Earth’s Aphelion 2015

The Earth will reach its Aphelion today at 19:41 UTC, which is 3:41 PM Eastern Daylight Time. This is the point in the planet’s orbit where it is farthest from the Sun.

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

Image Credit: mydarksky.org

Seasonal Temperature Lag

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.

Searching for the End of a Rainbow

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.

What is a Blizzard?

A blizzard is expected to blast the northeastern United States over the next two days. Different than a typical winter storm, a blizzard is characterized more by its winds than the amount of snow it produces.

According to the National Weather Service, a blizzard means the following conditions prevail for three hours or longer:

  • Sustained winds or frequent gusts of 35mph or higher, and
  • Considerable falling and/or blowing snow that frequently reduces visibility to ¼ mile or less.

These conditions heighten the risk for power outages and often produce whiteout conditions on roadways, making travel extremely dangerous.

The Dead of Winter: Coldest Part of the Season

The “Dead of Winter” is an old saying that refers to the coldest part of the winter season. This annual chilly period, statistically, begins today.

While actual daily weather varies, historical average temperatures in most of North America reach their lowest point of the year between January 10th and February 10th.  This cold period does not begin on the winter solstice, the day we receive the least amount of solar energy, because of a phenomenon known as seasonal temperature lag.

Air temperature depends on both the amount of heat received from the sun and the amount of heat lost or absorbed by the oceans and continents. From the start of winter through mid-February, both the oceans and land are losing more heat than they gain.

These few cold weeks are the climatological opposite of the “Dog Days of Summer.”

Earth’s Perihelion 2015

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

Winter Solstice 2014

Today is the December Solstice, the first day of winter in the northern hemisphere. The new season officially begins at 23:03 UTC, which is 6:03pm EST here in New York City.

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 winter months, the northern half of the Earth is tilted away from the sun. This position brings the hemisphere the least amount of solar energy and its coolest temperatures of the year.

Since the summer solstice in June, the arc of the sun’s daily passage across the sky has been dropping toward the southern horizon and daylight hours have been decreasing. Today, it reached its southern most position at the Tropic of Capricorn (23.5° south latitude) marking the shortest day of the year. This observable stop is where today’s event takes its name.  Solstice is derived from the Latin words “sol” for sun and “sisto” for stop.

Now, the sun will move northward again in our sky and daylight hours will slowly start to increase. Marking this transition from darkness to light, the winter solstice has long been a cause for celebration across many cultures throughout human history.

Seasons

Earth has seasons because it is tilted on an axis relative to its orbit around the sun.  Image Credit: NASA

winter_solstice-1

The Sun is directly overhead at the Tropic of Capricorn on the Winter Solstice.                     Image Credit: NASA

How Greenhouse Gases Influence Climate

The latest round of UN climate change talks is currently underway in Lima, Peru. Representatives from nearly 190 countries are meeting to discuss ways to reduce greenhouse gas emissions, the main driver of global warming.

Earth’s atmosphere is made up of a variety of gases, mostly nitrogen and oxygen by volume. The greenhouse gases, including water vapor, carbon dioxide, and methane, represent a smaller percentage, but are also a natural part of the mix. Acting like the windowpanes of a traditional glass greenhouse, these gases allow the sun’s energy (shortwave radiation) to pass through the atmosphere during the day and heat the Earth’s surface. At night, the greenhouse gases trap some of the heat (long-wave radiation) that the surface emits as it cools.  In essence, greenhouse gases function like a blanket that help keep the planet warm. Without them, the average surface temperature of the Earth would be 0°F – a temperature at which all the water on the planet would be frozen and life as we know it would not exist. Having too many greenhouse gases is also a problem – one that we are currently facing.

Simply put, more greenhouse gases in the atmosphere trap more heat and increase the planet’s average temperature. During the last century, according to the IPCC, Earth’s mean temperature rose 1.5°F.  As temperatures continue to rise, long established weather patterns and storm tracks are shifting. Different regions, in turn, are being affected in different ways. Some areas are getting wetter, while others are getting dryer, and coastal communities are feeling the impacts of rising sea levels.

Scientists say that while some greenhouse gases come from natural sources like volcanic eruptions, the vast majority entering our atmosphere today come from human activities that burn fossil fuels. Before the industrial revolution in the late 1700’s, atmospheric carbon dioxide levels were 280 parts-per-million (ppm). This year, it passed 400ppm for the first time in human history. In addition, according to NOAA, 2014 is on track to be the planet’s warmest year on record.

Any agreements reached in Lima on reducing greenhouse gas emissions will be used as the framework for a binding global treaty at the UN Climate Conference in Paris next year.

Source: dec.ny.gov

Source: dec.ny.gov

Lake-Effect Snow

A relentless snowstorm buried the Buffalo area of western New York State with more than 5 feet of snow this week. Situated on the shore of Lake Erie, the impressive accumulation was the product of a meteorological phenomenon known as “Lake-Effect Snow.”

Lake-effect snowstorms, according to NOAA, develop when cold air blows across the warmer waters of a large unfrozen lake. The bottom layer of the air mass is warmed by the water and allows it to evaporate moisture, which forms clouds. When the air mass reaches the leeward side of the lake its temperature drops again, because the land is cooler than the water. This releases the water vapor as precipitation and enormous amounts of snow can accumulate. The effect is enhanced if the air is lifted upward by local topography.

With the clouds typically forming in bands, the snowfall is highly localized. Some places can see the snow come down at a rate of more than 5 inches per hour, while others will only get a dusting. The shape of the lake and the prevailing wind direction determines the size and orientation of these bands.

Fetch, the distance wind travels over a body of water, also plays a key role. A fetch of more than 60 miles is needed to produce lake effect snow. In general, the larger the fetch, the greater the amount of precipitation, as more moisture can be picked up by the moving air.

The massive surface area of the Great Lakes in the northern United States make them excellent producers of lake-effect snow. With northwesterly winds prevailing in the region, communities along the southeastern shores of the lakes are often referred to as being in the “Snowbelt.”

Credit: NOAA

Credit: NOAA

The Polar Vortex: A Widely Misused Weather Term

An early season arctic blast has sent temperatures across most of this country plummeting well below average this week and brought last winter’s buzzword, the polar vortex, back into the spotlight. From advertisements for winter coats to social media hashtags for almost anything cold, this technical meteorological term is being widely misused.

A polar vortex, according to NOAA, is a massive and persistent high altitude low-pressure system present over both poles of this planet. Basically a whirlpool-like wind pattern, the northern hemisphere’s polar vortex is anchored above the Arctic. It, as a whole, does not move south over the US.  That said, pieces of it can ocassionally meander southward and influence our weather via the position of the polar jet stream.

When the polar vortex is strong, the jet stream generally flows in a smooth circular pattern from west to east and bottles up the Arctic’s coldest air.  When weak or displaced by an area of high pressure, the shape of the jet stream distorts into a wavy, more north to south pattern.  This allows cold air influenced by the polar vortex to push southward.  When this happens, it is called a polar outbreak.

This week’s unseasonably cold temperatures are the result of a large ridge in the jet stream to our west that was enhanced by former typhoon Nuri in the Pacific.  It, in turn, has caused a sizable trough to develop east of the Rocky Mountains and allowed cold arctic air to flow deep into the US.

In the video below, Dr. Mark Serezze, Director of the National Snow and Ice Data Center, explains exactly what the polar vortex is, how it works, and how climate change may play a role in its future.

Video credit: EarthVisionTrust and YouTube