Tag Archives: explainer

Weather Lingo: Downburst

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Thunderstorms pose a number of familiar hazards, such as lightning and hail. The lesser-known downburst, however, is also a serious threat to life and property.

A downburst is a strong downward current of air that causes damaging winds on or near the ground. They initiate high up in the atmosphere, where relatively dry air is entrained inside of an intense thunderstorm. The dry air evaporates some of the storm’s raindrops, which has a cooling effect. Since this cooler air is denser than the warm air that surrounds it, it sinks rapidly toward the surface. When it hits the ground, it spreads out radially – in straight lines in all directions. Reaching speeds in excess of 100mph, a downburst will knock down trees and other obstacles leaving a trail of debris all facing the same direction.

These straight-line wind events, according to the NWS, can vary in size and duration. When they cover an area less than 2.5 miles, they are referred to as microbursts. These typically last between 5 and 15 minutes. Larger events, known as macrobursts, affect an area greater than 2.5 miles and last from 5 to 30 minutes.

While short-lived, these powerful winds can pose a threat to property on the ground as well as airplanes in the process of taking off or landing.

Credit: NWS

Volcanic Smog: Kilauea’s Other Threat

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Lava is not the only thing flowing out of the fissures of Kilauea on Hawaii. Sulfur dioxide, a foul-smelling and toxic gas, has also driven people from their homes.

According to the USGS, sulfur dioxide levels near the volcano have been measured above 100 parts per million. That is a level considered dangerous to human health. Noxious on its own, sulfur dioxide is also the main ingredient in volcanic smog. Known as vog in Hawaii, it can have a variety of adverse health effects.

Vog occurs when the sulfur dioxide spewing from a volcano reacts with oxygen, moisture, and other particles in the air in the presence of sunlight. It is considered a form of air pollution, not unlike that given off by power plants burning sulfurous coal. Looking back to December 1948, a similar type of toxic smog caused by unregulated industrial pollution killed 26 people and sickened thousands of others in Donora, PA.

Volcanic smog can irritate the skin, eyes, nose, throat, and lungs. Shortness of breath and dizziness can also occur.  Its effects can be even worse for anyone with respiratory problems or lung disease.

This threat, on top of the flowing lava, has led public health officials to order an evacuation of areas around the fissures, such as the hard hit Leilani Estates.  Other parts of the Big Island, however, have reported moderate to good air quality. This is largely because the region’s prevailing northeast trade winds have been pushing the vog offshore. If those winds slacken and a southeasterly flow emerges, the vog could impact a wider area, including other islands in the Hawaiian chain.

Volcanic eruptions spew gas as well as lava. Credit: GoVisitHawaii

Do April Showers Really Bring May Flowers?

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The phrase, “April showers bring May flowers “ has been around for centuries. It is derived from a poem written in the 1500s by Thomas Tusser – an English poet and farmer. This old adage, however, does not hold true in the northeastern United States.

Coming on the heels of the snowy months of winter, April typically produces more rain than snow. Many people, therefore, consider it a rainy month. Since water is necessary for the overall survival of plants, they also associate it with the bloom of flowers in May. Nevertheless, according to botanists, perennials – the plants that go dormant in winter and re-grow in the spring – are more dependent on the soil moisture derived from winter snowmelt and the long-term local precipitation pattern.

In the end, though, temperature is the most significant factor in determining when a flower will bloom. As soon as the weather becomes more spring-like, flowers will start to blossom, regardless of how much it rained in April or whatever the prior month was. That said, a “false spring” – a warm spell that triggers flowering but is followed by a hard frost – can kill the fragile blooms.

It is also worth noting that April is not typically the wettest month of the year for most places in the US. In New York City, July, on average, takes that honor because of the downpours associated with its strong summer thunderstorms.

Peonies in bloom. Credit: Melissa Fleming

First Day of Spring 2018

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Today is the Vernal Equinox, the first day of spring in the northern hemisphere. The new season officially begins at 16:15 UTC, which is 12:15 PM 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 spring months, 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

How Rainbows Form

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St. Patrick’s Day is a holiday often associated with images of rainbows promising a path to a leprechaun’s pot of gold. For most people, however, just spotting a rainbow is enough to brighten a day.

These amazing displays of nature form when raindrops, which act like prisms, scatter sunlight. To see one, an observer must be facing a moisture source like rain or mist with the sun at their back. The sun also needs to be at a low angle in the sky, less than 42° above the horizon. The lower the sun angle, the more of a rainbow’s arc will be visible.

Refraction and reflection inside a raindrop. Credit: Met Office

Passing from the air into a denser raindrop, the light slows and refracts. Since the different wavelengths of light bend by different amounts, the white light is dispersed into the colors of the visible spectrum: red, orange, yellow, green, blue, indigo, and violet. Red, which has a long wavelength, is refracted the least and is always on the top of a single rainbow. Violet, with a shorter wavelength, is refracted the most and is always on the bottom.

The light also needs to reflect off the back wall of the raindrop towards the viewer at the critical angle of 48° before it refracts again when it re-enters the air. A lesser angle will let the light pass through the raindrop and a larger angle will allow the light to reflect straight back out of the drop.

A double rainbow is seen when the light reflects twice inside the raindrop. Since each reflection weakens the intensity of the light, the second bow appears dimmer. The order of the colors is also reversed, with blue on top and red on the bottom.

Rainbow and faint second rainbow form after a rainstorm in Bermuda. Credit: Melissa Fleming

Why Are We Seeing So Many Nor’easters?

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The east coast of the United States has been slammed with three nor’easters in just eleven days – March 2, March 7, and March 13. The reason for this barrage of storms involves something called the North Atlantic Oscillation (NAO).

Based in the North Atlantic Ocean, this weather pattern 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 areas and a robust jet stream that keeps cold air bottled up in the northern latitudes.

Three nor’easters in eleven days. Credit: NOAA

Fluctuating between positive and negative, the strength and duration of these phases vary. Since late February, however, a strong negative phase has been locked in place. With an area of high pressure over Greenland, the jet stream is blocked and therefore dipping southward over the eastern US. As the jet stream is essentially a storm track, this pattern has allowed areas of low pressure to be steered over the warm waters of the Gulf Stream off the eastern seaboard, where they have intensified into nor’easters.

In terms of climate change, the connection between the warming Arctic and the storm track across the mid-latitudes is an active area of research. Sea level rise, however, is clearly amplifying the coastal flooding associated with these powerful storms.

NAO Patterns. Credit: NOAA

Weather Lingo: Venturi Effect

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March, a transitional month between winter and spring, is well known for its winds. In large cities like New York, however, the wind can be accelerated by something called the Venturi Effect.

Tall buildings and straight, grid-like streets essentially create man-made canyons that affect how the wind moves through a city. Funneled through the buildings, the wind is constricted and forced to speed up. The same process is seen when you put your thumb over the mouth of a hose to create a choke point and make the water flow faster.

The Venturi Effect is named for Giovanni Battista Venturi, an 18th-century Italian physicist.

Credit: Currents

Weather Lingo: Nor’easter

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The winter season can produce a number of different types of storms. One of these is a nor’easter.

These intense systems generally affect the east coast of the United States from the mid-Atlantic to New England. They traditionally develop when a strong area of low pressure to the south moves up the coast and meets cold air pushing down from Canada. With a plentiful supply of moisture from the Atlantic, these storms are notorious for producing copious amount of precipitation. The exact type – rain or snow – depends on the temperature at the time of the storm. They are also known for their strong onshore winds that can cause coastal flooding and beach erosion.

Spinning counterclockwise, these storms take their name from the steady northeasterly wind they produce. 

Credit: NOAA

From Snow to Ice, Winter Precipitation Can Take Several Forms

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The winter season can produce various types of precipitation – rain, freezing rain, sleet, or snow. It depends on the temperature profile of the lower atmosphere.

All precipitation starts out as snow up in the clouds.  But, as it falls toward the Earth, it can pass through one or more layers of air with different temperatures.  When the snow passes through a thick layer of warm air – above 32°F – it melts into rain.  If the warm air layer extends all the way to the ground, rain will fall at the surface.  However, if there is a thin layer of cold air – below 32°F – near the ground, the rain becomes super-cooled and freezes upon impact with anything that has a temperature at or below 32°F.  This is known as freezing rain.  It is one of the most dangerous types of winter precipitation, as it forms a glaze of ice on almost everything it encounters, including roads, tree branches, and power lines.

Sleet is a frozen type precipitation that takes the form of ice-pellets. Passing through a thick layer of sub-freezing air near the surface, liquid raindrops are given enough time to re-freeze before reaching the ground. Sleet often bounces when it hits a surface, but does not stick to anything.  It can, however, accumulate.

Snow is another type of frozen precipitation.  It takes the shape of six-sided ice crystals, often called flakes.  Snow will fall at the surface when the air temperature is below freezing all the way from the cloud-level down to the ground.  In order for the snow to stick and accumulate, surface temperatures must also be at or below freezing.

When two or more of these precipitation types fall during a single storm, it is called a wintry mix.

Precipitation type depends on the temperature profile of the atmosphere. Credit: NOAA

Auroras: What Causes the Northern and Southern Lights

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Auroras occur throughout the year, but the long nights of winter at high latitudes provide an optimal environment in which to see this amazing natural phenomenon.

These colorful patterns of light that dance across the night sky are the result of charged particles from the sun interacting with the Earth’s atmosphere. Originating in a massive explosion on the sun known as a coronal mass ejection, the protons and electrons travel nearly 93 million miles before some of them reach the Earth. When they encounter the planet’s magnetic field, they are pulled toward the poles, where the magnetic force is strongest. There, they interact with atmospheric gases and produce the variety of colors we see. The main factor in which colors are displayed, however, is altitude. Different gases, such as nitrogen and oxygen, vary in concentration at different levels of the atmosphere.

Oxygen molecules can generate green auroras up to 150 miles above sea level and red auroras further up. Nitrogen molecules produce blue lights up to 60 miles above the ground and violet colored lights at higher levels. Auroras, in general, extend from 50 miles to as high as 400 miles above the Earth’s surface.

The word aurora is Latin for dawn. Therefore, the “aurora borealis”, the northern lights, means “dawn of the north”. At the South Pole, the lights are known as the “aurora australis”, which means “dawn of the south”.

Aurora Borealis over Alaska. Credit SmithsonianMag