air traveling poleward from the tropics. As the air cools, it descends around 30N latitude. It compresses and warms as it sinks, making the air aloft warmer than the cold, moist air at the surface. Since air temperatures normally decrease with height, this situation acts like a cap on the cool air below and prevents it from rising any higher.
When the air under the inversion layer, known as the marine layer, is cooled to the point where the moisture condenses, an expansive sheet of low level stratus clouds form. The region’s prevailing westerly winds, as well as the sea-breeze circulation that often develops during the summer months, carries these clouds inland. While they create overcast conditions and some light drizzle, the clouds do not produce any significant rain. They also tend to dissipate by the afternoon as the land heats up.
The thickness and inland extent of the marine layer clouds depend on the strength of the high-pressure system. A stronger high will thin the clouds and keep them confined to the coast. A weaker high with allow the clouds to thicken and move further inland. Separated by only a few miles, the cloud-covered coast can be significantly cooler than sunny areas further east.
These conditions are most common in June, but are not necessarily limited to the month. They have been known to develop in May and last on and off through August. The monikers for these events include “May Gray”, “No Sky July”, and “Fogust”. However, high pressure usually builds over southern California in July, decreasing the impact of the marine layer or eliminating it altogether.
“June Gloom” clouds along west coast. Credit: NWS/UCSD
First published in 1896, the atlas is considered the most authoritative and comprehensive reference volume for identifying clouds. This is the first time it has been updated in thirty years.
Clouds, like flora and fauna, have an official classification system. There are ten different genera, which are defined by altitude and appearance. These are then subdivided into species based on a cloud’s shape and structure. Within these, there are varieties that describe the arrangement and transparency of different clouds. Whittling things down even further, there are also supplementary features/accessory clouds that merge with or attach to the main cloud body. In total, there are about one hundred combinations.
This new version of the Cloud Atlas recognizes one new species called volutus, but it is more commonly known as a roll cloud. This tube-shaped cloud appears to roll around a horizontal axis and is typically associated with the leading edge of a thunderstorm. But, on occasion, advancing cold fronts can also trigger their formation.
Six new supplementary/accessory features were also added. For avid sky-watchers, however, they are already widely known by their common names. These include:
Furthermore, five new “special clouds” were also part of the update. These form because of unique localized factors, including human activity such as exhaust from jet engines.
Of all these new additions, the asperitas (formerly known as undulatus asperatus) has garnered the most attention. These low-level clouds are caused by weather fronts that create rolling waves in the atmosphere and resemble the underside of a turbulent sea. It was first photographed in 2006 by a cloud-watcher in Iowa. Then in 2008, after several other sightings around the world, the Cloud Appreciation Society, an international group of cloud enthusiasts, began to lobby the WMO to acknowledge it as a new cloud type.
Available in digitized form for the first time, the WMO hopes this new edition of the International Cloud Atlas will help to increase public understanding of the critical role clouds play in the atmosphere. “If we want to forecast weather we have to understand clouds. If we want to model the climate system, we have to understand clouds. And if we want to predict the availability of water resources, we have to understand clouds”, says WMO Secretary-General Petteri Taalas.
The WMO cloud classification system can be traced back to Luke Howard, the so-called father of meteorology. In 1803, he published “The Essay on the Modifications of Clouds” which organized its then nebulous subject using a Latin nomenclature.
Fueled by drought, wildfires have been blazing across the American West all summer. Sixteen are currently burning in California alone. While hiking in Kings Canyon National Park in the state’s rugged Sierra Nevada Mountains recently, I crossed paths with the “Rough Fire” and saw it produce a billowing pyrocumulus cloud.
Pyrocumulus clouds form when intense heat at the surface – usually from a wildfire or volcanic eruption – causes air to rise rapidly. As it travels upward, water vapor in the air condenses into droplets and forms a cloud. Filled with ash and smoke, the swelling cloud generally appears more grey than white.
Ignited by lightning over three weeks ago, the Rough Fire continues to spread and has even caused parts of Kings Canyon National Park to close. According to the NPS, smoke from the massive fire has also impacted the air quality in and around the park. To date, the fire has charred close to 50,000 acres and is only 17% contained.
Pyrocumulus cloud rising over California’s Rough Fire in Sierra National Forest and Kings Canyon National Park, August 2015. Credit: The Weather Gamut.
There may be a new variety of cloud floating in the skies overhead – the undulatus asperatus.
According to the Cloud Appreciation Society (CAS) – an international group of weather enthusiasts – this new cloud resembles an agitated wave that covers most of the sky. First photographed in 2006 by a cloud-watcher in Iowa, it has since been spotted several times around the globe. The CAS is now lobbying the World Meteorological Organization (WMO) to acknowledge the undulatus asperatus as an official new variety of cloud.
There are currently ten cloud types officially recognized by the WMO. Most of these have a number of sub-varieties – specific Latin adjectives to describe their formation. If the undulatus asperatus is accepted as a new variety, it will be the first new cloud added to the WMO’s International Cloud Atlas since 1951.
While traveling among the Hawaiian Islands, I had the opportunity to visit Haleakalā National Park. Ascending its volcanic slopes, I was struck by its summit region known as “kua mauna”, the land above the clouds. Its unique view is made possible by an elevated temperature inversion.
In the troposphere, the weather layer of our atmosphere, air temperature usually decreases with height. An inversion occurs when something causes that situation to reverse and allows air temperature to increase with height.
At Haleakalā , the inversion is caused by a large-scale subsidence in the Trade Winds. Blowing from centers of high pressure across the Pacific, cool, dense air aloft is warmed by compression as it descends to lower altitudes. In opposition, solar heating warms air near the surface allowing it to rise and cool, forming clouds. When these cool clouds meet the warmer air above them, an inversion layer is formed.
The inversion layer acts like a cap for cloud convection. Therefore, the summit of Haleakalā (10,023 feet), rising above the inversion altitude, stands out like an island in a sea of clouds.
Clouds are visual indicators of what is going on in the atmosphere. They are also aesthetically interesting.
Mammatus clouds are one of the more striking sights in the sky. Technically, they are a supplementary feature of a variety of other large clouds. However, they are most dramatic when they line the underside of a cumulonimbus. Shaped like giant udders, they form when parts of the anvil cool and sink into warmer air. In general, the more plump the udders, the more severe the recent or nearby thunderstorm.
Below is a photo of the remnants of the first Mammatus cloud I ever saw in person. It was taken in New York City shortly after a violent summer thunderstorm a few years ago.
On a recent visit to the Yale Center for British Art, I was enchanted by the cloud studies of John Constable. He was an English Romantic painter who believed, “…the sky is the source of light in nature, and governs everything.”
Constable is well known for his grand landscapes that emphasize atmospheric phenomena and mood. His sketch canvases, however, tell us even more about his fascination with the weather. On the back of his cloud studies, he recorded the weather conditions that existed while he painted. For example, on the back of the cloud study painted on September 13, 1821, the artist wrote, “1 o’clock, slight wind at NW, which became tempestuous in the afternoon, with rain all the night following.”
Interested in the developments of the atmospheric sciences of his time, Constable is reported to have owned a copy of Thomas Forster’s Researches About Atmospheric Phenomena. This book included Luke Howard’s 1802 scientific paper, Essay on the Modification of Clouds, which outlines the classification system for clouds that is still used today. A self described “man of clouds”, Constable believed, “we see nothing truly till we understand it.”
After two days of cloudy skies and heavy rain, it was nice to have a bright sunny day for the Thanksgiving holiday here in New York City.
This quick turnaround in the weather reminded me of a piece of poetry that I recently came across. The poem is called The Cloud and was written by Percy Shelley in 1820. It uses the weather as a metaphor, but highlights the fact that the atmosphere is in a continuous state of change. Clouds, visual indicators of atmospheric activity, are diverse in their forms and functions, but are nonetheless ephemeral phenomena. A cumulus cloud, for example, usually only lasts between five and forty minutes, depending on the wind and other environmental conditions. Enjoy the poem.
I am the daughter of Earth and Water,
And the nursling of the Sky;
I pass through the pores of the ocean and shores;
I change, but I cannot die.
For after the rain when with never a stain
The pavilion of Heaven is bare,
And the winds and sunbeams with their convex gleams
Build up the blue dome of air,
I silently laugh at my own cenotaph,
And out of the caverns of rain,
Like a child from the womb, like a ghost from the tomb,
I arise and unbuild it again.
Clouds are mostly made of water-vapor, but a dash of something extra is needed to complete their formation.
As air rises through the atmosphere, it cools. When it reaches its dew point, it is ready to form a cloud if it can find a condensation nuclei. These are free floating microscopic particles in the air, like dust, salt, or smoke . A cloud droplet is born when a water-vapor molecule lands on one of these tiny non-gaseous surfaces and condenses. As cloud droplets gather together in massive numbers, they become visible.
There are a variety of potential condensation nuclei in the air. Their size, composition, and capacity to attract and hold water molecules impact their ability to form clouds. Salt particles, for example, are very good at absorbing water while carbon based particles are not. Without the presence of condensation nuclei in the air, cloud development would be limited to areas of significantly colder air temperatures.