Tag Archives: explainer

Weather Lingo: The Brown Ocean Effect

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Tropical cyclones are fueled by warm ocean water and typically peter out over land. Sometimes, however, their lives are extended by something called the “brown ocean effect”.

This is a phenomenon where a storm derives energy from the evaporation of abundant soil moisture deposited by previous rainfall. Essentially, the saturated soil mimics the role of the ocean allowing a tropical cyclone to maintain its strength or even intensify after making landfall.

For the brown ocean effect to occur, according to a NASA funded study by Theresa Andersen and Marshall Shepherd of the University of Georgia, three criteria need to be met:

  • The soil needs to contain copious amounts of moisture.
  • Atmospheric conditions near the ground must have tropical characteristics with minimal variation in temperature.
  • Evaporation rates must be high enough to provide the storm with sufficient latent heat that it uses for fuel, at least 70 watts averaged per square meter.

Although this process supplies less energy than the ocean, it is enough to sustain a storm for a longer period than normal over land. It was first noticed in 2007 after Tropical Storm Erin made landfall in Texas and then intensified as it traveled inland. It formed an eye over Oklahoma and unleashed a massive amount of rainfall.

Storms that are impacted by the brown ocean effect maintain a warm-core and are known as Inland Tropical Cyclone Maintenance and Intensification events (TCMIs). While rare, they are most common in the US, China, and Australia.

Credit: NBC/TWC

Weather Lingo: Invest

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The Hurricane Season officially began in June, but August is when things typically start to ramp up. It is also when the word “invest” (short for an investigative area) becomes more prevalent in weather forecasts.

When the National Hurricane Center (NHC) wants to take a closer look at an area of disturbed weather that could possibly develop into a tropical cyclone, it designates it as an invest. This opens up specialized resources, such as computer models and satellites that provide forecasters with additional data on the area in question. That said, an invest does not always become a tropical system.

More than one invest can exist at any given time, so the NHC has a special way to identify them. They are numbered from 90 to 99, followed by a letter. If the invest is in the Atlantic, the letter will be “L” and if it is in the Eastern Pacific, it will be an “E”. The numbers can be reused throughout a season, as necessary.

The Central Pacific Hurricane Center and the Joint Typhoon Warning Center follow a similar system. Letters for the basins they cover include “C” for the Central Pacific, “W” for the Western Pacific, “A” for the Arabian Sea, and “B” for the Bay of Bengal.

If an invest in any basin develops into a tropical storm, it is reclassified and given a name from that season’s pre-determined list.

An example of how the National Hurricane Center monitors and forecasts the development track of an Invest. Red is the current Invest 99L and Yellow is 90L Credit: NHC

What are the Dog Days of Summer?

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The “Dog Days” of summer have arrived. This popular saying refers to what are traditionally the hottest and most oppressive days of the season.

Rooted in astronomy, the phrase is linked to Sirius, the brightest star seen from Earth. As part of the constellation Canis Major, it is known as the Dog Star.  During most of July and August, Sirius rises and sets with our Sun. Ancient Greeks and Romans believed it acted like a second Sun, adding extra heat to summer days. Today, we know that light from this distant star does not affect our weather, but the name has endured.

Varying by latitude around the globe, the so-called “Dog Days” of summer typically run from July 3 to August 11 in the United States.

Sirius, the “Dog Star”.  Credit: EarthSky/Tom Wildoner

Aphelion 2017: Earth Farthest from Sun Today

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The Earth will reach its farthest point from the Sun today – an event known as the aphelion. It will officially take place at 20:11 UTC, which is 4:11 PM Eastern Daylight Time.

This annual event is a result of the elliptical shape of the Earth’s orbit and the slightly off-centered position of the Sun inside that path. The exact date of the Aphelion differs from year to year, but it’s usually in early July – summer in the northern hemisphere.

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, Earth is about 152 million kilometers (94 million miles) away from the Sun. That is approximately 5 million kilometers (3 million miles) further than during the perihelion in early January. That means the planet will move more slowly along its orbital path than at any other time of the year. As a result, summer is elongated by a few days in the northern hemisphere.

The word, aphelion, is Greek for “away from the sun”.

Earth is farthest from the Sun during summer in the northern hemisphere. Credit: TimeandDate.com

Summer Solstice 2017

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Today is the June Solstice, the first day of summer in the northern hemisphere. The new season officially began at 04:24 UTC, which is 12:24 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. During the summer months, the northern half of the Earth is tilted toward the sun. This position allows the northern hemisphere to receive the sun’s energy at a more direct angle and produces our warmest temperatures of the year.

Since the winter solstice in December, the arc of the sun’s apparent daily passage across the sky has been moving northward and daylight hours have been increasing. Today, it reached its northernmost position at the Tropic of Cancer (23.5° north latitude) marking the “longest day” of the year. This observable stop is where today’s event takes its name. Solstice is a word derived from Latin and means “the sun stands still”.

While today brings us the greatest number of daylight hours all year (15 hours and 5 minutes in NYC), it is not the warmest day of the year.  The hottest part of summer typically lags the solstice by a few weeks. This is because the oceans and continents need time to absorb the sun’s energy and warm up – a phenomenon known as seasonal temperature lag.

Earth’s solstices and equinoxes. Image Credit: NASA

Severe Thunderstorms: Watches vs Warnings

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A severe thunderstorm is forecast for the New York City area on Monday afternoon. In addition to lightning, it could bring strong winds, heavy rain, hail, and the possibility of a tornado. Simply put, this is the type of weather that can cause property damage and loss of life. Therefore, it is important to understand the difference between the various alerts issued by the National Weather Service. They include advisories, watches, and warnings.  All should be taken seriously.

  • Advisory: Issued when significant, but not necessarily hazardous, weather conditions are likely to occur. Residents should exercise caution.
  • Watch: Issued when dangerous weather conditions are possible over the next several hours.  They generally cover a large geographic area.  Residents should be prepared to take action.
  • Warning: Issued when dangerous weather is imminent or already occurring.  They cover a smaller, more specific geographic area.  Residents should take action immediately.

The Eastern Pacific Hurricane Season is Underway

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Hurricane season in the eastern Pacific begins today.

Tropical cyclones, known as hurricanes in the United States, develop around the globe at different times of the year. In the northeastern Pacific, they tend to form between May 15 and November 30. This early start is related to the basin’s warm sea surface temperatures and relatively low wind shear.

While powerful, these Pacific storms are generally not as familiar to Americans as those that form in the Atlantic. This is because they rarely make landfall in this country. In fact, it has only happened twice. A hurricane slammed San Diego, CA in 1858 and a tropical storm battered Long Beach, CA in 1939. This low rate of occurrence is attributed to the cold water of the California Current that flows south along the west coast. Nonetheless, Pacific hurricanes can still impact the US.

Developing in the tropics, Pacific storms deteriorate as they travel north to cooler waters and in some cases over the mountains of Mexico. However, their remnants are still laden with moisture when they reach the southwestern US, where they often unleash flooding rains.

East Pacific storms can also cross into the Central Pacific and affect Hawaii. (The dividing line between the two basins is 140°W longitude.) One such storm was Hurricane Iniki in 1992, the worst hurricane in the state’s history. With wind speeds measured up to 145mph, it was rated category-4 on the Saffir-Simpson scale.

This year, the eastern Pacific hurricane season got off to a record early start, with the development of Tropical Storm Adrian on May 9. No other storm in the basin has formed earlier during the satellite era.

Source: WMO

The Different Ways a Nor’easter Can Develop

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In the northeastern United States, nor’easters are well known for producing heavy precipitation, strong winds, and coastal flooding. Despite approaching the region from the south, they take their name from the steady northeasterly winds that blow in off the ocean.

Nor’easters can occur any time of the year but are most common between November and March. This is because the cold air that dips south in a jet stream trough during fall and winter often meets warmer air moving north over the Gulf Stream, which is just off the east coast. This contrast in temperature strengthens the storms. That said, there are a few different ways in which they can develop.

The first is called a “Miller Type-A” storm and is considered the “classic” nor’easter. These occur when an area of low pressure develops along the Gulf Coast or Atlantic Coast and intensifies as it tracks northward along the eastern seaboard.

The second type originates in the mid-west and is known as a “Miller Type-B” storm. Traveling east, these low-pressure systems weaken when they run into the Appalachian Mountains and transfer their energy to the coast. Often called a “center jump”, this transfer strengthens or creates a secondary low on the lee side of the mountains, which then moves northward. These types of storms are notoriously difficult to forecast as everything depends on the timing and location of the energy transfer between the two lows.

Both Miller “types” are named after James E. Miller, an atmospheric scientist who studied storm formation in the Atlantic coastal region during the mid 20th century.

Typical Miller Type A and B storm tracks. Credit: NOAA

Twelve New Cloud Types Added to International Cloud Atlas

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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

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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