Tag Archives: explainer

A Look at the Different Types of Lightning

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Summer is the season for thunderstorms and the lightning they produce can light up the sky in a variety of ways. Here is a quick look at the different types of lightning.

While there are variations within each, these are the four primary categories:

Intra-Cloud: This is the most common type of lightning. It happens completely inside a single cloud, jumping between regions with different charges. It is sometimes called “sheet” lightning.

Cloud to Cloud: This is lightning that occurs between two or more separate clouds.

Cloud to Air: This type of lightning occurs when positive charges at the top of a cloud reach out to the negatively charged air around it.

Cloud to Ground: This lightning occurs between the cloud and the ground. It can be either positively or negatively charged.

While thunderstorms can be fascinating things to watch, is important to remember that all lightning is dangerous and strike locations are unpredictable. So, as NOAA recommends, “When Thunder Roars, Go Indoors.”

Different types of lightning. Credit: mshuntergi.com

Different types of lightning. Credit: mshuntergi.com

Seasonal Temperature Lag

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The June Solstice marks the official beginning of the summer season in the northern hemisphere. It is famous as the longest day of the year – a day when we receive the greatest amount of incoming solar radiation. The reason it is not also the warmest day of the year relates to a phenomenon known as seasonal temperature lag.

Air temperature depends not only the amount of energy we receive from the sun, but also the amount of energy absorbed by the planet’s landmasses and oceans. The heat capacity of both, which is defined as the amount of heat required to raise the temperature of a substance 1°C, play a major role. Given the fact that water has a much higher heat capacity than land and that oceans cover 71% of the Earth’s surface, it takes awhile for the atmosphere to warm up. Here in the mid- latitudes, we usually see our warmest days of the year in mid-July which are often referred to as the Dog Days of Summer.

In winter, the process works in reverse. The oceans take time to lose their heat. So, the coldest days generally lag the winter solstice by a few weeks.

How a Circumhorizontal Arc Forms

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The sky puts on an amazing light show everyday. But sometimes, it produces something special like a circumhorizontal arc.

Often mistaken for a rainbow, a circumhorizontal arc is an entirely different optical phenomenon. It is formed by the refraction, or bending, of sunlight through plate-like hexagonal ice crystals that are situated horizontally in cirrus or cirrostratus clouds. More specifically, light enters through the vertical side of the crystals and exits through their horizontal bottoms.This angled pathway produces the well-separated colors of the spectrum that we see in the sky.  They are brightest where the cirrus clouds are thickest. Oriented parallel to the horizon, a circumhorizontal arc always sits below the sun.

Rainbows, by contrast, are produced by the combination of refraction and reflection of sunlight in liquid water droplets. These arching bands of color always appear in the part of the sky that is opposite the sun.

Circumhorizontal arcs are somewhat rare. In addition to the appropriate cloud conditions, they require the sun to be very high in the sky – at least 58° above the horizon.  They are usually only seen during the summer in the mid-latitudes.

A circumhorzontal arc seen by the author last summer in Colorado. Image Credit: Melissa Fleming

A circumhorizontal arc seen by the author last summer in Colorado.  Credit: Melissa Fleming

Vernal Equinox 2015

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Today is the Vernal Equinox, the first day of spring in the northern hemisphere. The new season officially begins at 22:45 UTC, which is 6:45 pm 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. 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 daily passage across our sky has been moving northward 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 largest increase in average daily temperature, however, usually lags the equinox by a few weeks.

Earth’s solstices and equinoxes. Image Credit: NASA

Earth’s solstices and equinoxes. Image Credit: NASA

The Earth’s axis is tilted neither toward nor away from the sun on the Vernal Equinox. Image Credit: NASA

The Earth’s axis is tilted neither toward nor away from the sun on the Vernal Equinox. Image Credit: NASA

Searching for the End of a Rainbow

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At the end of a rainbow, according to Irish folklore, lies a leprechaun’s pot of gold. In reality, however, the true end of a rainbow is impossible to locate.

A rainbow is an optical phenomenon that forms when water droplets in the air both refract and reflect sunlight to reveal the colors of the visible spectrum in an arch formation. It is not a physical entity that can be touched or approached. To see them, the National Center for Atmospheric Research says you need to be both facing the source of moisture and be standing at a 42° angle to the sun’s rays.

This specific line of sight means that no two people will ever see the exact same rainbow. It also means that as you attempt to move closer to the rainbow, the further away it will appear. So, try as you might, you will never get close enough to see a rainbow’s true terminus.

In the end, rainbows are all about perception.  For many people, even without the promise of a pot of gold, the joy of sighting a beautiful rainbow is reward enough.  Happy Saint Patrick’s Day!

Rainbow appears to end in the Atlantic Ocean off Bermuda's coastline. Image Credit: The Weather Gamut

A rainbow appears to end in the Atlantic Ocean off Bermuda’s coastline.                      Image Credit: The Weather Gamut.

The Different Shapes of Snowflakes

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There is an old saying: “no two snowflakes are alike.” While that may well be true, their basic shapes are driven by temperature and atmospheric moisture content. Here is a look at their general categories.

snow-morphology-diagram

Snowflake morphology diagram.  Image Credit: Kenneth Libbrecht/CalTech

How Icicles Form

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Icicles are a classic symbol of cold winter weather. In order to form, however, they need a mix of both warm and cold conditions.

These hanging pieces of tapered ice develop when the air temperature is below freezing, but there is enough heat from the sun – or in the case of a building, a  poorly  insulated roof – to thaw some snow. As the melt water runs off the edge of a surface, it re-freezes in the cold air. Starting with only a few water droplets, an icicle can begin to form. Over time, as melt water continues to drip and re-freeze, an icicle gains both layers of thickness and length.

Often seen forming along the edge of roofs, icicles can also be found on tree branches, power lines, and rocks where water seeps out of the ground. The size and shape of an icicle, according to experts at the National Snow and Ice Data Center, depends on a few different factors. These include the shape of the surface on which it forms and the number of different directions from which melt water approaches the growing icicle.

Icicles can range in size from a few inches to tens of feet.

Icicles in the sunshine. Credit: MLE

Icicles in the sunshine. Credit: MLE

Earth’s Perihelion 2015

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The Earth reached its Perihelion today at 6:36 UTC, which is 1:36 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 about 146 million kilometers away from the Sun. That is approximately 5 million kilometers (3 million miles) closer than 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”.

Image Credit: mydarksky.org

Image Credit: mydarksky.org

The Polar Vortex

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The Polar Vortex has been making headlines across most of the United States recently.  But, what exactly is it?

According to NOAA, the northern hemisphere polar vortex is a high altitude low-pressure system anchored over the Arctic.  More specifically, it is “the pattern of winds around the North Pole.”  It is always present, but tends to be stronger in the winter.

The configuration of the polar vortex – smooth or wavy – determines how much cold air escapes the region. Driven by the temperature difference between north and south, these winds typically circle the pole from west to east in a smooth pattern that bottles up the Arctic’s cold air.   When these winds weaken, the pattern becomes wavy and cold air pushes southward.

The connection between warming conditions in the Arctic (decreasing the temperature difference between north and south) and extreme weather events in the mid-latitudes is an active area of research. Some scientists suggest that increasing Arctic temperatures may be responsible for disrupting the pattern of the polar vortex.

The polar vortex is shown in purple. Image Credit: NOAA

Polar Vortex configuration: Smooth vs Wavy.  Image Credit: NOAA

Today is Earth’s Perihelion

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The Earth reached its Perihelion today at 12 UTC, which is 7AM 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 about 146 million kilometers away from the Sun.  That is approximately 5 million kilometers closer than 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”.

Image Credit: Academy Artworks

Image Credit: Academy Artworks