Twelve New Cloud Types Added to International Cloud Atlas

Look up! There are some “new” clouds in the sky. The World Meteorological Organization (WMO) – the UN’s weather agency – announced twelve new additions to its prestigious International Cloud Atlas on Thursday.

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.

Asperitas Cloud. Credit: WMO

First Day of Spring 2017

Today is the Vernal Equinox, the first day of spring in the northern hemisphere. The new season officially began at 10:29 UTC, which is 6:29 AM Eastern Daylight Time.

Our astronomical seasons are a product of the tilt of the Earth’s axis – a 23.5° angle – and the movement of the planet around the sun. Today, as spring begins, the Earth’s axis is tilted neither toward nor away from the sun. This position distributes the sun’s energy equally between the northern and southern hemispheres.

Since the winter solstice in December, the arc of the sun’s apparent daily passage across the sky has been getting higher and daylight hours have been increasing. Today, the sun appears directly overhead at the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

As a transitional season, spring is a time when the chill of winter fades away and the warmth of summer gradually returns. The most noticeable increases in average daily temperature, however, usually lag the equinox by a few weeks.

Earth’s solstices and equinoxes. Image Credit: NASA

A Look at Rainbows and their Legendary Pots of Gold on this St Patrick’s Day

According to Irish folklore, a pot of gold can be found at the end of a rainbow. In reality, however, it is impossible to locate the terminus of this optical phenomenon.

For a rainbow to form, rain has to be falling in one part of the sky while the sun is out in another. The water droplets in the air act like prisms that refract and reflect the sunlight, revealing the colors of the visible spectrum. Red is refracted the least and is always on the top of the bow while blue is on the bottom. Since we only see one color from each drop, it takes a countless number to produce a rainbow.

That said, these colorful arcs are not physical entities that can be approached. No matter how close they appear to be, they are always tantalizingly out of reach. Nevertheless, most people consider seeing one to be a treasure with no gold required.

With a little luck, you can spot a rainbow if you face a moisture source – rain or mist from a waterfall – while the sun is at your back.

Happy Saint Patrick’s Day!

Rainbow after a rainstorm in Bermuda. Credit: Melissa Fleming

Weather Lingo: Bombogenesis

The weather world has some interesting words and phrases. One of these is “bombogenesis”.

Sounding rather ominous, it is a combination of the words cyclogenesis (storm formation) and bomb. It refers to the explosive or rapid intensification of an area of low pressure. More specifically, it means the central pressure of a storm system drops at least 24 millibars in 24 hours.

Air pressure is measured in millibars (mb) and the lower it is, the stronger the storm.

Taking place along steep temperature gradients, bombogenesis is most common along the east coast where cold continental air masses meet the relatively warm waters of the Gulf Stream. Disturbances in the jet stream above this type of temperature contrast help the air to rise and the pressure to drop.

This process can develop any time of the year but is most likely between October and March. When a system “bombs out” – a variation on the original phrase – strong winds, heavy precipitation, and even lightning can be expected. Nor’easters often become “weather bombs” – another popular variation – as they move up the coast.

Credit: TWC

The Difference Between a Snowstorm and a Blizzard

The biggest snowstorm of the year is expected to blast the northeastern US on Tuesday. In New York City, on top of the significant snow totals that have been forecast, a blizzard warning is in effect.

Different than a typical winter storm, a blizzard is characterized more by wind speeds and reduced visibility than the amount of snow it produces. According to the NWS, the three main factors for blizzard conditions are:

  • Wind – Sustained winds or frequent gusts of 35mph or higher.
  • Visibility – Falling and/or blowing snow that reduces visibility to ¼ mile or less.
  • Time – High winds and reduced visibility must prevail for at least 3 hours.

These conditions heighten the risk of power outages and often produce whiteout conditions on roadways, making travel extremely dangerous.  Stay Safe!

A blizzard warning is in effect for NYC. Credit: NWS

Why This Winter Has Been So Warm in the Eastern US

The weather usually associated with winter in the eastern United States has not really taken hold this year. One of the reasons for this involves something called the North Atlantic Oscillation (NAO).

This is a natural phenomenon that affects the position of the jet stream and weather patterns thousands of miles away. Based in the North Atlantic Ocean, it is driven by the pressure differences between the semi-permanent Icelandic Low and Azores/Bermuda High.

When the pressure difference between these two systems is low, the NAO is said to be in a negative phase. This means the winds of the jet stream are relatively relaxed and cold air from the north can spill down into the eastern US. The positive phase of NAO is characterized by a strong pressure difference between the two systems and a robust jet stream that keeps cold air bottled up in the northern latitudes.

Fluctuating between positive and negative, the strength and duration of these phases vary. This winter, however, the positive phase has been occurring more often and lasting longer than the negative phase. That is why the eastern US has been experiencing prolonged warm spells separated by a few brief blasts of cold air.

Unsurprisingly, this season’s soaring temperatures have sparked many important conversations about global warming. But as weather is extremely variable, no single warm day or week can be linked (at this time) to our changing climate. That said, anomalously warm events are happening more often, which is consistent with the long-term trend of human-caused climate change.  2016, for example, was this planet’s third consecutive warmest year on record.

Typical impacts associated with the positive phase of NAO. Credit: NOAA/NCDC

NAO observations, Nov 2016 to date. Credit: NOAA/CPC

Weather Lingo: The Benchmark

Just like real estate, weather is all about location. In the northeastern US, a special set of coordinates known as “The Benchmark” (40°N 70°W) can help identify the type of impacts a winter storm will have on the region.

When a low-pressure system travels west of this position, coastal areas will see more rain than snow as the storm pulls relatively warm marine air onshore. Further inland, where the air is colder, snow is more likely.

If a storm tracks east of the benchmark, it is essentially moving away from land and less warm air is pulled onshore. Some light snow will fall along the coast, but usually not very much.

When a system moves directly through the crosshairs of the benchmark, coastal communities in the region can expect a major snow event. This is exactly what happened with the storm on Thursday that dumped heavy snow across the area.

The 40/70 Benchmark. Credit: Wx4cast

Climate Science is Not New

As someone who both writes and gives talks on climate change, I often meet people with doubts about the subject who ask: “Climate science is so new, how can we trust it?” The answer is simple. It is not new. In fact, the fundamentals of climate science have been understood for close to 200 years.

One of the first scientists to look into the planet’s energy balance was Joseph Fourier, a French physicist, in the 1820s. Given the Earth’s distance from the Sun, he was curious to know why its temperature was not cooler. Fourier felt that something other than incoming solar radiation was keeping the planet warm and hypothesized that the atmosphere was somehow acting like an insulating blanket. Working with the limited technology of the day, however, he was unable to make the detailed measurements needed to carry his idea further.

Decades later, in the 1860s, an Irish scientist named John Tyndall picked up Fourier’s theory. An alpine adventurer, he was interested in glaciers and the then controversial idea of ice ages. Wanting to know more about how they formed, he devised an experiment to see if the Earth’s atmosphere was acting like a thermostat. For this, he built a spectrophotometer – an instrument that measures the amount of heat that gases can absorb. His experiments showed that water vapor, carbon dioxide (CO2), and methane were all very efficient at trapping heat. This essentially proved Fourier’s idea of a greenhouse effect.

In the 1890s, Svante Arrhenius, a Swedish physicist, followed up on Tyndall’s idea of an atmospheric thermostat and ran with it. Ruling out water vapor as too transitory, he focused on carbon dioxide, which tends to linger in the atmosphere for a long time. His calculations showed that doubling the amount of carbon dioxide in the atmosphere would raise the average global temperature by 5°C (9°F).

To understand if such a large-scale change in atmospheric CO2 was possible, he turned to Arvid Hogbom, a colleague studying the global carbon cycle. This is the natural geochemical process where volcanic eruptions and the chemical weathering of rocks release CO2, while plants and oceans absorb it. Hogbom confirmed that CO2 levels could change dramatically over long periods of time. However, he also noted that industrial processes were releasing a significant amount of CO2 relatively quickly. Using this information, Arrhenius calculated that human activities, such as burning fossil fuels, could alter the composition of the atmosphere and increase global temperatures. In the 1890’s, however, fossil fuel use was only a fraction of what it is today and he believed it would take more than 1,000 years for the level of atmospheric CO2 to double.

Jumping ahead to the 1950s, Charles David Keeling, a researcher at the Scripps Institution of Oceanography in California, found a way to directly monitor levels of CO2 in the atmosphere. He created an instrument called a gas chromatograph and installed it on top of Mauna Loa in Hawaii. At an elevation of more than 11,000 feet in the middle of the Pacific Ocean, it is removed from both direct CO2 sources like factories and sinks such as forests that could skew the data. Still in operation today, the information recorded at this station is known as the Keeling Curve. It shows the steady increase in CO2 levels in the atmosphere from 1958 to present.

Keeling’s measurements provided solid evidence that CO2 levels were rising and validated the theories of Tyndall and Arrhenius. More recently, scientists were able to extend his curve back in time by analyzing ancient air bubbles trapped in ice-cores from Greenland and Antarctica. This lengthy record shows that pre-industrial CO2 levels in the atmosphere were about 280 ppm. Today, they are over 400ppm – the highest they have been in more than 800,000 years.

Seeing this dramatic rise in CO2 and realizing the impact that a warming climate could have on society, the UN formed the Intergovernmental Panel on Climate Change (IPCC) in 1988. They assess the peer-reviewed research of thousands of scientists from around the world and publish a synthesized view of the current science. The latest IPCC report (AR5 published in sections in 2013/2014) unconditionally states that human activities are the main drivers of modern climate change.

Therefore, while it is the nature of all science to evolve with time and research, it is safe to say that climate science is not a new subject. It is only relatively new to those in the political sphere.

Giants in the history of climate science.

Weather Lingo: Rain Shadow

The world of weather has some interesting words and phrases. One of these is “Rain Shadow”.

While it sounds rather poetic, a rain shadow refers to the land area on the leeside of a mountain that is exceptionally dry. Mountains act as barriers for weather systems traveling in a region’s prevailing winds, forcing them to drop most of their moisture on the windward side before they can pass.

As an air mass rises up and over a mountain, it enters an area of lower atmospheric pressure where it expands and cools. As a result, the moisture it contains condenses, clouds form, and precipitation falls. After the air mass moves over the mountain, it starts to descend the other side. The air is warmed by compression and the clouds dissipate. This means little to no rain falls on the leeward side.

Rain shadows are found all over the world, from the Tibetan Plateau in Asia to the Atacama Desert in South America. Here in the US, Death Valley is a famous example as it lies in the rain shadow of four different mountain ranges.

The Rain Shadow Effect. Credit: Kagee Commons

Earth’s Perihelion 2017

The Earth will reach its Perihelion today at 14:18 UTC, which is 9:18 AM Eastern Standard Time. This is the point in the planet’s orbit where it comes closest to the Sun.

This annual event is due to the elliptical shape of the Earth’s orbit and the off-centered position of the Sun inside that path. The exact date of the Perihelion differs from year to year, but it’s usually in early January – winter in the northern hemisphere. The Earth will be furthest from the Sun in July.

While the planet’s distance from the Sun is not responsible for the seasons, it does influence their length. As a function of gravity, the closer the planet is to the Sun, the faster it moves. Today, the Earth is 147.1 million kilometers (91.4 million miles) away from the Sun. That is approximately 5 million kilometers (3 million miles) closer than it will be in early July. This position allows the planet to speed up by about one-kilometer/second. As a result, winter in the northern hemisphere is about five days shorter than summer.

The word, perihelion, is Greek for “near sun”.

Earth is closest to the Sun during the northern hemisphere’s winter. Credit: