Tag Archives: explainer

The Weather Gamut Becomes a Weather-Ready Nation Ambassador for NOAA

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It’s official! The Weather Gamut is now a Weather-Ready Nation (WRN) Ambassador for the National Oceanic and Atmospheric Administration (NOAA).

As a member of the WRN Initiative, The Weather Gamut is committed to working with NOAA to help increase awareness and strengthen resilience to our nation’s increasing vulnerability to extreme weather and climate events.

Be it hurricanes, tornadoes, snowstorms, heat waves, cold waves, drought, or flooding, every part of this country is subject to extreme weather. Therefore, it is vital that we, as a nation, improve our readiness, responsiveness, and resiliency to these environmental hazards that threaten lives and property.

Look for articles tagged “WRN” on our website to learn more about different types of extreme weather and climate events and how to prepare for them.

For more info on NOAA's WRN Initiative: http://www.nws.noaa.gov/com/weatherreadynation/

Weather-Ready Nation Ambassador™ and the Weather-Ready Nation Ambassador™ logo are trademarks of the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, used with permission. For more information, click  here.

The Azure Skies of Autumn

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Autumn is well known as the time of year when leaves change color. However, have you ever noticed the sky also changes shades with the season?

In general, we see the sky as blue because of Rayleigh scattering. This is a phenomenon where the molecules of nitrogen and oxygen that make up most of Earth’s atmosphere scatter the incoming light radiation from the sun. More to the point, they are most effective at scattering light with short wavelengths, such as those on the blue end of the visual spectrum. This allows blue light to reach our eyes from all directions and dictates the color we understand the sky to be.

The arc height of the sun’s apparent daily passage across our sky, which varies with the seasons, determines how much of the atmosphere the incoming light must pass through. This, in turn, affects how much scattering takes place. Simply put, the more Rayleigh scattering, the bluer the sky appears.

That said, humidity levels also play a role. Water vapor and water droplets are significantly larger than nitrogen and oxygen molecules, and therefore scatter light differently. Instead of sending light in all directions, they project it forward. This is known as Mie scattering and tends to create a milky white or hazy appearance in the sky.

During the summer months, when the sun is higher in the sky, light does not have to travel that far through the atmosphere to reach our eyes. Consequently, there is less Rayleigh scattering. The warm temperatures of summer also mean the air can hold more moisture, increasing the effect of Mie scattering. As a result, the summer sky tends to be a relatively muted or pale blue.

In autumn, the sun sits lower on the horizon, increasing the amount of Rayleigh scattering. The season’s cooler temperatures also decrease the amount of moisture the air can hold, diminishing the degree of Mie scattering. Taken together, these two factors produce deep blue skies.

When this azure hue is contrasted with the reds and yellows of the season’s famous foliage, all of the colors look even more vibrant.

Photo credit: Azure-Lorica Foundation

Yellow leaves pop against a deep blue autumn sky. Photo credit: Azure-Lorica Foundation

What is Indian Summer?

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Autumn is a season known for changing leaves and falling temperatures. Every once in a while, however, summer warmth makes a resurgence. When this happens, it is often dubbed an “Indian Summer”.

This weather phenomenon, according to the NWS glossary, is defined as “an unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.”  In the northeastern US, it is generally associated with an area of high pressure to the south that ushers warm air northward.

In popular use since the 18th century, the exact origins of the term “Indian Summer” are a bit foggy. One of the more reasonable explanations behind this unique phrase suggests a connection to when Native Americans began their hunting season, but no one knows for sure.

In other parts of the world, this summer-like weather goes by a variety of different names. In Europe, a number of countries associate the unusual warmth with the nearest saint’s day. It is known as “St. Luke’s Little Summer” if it develops in October or a “St. Martin’s Summer” if it occurs in November. In temperate parts of South America, it is simply known as “Veranico” (little summer).

Regardless of its name, the timing and intensity of these autumn warm spells vary from year to year. Nevertheless, when they do occur, they usually only last a few days. So, as we inevitably move toward winter, enjoy them while they last.

Autumnal Equinox 2016

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Today is the Autumnal Equinox, the first day of fall in the northern hemisphere. The new season officially begins at 14:21 UTC, which is 10:21 AM Eastern Daylight Time.

The 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 autumn 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 summer solstice in June, the arc of the sun’s apparent daily passage across our sky has been moving southward and daylight hours have been decreasing. Today, it crossed the equator and we have approximately equal hours of day and night. The word “equinox” is derived from Latin and means “equal night”.

With the sun sitting lower in the sky and daylight hours continuing to shorten, Autumn is a season of falling temperatures. According to NOAA, the average high temperature in most US cities drops about 10°F between September and October.

Earth’s solstices and equinoxes. Image Credit: NASA

Earth’s solstices and equinoxes. Image Credit: NASA

Why Hurricanes Hit the East Coast and Not the West Coast of the US

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It is mid-September and hurricane season is in full swing in both the Atlantic and Pacific. With these mighty oceans bordering both sides of the US, have you ever wondered why hurricanes only make landfall on the east coast?

The answer is two-fold, involving the direction of prevailing winds in the tropics and the difference in water temperature in the two basins.

Hurricanes develop at tropical and sub-tropical latitudes in both the Atlantic and Pacific, where water temperatures are at least 80°F. This part of the globe is also where the Trade Winds prevail, flowing from east to west.

In the Atlantic, storms traveling west-northwest often run into the east coast or Gulf Coast of the US. There, the warm waters of the Gulf Stream that flow along the eastern seaboard sustain them as they move northward.

In the Pacific, storms tend to be pushed out to sea by the Trade Winds.  Any hurricanes that manage to move north quickly dissipate when they encounter the cooler waters of the California current that flows southward along the west coast from Canada.

Only two tropical systems have ever made landfall on the west coast of the US. A hurricane slammed San Diego, CA in 1858 and a tropical storm battered Long Beach, CA in 1939. That said, hurricanes and tropical storms generally have indirect impacts on the western states. When a named storm makes it as far north as Baja California, remnants of it can travel across the border and cause heavy rain and flooding in parts of the American southwest.

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All North Atlantic and Eastern North Pacific hurricanes, through 2013. Credit: NOAA/NWS

What is a Heat Dome?

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Summer is the season for warm weather. So, when temperatures reach 5°F to 10°F above average, it can be excessively hot. When this type of weather lasts for multiple days, it is usually the result of a phenomenon known as a “heat dome”.

Although not an official meteorological term, it does help paint a picture of what is happening. To start, an area of high pressure develops under a ridge in the jet stream. Acting like a lid in the upper atmosphere, it forces warm air that would normally rise to sink back toward the surface. As it sinks, it compresses and warms even further. Unable to escape, the hot air is remains in place until the ridge breaks down or moves.

Heat domes are not rare events, but when they produce extended heat waves and poor air quality, they can pose serious dangers to human health.

A Heat Dome forms in the upper atmosphere. Credit: NOAA

A Heat Dome forms in the upper atmosphere. Credit: NOAA

Earth’s Aphelion 2016

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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 16:24 UTC, which is 12:24 PM Eastern Daylight Time.

This annual event is a result of the elliptical shape of the Earth’s orbit and the 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 sun”.

Image Credit: mydarksky.org

Earth is farthest from the Sun during summer in the northern hemisphere. Image Credit: mydarksky.org

Summer Solstice 2016

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Today is the June Solstice, the first day of summer in the northern hemisphere. The new season officially begins at 22:34 UTC, which is 6:34 PM Eastern Daylight Time.

The astronomical seasons are produced by 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 northern most 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 meaning, “sun stands still”.

While today brings us the greatest number of daylight hours all year (15 hrs, 5 min. 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.

The tilt of the Earth during different seasons. Image Credit: NASA

The tilt of the Earth during different seasons. Image Credit: NASA

If You Don’t Like the Weather in Iceland, Just Wait Five Minutes

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In Iceland, people are fond of saying, “If you don’t like the weather, just wait five minutes.” This popular expression is written on everything from t-shirts to coasters, and during the course of my recent visit I also found it to be true.

The notoriously changeable weather of this island nation is largely the result of its location. Situated just south of the Arctic Circle, Iceland sits at the border between the Arctic Ocean with cold air masses to the north and the Atlantic Ocean with milder air masses to the south. A branch of the warm Gulf Stream Current known as the Irminger Current also flows along the country’s southern and western shores moderating the climate. As this mild air interacts with cold arctic air, it produces frequent changes in the weather. A relatively mild, sunny day can quickly turn cold and rainy. Strong winds are also very common.

So, when in Iceland, heed the advice of the locals and be prepared for all four seasons on any given day.

Coaster for sale with the popular saying, " Welcome to Iceland. If you don't like the weather, just wait five minutes." Credit/Source: Gullfoss.is

Coaster for sale with the popular saying, “Welcome to Iceland. If you don’t like the weather, just wait five minutes.” Credit/Source: Gullfoss.is

How Does Hail Get to be the Size of a Grapefruit?

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Spring is considered severe weather season in the Central US and on Tuesday the power of Mother Nature was on full display across the region. More than 300 severe storm reports were counted and the vast majority included very large hail.

In Kansas and Nebraska, hailstones the size of a grapefruit were reported. Those are balls of ice measuring about four inches in diameter. According to the NWS, once a thunderstorm produces hail with a one inch diameter or more it is considered severe. So, how does hail get that big?

The answer to that question lies with the speed of a storm’s updraft. Basically, the stronger the updraft, the longer the ice remains suspended in the cloud where it can grow larger. Below is a chart that shows approximately how strong an updraft has to be to support different sizes of hail.

The largest hailstone ever recorded fell in Vivian, South Dakota on July 23, 2010 and measured eight inches in diameter – about the size of a volleyball. To support a hailstone that size, the updraft likely exceeded 150mph.

Credit: NWS

Credit: NWS